We uncover a relation between the scattering amplitudes of massive strings and the α′ expansion of the massless string amplitude at tree level. More precisely, the n-point tree amplitude of n−1 massless and one massive state is written as a linear combination of n+1 massless string amplitudes at the α′ square order.
Scattering amplitudes for pions in the non-linear sigma model (NLSM) have been the subject of careful study for many decades. A hallmark feature of NLSM amplitudes is their vanishing soft behavior (Adler zero) which only recently was crucial for the formulation of recursion relations at tree-level. In this contribution we present novel off-shell recursion relations valid for tree-level amplitudes and planar loop-integrands. Still leveraging the Adler zero as a guiding principle, we formulate a highly efficient Berends-Giele-type recursion for NLSM amplitudes and integrands based on a so-called "Soft Factor Expansion". This recursion also reinforces recently discovered close connections between NLSM amplitudes and those of the simple bi-adjoint cubic scalar (BAS) theory.
Much of our understanding of string theory relies on the perturbative window provided by the worldsheet. Nevertheless, non-perturbative effects, such as D-branes, play an essential role in furthering our understanding of both quantum gravity and string-based models of the universe. In this talk I will describe the origin of certain mysterious non-perturbative effects in heterotic string theories. I will argue that they arise from heterotic disk diagrams whose existence hinges on a non-trivial interplay between worldsheet and spacetime degrees of freedom.
Strings: Monday, 14/10/2024, 14:00, Room 226
Aritra Saha (Bogazici University, Istanbul)
Rotating AdS3 x S3 and Dyonic Strings in 6D from 3D
In this talk, we shall make a general Killing spinor analysis of a particular gauged supergravity in 3-dimensions that comes from a consistent S^3 reduction of N=(1,0) supergravity coupled to a chiral tensor multiplet in 6-dimensions. In particular, we shall focus on the supersymmetric solutions with a null Killing vector in 3D, and find three new ones. We shall then study their uplifts to 6D. As we shall see, two of these newly found solutions in 3-dimensions, namely a null warped AdS_3 solution and a charged domain wall solution, admit non-trivial gauge fields. The uplift of the first one will produce an interesting AdS_3 x S^3 solution with a non-trivial rotation. The uplift of the second one will lead to the well-known rotating dyonic string solution. Finally the uplift of the third solution, which is an uncharged domain wall in 3D, will result in a distribution of dyonic strings. The talk will be based on a recent work arXiv:2408.03197 done in collaboration with N. S. Deger, C. Deral and O. Sarioglu.
Strings: Monday, 26/08/2024, 14:00, Room 226
Matthew Blacker (DAMTP, Cambridge)
Cosmological Wheeler DeWitt States and their Holographic Interpretation
In the canonical quantization of gravity, the Hamiltonian constraint of general relativity is promoted to the Wheeler DeWitt (WDW) equation – the quantum cosmological analogue of the Schrödinger equation. Solving the WDW equation is challenging, but by assuming homogeneity and isotropy we can construct WDW states via the Hamilton-Jacobi formalism of classical mechanics. Such an approach lends itself towards a holographic interpretation, as the Hamilton-Jacobi equation also describes the holographic renormalization group flow. The first technical point of this talk is to construct a semiclassical wavefunction (in 3+1 dimensional pure gravity with a positive cosmological constant) which strongly localises on the de-Sitter Schwarzschild spacetime. The second aim is to conjecture a holographic duality between these WDW states and a family of quantum theories in the static patch, and extract information about those dual theories from the WDW state. These first two results are as presented in [2304.06865]. We will then, time permitting, establish a link between this approach and that of the TT-bar deformation, as presented in [2406.02508].
Strings: Monday, 05/08/2024, 14:00, Room 226
Sebastian Murk (OIST, Okinawa)
Light rings and causality for nonsingular ultracompact objects sourced by nonlinear electrodynamics
In this seminar, we will explore observational signatures of nonsingular ultracompact objects regularized by nonlinear electrodynamics (NED). Due to the phenomenon of birefringence, photons of different polarizations propagate with respect to two distinct metrics, which manifests itself in the appearance of additional light rings surrounding the ultracompact object. I will outline the physical consequences of this result and illustrate them based on three regular black hole models commonly considered in the literature. We will discover that nonsingular horizonless ultracompact objects sourced by NED possess an odd number of light rings and discuss the viability of NED as an effective description of their properties. Time permitting, we will compare the phase velocities of polarized light rays propagating in nonsingular NED geometries to that of the Schwarzschild spacetime and demonstrate that regularizing the singularity by means of a theory that does not adhere to the Maxwell weak-field limit may lead to the emergence of acausal regions. [Based on arXiv:2406.07957]
Strings: Monday, 17/06/2024, 14:00
Tung Tran (University of Vienna, Vienna)
Quantum higher-spin Yang-Mills from the IKKT matrix model
The one-loop effective action of the higher-spin gauge theory induced by the IKKT matrix model on an FLRW cosmological spacetime brane in the presence of compact fuzzy extra dimensions is discussed. We show that all non-abelian (hs-valued) gauge fields in this model acquire mass via quantum effects, thus avoiding no-go theorems. This leads to a massive non-abelian quantum higher-spin Yang-Mills theory below the Kaluza-Klein scale, whose detailed structure depends on the extra dimensions. The stabilization of these extra dimensions naturally induces a Kaluza Klein scale into the N = 4 SYM sector of the model, thus breaking the superconformal symmetry.
Strings: Thursday, 13/06/2024, 14:00
Daniel Junghans (TU Wien, Vienna)
Problems with de Sitter in string theory
The simplest explanation for the observed accelerating expansion of our universe is that we live in an approximate de Sitter space. On the other hand, there is growing evidence that various popular proposals for realizing de Sitter space in string theory do not self-consistently control neglected string corrections. In this talk, we discuss recent results on such control problems for some of these de Sitter models.
Cosmo: Wednesday, 12/06/2024, 11:00
Markus Fröb (University of Leipzig, Leipzig)
Invariant observables in quantum gravity and graviton loop corrections to the Hubble rate and the Newtonian potential
I present work done in the last years on the construction of dynamical coordinate systems for highly symmetric backgrounds, such as Minkowski, de Sitter, and FLRW cosmologies, and hich are needed in the relational approach to construct gauge-invariant observables in gravity. I show that it is possible to restrict the inevitable non-local contributions to the past light cone such that the obtained observables are causal. Lastly, I present some applications, namely the leading quantum gravitational corrections to the local expansion rate of our universe (the Hubble rate) and the Newtonian gravitational potential. Based on arXiv:1711.08470, arXiv:1806.11124, arXiv:2108.11960, arXiv:2109.09753, and arXiv:2303.16218.
Cosmo: Thursday, 06/06/2024, 15:00
Tony Padilla (Nottingham University)
From number theory to QFT (and beyond)
Inspired by the method of smoothed asymptotics developed by Terence Tao, we introduce a new ultra-violet regularisation scheme for loop integrals in quantum field theory which we call ηη regularisation. This allows us to reveal a surprising connection between the elimination of divergences in divergent series of powers and the preservation of gauge invariance in the regularisation of loop integrals in quantum field theory. In particular, we note that a method for regularising the series of natural numbers so that it converges to minus one twelfth inspires a regularisation scheme for non-abelian gauge theories coupled to Dirac fermions that preserves the Ward identity for the vacuum polarisation tensor.
Cosmo: Wednesday, 05/06/2024, 11:00
Florian Kühnel (Max Planck Institute, Munich)
Primordial Black Holes - Positivist Perspective and Quantum Quiddity
Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes those black holes recently discovered by LIGO/Virgo/KAGRA, and (part of) these may conceivably be of primordial origin. After a general introduction on primordial black holes, I review the observational hints for their existence -- from a variety of lensing, dynamical, accretion and gravitational-wave effects. As I will show, all of these (over 20) may be explained by a single and simple unified model, naturally shaped by the thermal history of the Universe. If time permits, I will comment on vorticity, which we recently conjectured to be a novel feature of (near-extremally rotating) black holes, this possibly yielding the very first astrophysical observable for quantum effects in these compact bodies.
Cosmo: Thursday, 30/05/2024, 15:00
Oleg Zaslavskii (Kharkiv National University)
Black-hole bomb, confined Penrose process and particle decay near singularity
We consider the decay of a particle with some energy E_0 > 0 inside the ergosphere of a black hole. After the first decay, one of the particles with the energy E_1 < 0 falls towards a black hole while the second one with E_2 > E_0 moves in the outward direction. It bounces back from a reflecting shell and, afterwards, the process repeats. For radial motion of charged particles in the Reissner-Nordstrom metric, the result depends strongly on a concrete scenario. In particular, an indefinitely large growth of energy inside a shell is possible that gives rise to a black-hole bomb. We also consider a similar multiple process with neutral particles in the background of a rotating axially symmetric stationary black hole. We demonstrate that, if particle decay occurs at the turning point, a black-hole bomb in this case is impossible at all. For a generic point inside the ergoregion, there is a condition for a black-hole bomb to exist. It relates the ratio of masses before and after decay and the velocity of a fragment in the center of mass frame. One more process (decay near naked singularity) is also considered).
Cosmo: Wednesday, 29/05/2024, 11:00
Nicola Menadeo (Max Planck Institute, Potsdam)
Lensing of gravitational waves beyond General Relativity
The nature of gravity can be tested by how gravitational waves (GWs) are emitted, detected and propagate through the universe. Propagation tests are powerful, as small deviations compound over astronomical distances. However, tests of theories Einstein's general relativity (GR) are limited by the high degree of symmetry of the cosmological spacetime. Deviations from homogeneity, aka gravitational lenses, allow for new interactions to emerge, thus offering a path towards novel tests of gravity with GW propagation. This talk aims to delve into the regime of beyond geometric optics (bGO) whose analysis is still largely unexplored in beyond GR theories. Here we present the theory of GW propagation beyond GR (aka GW lensing beyond GR) in the short-wave expansion, including corrections to the leading order amplitude and phase for the first time. As an example, we will show the dispersive corrections to all metric and scalar field perturbations in Brans-Dicke, the simplest modified theory exhibiting GW dispersion.
Strings: Monday, 27/05/2024, 14:00
Libor Snobl (Czech Technical University, Prague)
Pairs of commuting quadratic elements in the enveloping algebra and integrable systems with magnetic fields
Motivated by the consideration of integrable systems in three spatial dimensions in Euclidean space with integrals quadratic in the momenta we first classify three dimensional Abelian subalgebras of quadratic elements in the universal enveloping algebra of the Euclidean algebra. This classification allows leading order terms of the integrals that cannot be written in the famous classical form of [Makarov, Smorodinsky, Valiev and Winternitz, Il Nuovo Cimento A 10 (1967) 106184] which holds only for integrable systems with scalar potentials. We show explicit examples of integrable and superintegrable systems with these more general leading order terms, some of which are of actual physical interest, e.g. helical undulator in solenoidal magnetic field. Next, we introduce recently classified integrable systems of ellipsoidal and paraboloidal type and compare their forms with and without magnetic field. We describe difficulties encountered in a similar analysis for conical type integrals. In collaboration with A. Marchesiello, O. Kubu and M.F. Hoque.
Cosmo: Wednesday, 22/05/2024, 11:00
Angelica Albertini (Astronomical Institute of the Czech Academy of Sciences, Prague)
An effective-one-body gravitational waveform model for large-mass-ratio inspiralling black hole binaries
The effective-one-body (EOB) approach is a powerful formalism that maps the two-body problem in general relativity into the motion of a single body in an effective metric. This provides a general framework that allows to exploit results from many other approaches, as post-Newtonian computations, numerical relativity, and gravitational self-force theory. EOB-based gravitational waveform models are nowadays providing fast and accurate templates for coalescences of comparable-mass compact binaries, that constitute the only type of event observed by the currently operating detectors. The next generation of detectors will instead allow us to receive the gravitational signal of a wider collection of sources, among which are large-mass-ratio inspirals. These are systems made up of a stellar-mass compact object orbiting around a massive or super massive black hole, and require a more accurate modelling. After briefly introducing the EOB formalism, I will discuss ongoing efforts in adapting an EOB waveform model in order to efficiently describe the evolution of large-mass-ratio binaries.
Strings: Monday, 20/05/2024, 14:00
Marcello Ortaggio (Institute of Mathematics of the Czech Academy of Sciences, Prague)
A new look at AdS black holes with conformal scalar hair
In the context of the "no hair conjecture", an early asymptotically flat, spherical black hole supporting a conformal scalar was discovered by Bocharova, Bronnikov, Melnikov and Bekenstein (BBMB) in the 70s. A solution in the presence of a positive cosmological constant has been obtained more recently by Martinez, Troncoso and Zanelli (MTZ) - provided a fine-tuned quartic self-interaction is also added to the theory - and various other black holes have been obtained since then. In this talk we revisit static, spherically symmetric solutions to AdS-Einstein gravity with a conformally coupled scalar field. After noticing that a convenient choice of coordinates leads to a significant simplification of the field equations, one is able to characterize analytically various branches of distinct solutions. Among these, we focus on a 2-parameter black hole (thus carrying primary hair) obtained as an infinite power series around its event horizon. Its properties are illustrated for various values of the parameters and compared with previous numerical results by Radu and Winstanley. In particular, we will make a few comments on the presence of a photon sphere and the corresponding shadow. More generally, since the method employed here (first put forward by Podolsky, Pravda, Pravdova and Svarc in quadratic gravity) can be used also in other contexts, both its advantages and limitations will be discussed. 2403.18048 [gr-qc]
Cosmo: Wednesday, 15/05/2024, 11:00
Giovanni D'Addario (University of Nottingham, Nottingham)
Ringdowns for black holes with scalar hair: the large mass case
Deviations from General Relativity can be probed with black hole spectroscopy, as the quasi-normal mode (QNM) frequency spectrum of a black hole with additional ‘hair’ is expected to differ from that of a Kerr black hole. We construct an effective field theory scheme for QNMs to capture deviations from Kerr for black holes in theories with a coupling between a shift-symmetric scalar and the Gauss-Bonnet invariant. I will explain how our analysis, which is particularly suited for black holes in the LISA range, places limits on the prospects of detecting evidence of scalar hair in ringdown signals.
Strings: Monday, 06/05/2024, 14:00
Filip Blaschke (Silesian University, Opava and Czech Technical University, Prague)
Amplitude modulations and resonant dynamics of oscillons in 1+1 dimensions
Although the oscillons - long-living quasi-periodic objects that are prevalent in non-linear scalar field theories - have been studied for a long time, there are still pertinent mysteries surrounding even their most basic properties. Due to their non-topological origin, it is hard to formulate precise conditions for their existence together with their impact on the dynamics of other solitons, like kinks and spharelons. For instance, oscillons typically carry two characteristic frequencies, one of which is very close to the mass threshold, manifesting in amplitude modulations of their envelopes. By studying oscillons in quasi-quadratic models, in which they are the only soliton-like excitations, we explain the origin of the modulation due to small amplitude oscillons being well approximated by a composite state of sine-gordon breathers. Moreover, for higher initial amplitudes, we discover that the field decays into several constituent oscillons that resonantly interact with each other in a fractal pattern of bouncing windows that is virtually identical to the same pattern found in scattering states of kinks. We argue that this is due to amplitude modulation playing the analog role of kink's shape mode in storing the kinetic energy of constituent oscillons, allowing for the resonant transfer of energy between the bounces. This talk is based on the paper: arXiv:2403.00443 [hep-th].
Strings: Monday, 29/04/2024, 14:00
Salomon Zacarias (Masaryk University, Brno)
Aspects of AdS(3) solutions and holography
In this talk I will discuss AdS preserving deformations in massive IIA supergravity. In particular I will focus on warped AdS(3)xS2xM5 solutions preserving 8 supercharges with M5 an SU(2) structure manifold. The new solutions preserve supersymmetry and lie outside the class of the original solutions. I will study the G-structure characterizing the new solutions and show that they are generalized to either a dynamical SU(2) or an identity structure on M5. Comments of the dual CFT realization of the deformed solutions will be given by studying some observables and probe branes on the deformed backgrounds.
Cosmo: Wednesday, 24/04/2024, 11:00
Massimiliano Maria Riva (DESY, Hamburg)
Vanishing of Nonlinear Tidal Love Numbers of Schwarzschild Black Holes
Love numbers describe the induced conservative response of an object subject to the action of an external gravitational field. They offer insight into the body's internal structure, and can be used to test gravity in the strong-field regime. In particular, Linear Love numbers are known to be zero for Schwarzschild black holes in four dimensions. In this talk I show how to extend this result to include nonlinear effects. I define these coefficients as Wilson couplings in the context of the point-particle effective field theory (EFT). After finding an explicit solution for the (static) response of a perturbed Schwarzschild black hole, I perform a matching with the aforementioned EFT, showing that: (i) the vanishing of the linear Love numbers is robust against nonlinear corrections and (ii) the quadratic Love number couplings also vanish.
Strings: Monday, 15/04/2024, 14:00
Alena Pravdova (Institute of Mathematics of the Czech Academy of Sciences, Prague)
Kerr-Schild double copy for Kundt spacetimes of any dimension
Although in contrast with the Maxwell equations, general relativity is a non-linear theory, analogies between solutions of the Einstein and Maxwell equations have been studied recently. These ideas originate from the double copy for scattering amplitudes and seem to be perturbative in nature. Remarkably, it has been found that double-copy type relations between the Maxwell and Einstein equations hold even in the fully non-linear regime for some exact solutions, including Kerr black holes. In this talk, we will discuss classical double copy for exact solutions of Einstein equations. In the original part of the talk we will focus on double copy for Kundt spacetimes and present selected results from [Ortaggio, Pravda, Pravdova, 2024(02):069].
Strings: Monday, 04/03/2024, 14:00
Ulisses Portugal (IFT, Sao Paulo)
A two parameter family of hyperbolic string vertices
String vertices are an essential element in the construction of string field theories, providing a decomposition of moduli spaces of Riemann surfaces compatible with Feynman rules. One explicit construction of such vertices are the hyperbolic vertices, defined by hyperbolic metrics on surfaces with fixed boundary lengths. I will review this construction and generalize it to a two-parameter family of vertices, and then show how the hyperbolic, polyhedral and light-cone vertices are obtained as different limits of this family.
Strings: Monday, 12/02/2024, 14:00, Room 226
Christian Northe (Ben-Gurion University, Israel)
Entanglement Resolution and BCFT
Conformal field theory is playing an essential role in developing our understanding of entanglement in many-body systems and field theory. I will discuss how it allows one to access the entanglement spectrum, which is known to encode defining characteristics of the system, such as topological signatures. In order to study these spectra in-depth in the presence of symmetries, a refined notion, called symmetry-resolved entanglement, can be introduced. I will demonstrate the lessons this holds when the system’s symmetry is generated by a U(1) Kac-Moody algebras and the Virasoro algebra.
Strings: Monday, 15/01/2024, 14:00, Room 226
Jakub Vosmera (IPhT, Saclay)
Topological defect lines in symmetric-product orbifolds and tensionless holography
We investigate certain maximally-symmetric realizations of topological defect lines in the symmetric-product orbifold CFTs. In the particular case when the seed CFT is taken to be the theory of four free fermions and bosons, we identify the AdS3 string worldsheet duals of various defect configurations in the 2d spacetime CFT.
Strings: Monday, 18/12/2023, 14:00, Room 226
Jan Pulmann (University of Edinburgh, Scotland)
Chern-Simons observables, Poisson structures and deformation quantization
Operators of a topological field theory supported on submanifolds of a fixed (co)dimension can be organized into a suitable category. For line observables in the Chern-Simons theory, one obtains a deformation of the category of representations of the underlying Lie group. I will review the construction by Drinfeld of one version of this category, explain how deformation quantization fits into this setting and present a similar issue for an even-dimensional Chern-Simons. Based in part on joint works with Anton Alekseev, Florian Naef and Pavol Severa.
Strings: Monday, 11/12/2023, 14:00, Room 226
Paolo Rossi (CEICO, Institute of Physics, Czech Academy of Sciences & Charles University)
Wormholes and surface defects in rational ensemble holography
I will briefly review the idea of ensemble holography and the role of Euclidean wormholes in this context. Then I will discuss the example of a specific ensemble of 2d rational CFTs and their putative 3d dual theory, focusing on geometries connecting two toroidal boundaries which should capture the variance of the ensemble distribution. The analysis leads to (1) nontrivial constraints on the ensemble distribution and (2) the requirement of contributions from "exotic" wormhole geometries in the bulk, which arise from the inclusion of topological surface defects. Based on arXiv:2312.02276.
Strings: Monday, 27/11/2023, 14:00, Room 226
Vit Tucek (Huawei Technologies, Sweden)
Grassmannian signalling
After a brief introduction to modern wireless communication I will explain how the complex Grassmannians can be used to circumvent some of the issues that arise in the next generations of mobile networks. That however brings its own set of mathematical challenges. We will concentrate on optimal distributions of a fixed number of points on a complex Grassmannian which can be seen as a generalization of the Thomson problem.
Strings: Monday, 20/11/2023, 14:00, Room 226
Petr Vasko (Charles University)
Introduction to celestial holography
I will give an introduction to celestial holography. First, I will review the original motivation connecting two areas of research that were developing independently until Strominger realized in 2014 that they are actually dual to each other: asymptotic symmetries and soft theorems for scattering amplitudes. To make this connection as manifest as possible, it is convenient to change the basis of scattering states to a "conformal basis" that I will introduce. S-matrix elements in this new basis define celestial amplitudes, which transform as correlation functions of primary operators in a 2D CFT living on the celestial sphere. Finally, I will discuss how certain properties of momentum space amplitudes translate to celestial amplitudes.
Strings: Monday, 13/11/2023, 14:00, Room 226
David Kubiznak (Charles University)
Shall Bekenstein's Area Law Prevail ?
According to Bekenstein's area law, the black hole entropy is identified holographically with one quarter of the horizon area. However, it is commonly believed that such a law is only valid in Einstein's theory and that higher curvature corrections generically give rise to its modifications. This is for example the case of black holes in Lovelock gravities, or their four dimensional cousins in the recently discovered 4D scalar-tensor Gauss-Bonnet gravity where one naively 'finds' (classical) logarithmic corrections to Bekenstein's law. We will argue that such logarithmic corrections originate from ignoring the shift symmetry of the 4D Gauss-Bonnet gravity. When this symmetry is properly taken into account, there is no longer any departure from the area law in this theory. Moreover, the first law remains valid upon modifying the black hole temperature, which can be derived via the Brown-York procedure but is no longer given by the surface gravity. Interestingly, we show that upon similar modification of the black hole temperature the area law can also prevail for black holes in higher-dimensional Lovelock gravities.
Strings: Friday, 10/11/2023, 11:15, Room 226
Shai Chester (Imperial College, London)
Gluon scattering in AdS at finite string coupling
We consider gluons scattering in Type IIB string theory on AdS5 x S^5/Z2 in the presence of D7 branes, which is dual to the flavor multiplet correlator in a certain 4d N=2 USp(2N) gauge theory with SO(8) flavor symmetry. We compute this holographic correlator in the large N and finite string coupling tau expansion using constraints from derivatives of the mass deformed sphere free energy, which we compute to all orders in 1/N and finite tau using localization. In particular, we fix the F^4 correction to gluon scattering on AdS in terms of Jacobi theta functions, and the D^2F^4 correction in terms of a non-holomorphic Eisenstein series. At weak string coupling, we find that the AdS correlator takes a remarkably similar form as the flat space Veneziano amplitude. Finally, we combine the numerical conformal bootstrap with the localization constraints to study the correlator at finite N and tau.
Strings: Monday, 06/11/2023, 14:00, Room 226
Tomas Prochazka (CEICO, Institute of Physics, Czech Academy of Sciences)
Bethe equations and 2d conformal field theory
The usual approach to 2d conformal field theory relies on the underlying Virasoro algebra. I want to discuss how this fits into a broader framework of integrable systems. In particular, the states in the Hilbert space can be labeled by algebraic equations, the Bethe equations. Perhaps surprisingly, Bethe equations associated to Virasoro algebra and its higher spin generalizations are very closely related to those of simplest Heisenberg SU(2) XXX spin chain. If time permits, I can discuss the set of Bethe equations associated to 2d free boson which is closely related to solvable generalizations of the quantum mechanical harmonic oscillator and classical system of Calogero particles.
Strings: Monday, 30/10/2023, 14:00, Room 226
Gideon Vos (CEICO, Institute of Physics, Czech Academy of Sciences)
Emergent KMS-states in 2d CFTs with large central charge
In this talk we will exploit a factorization property of conformal blocks in 2d CFTs with large central charge in order to reduce the expectation values of simple observables on high-energy states to vacuum expectation values on an effective background geometry. We will see that in the the high-temperature regime (T>>1) these vacuum expectation values manifest an emergent KMS-condition. As a consequence we will see a direct manifestation of an analog of the Hagedorn transition in large-N gauge theory in terms of operator product limits.
Strings: Monday, 16/10/2023, 14:00, Room 226
Canberk Sanli (CEICO, Institute of Physics, Czech Academy of Sciences)
Superconformal index for quivers
I will review the N=4B multi-particle (conformal) quantum mechanics which captures the microscopics of the (scaling) BPS bound states in d=4, N=2 supergravity, and then summarize our recent results on the index computations for N=2B superconformal quantum mechanics.
Strings: Monday, 02/10/2023, 14:00, Room 226
Ruggero Noris (CEICO, Institute of Physics, Czech Academy of Sciences)
Supergravity and the embedding tensor formalism
In this talk, I will review the basic ingredients of the geometric approach to supergravity. I will then present its application, within the embedding tensor formalism, to the construction of the gauged D = 4, N = 4 (half-maximal) supergravity coupled to an arbitrary number of vector multiplets.
Cosmo: Tuesday, 15/08/2023, 14:00
Andrei Frolov (Simon Fraser University, Canada)
Classical mechanics with inequality constraints
I will talk about mechanical systems with inequality constraints which restrict evolution to a region of phase space. Such constraints can be taken into account by modification of the action which describes the original unconstrained dynamics. To illustrate this approach I will describe a harmonic oscillator in the model with limiting velocity, and compare its behavior to a relativistic oscillator. When the amplitude of the oscillator is large, the properties of both types of oscillators are quite similar. Inequality constraints are interesting in the context of general relativity, where one could try to avoid formation of singularities by imposing a limit value on spacetime curvature.
Strings: Tuesday, 08/08/2023, 14:00, Room 226
Robert Brandenberger (McGill University, Montreal)
Emergent Metric Space-Time and Early Universe Cosmology from the BFSS Matrix Model
I will argue that effective field theory methods are inapplicable if we want to understand the evolution of the very early universe, and a nonperturbative approach is needed. The BFSS matrix model is a proposed non-perturbative definition of superstring theory. I will present an approach to obtain emergent space-time, an emergent metric and an emergent early universe cosmology, starting with a high temperature state of the BFSS matrix model. Thermal fluctuations yield scale-invariant spectra of cosmological fluctuations and gravitational waves, without the need for a phase of cosmological inflation.
Strings: Monday, 26/06/2023, 14:00, Room 226
Siddharth Prabhu (TIFR, Mumbai)
Holography and boundary observables in asymptotically flat spacetimes
We discuss progress in the study of asymptotically flat spacetimes on two fronts - the nature of holography and the construction of boundary observables. We formally recast the bulk S-matrix as a boundary observable. This naturally leads us to a study of position space correlators. We study their analytic structure, and provide a description of the submanifold in the space of all insertions that scatter in the bulk. We explicitly obtain the S-matrix from boundary correlators for massless as well as massive fields. Our study of position space correlators reveals that even though these correlators are not conformal, they can be recast as correlators in a conformal theory. This provides a new way to compute these correlators exactly.
Cosmo: Wednesday, 21/06/2023, 11:00, room 117
Dina Traykova (Potsdam Max Planck Institute, Germany)
Relativistic drag forces on black holes from scalar dark matter clouds
Gravitational interactions with compact objects may be one of the most promising tools for investigating dark matter properties. In particular, the presence of dark matter around black holes may lead to a distinctive dephasing of the gravitational wave signal from a merger due to dynamical friction and accretion. In this talk I will present the numerical results from calculating the dynamical friction and momentum accretion on a black hole generated by a cloud of ultra-light scalar field, confirming the regime of validity of semi-analytic expressions derived from first principles. I will show how both of these drag forces transition between different regimes of validity and identify the scalar masses at which this transition occurs.
Strings: Tuesday, 20/06/2023, 14:00, Room 226
Jaroslav Trnka (Charles University, Prague)
Non-perturbative Amplituhedron Geometries
I will discuss some new developments in the context of the Amplituhedron construction for planar N=4 SYM scattering amplitudes. I will define a negative Amplituhedron geometry which provides an all-loop order definition for a certain IR finite quantity, related to the ratio of null polygonal Wilson loops. Then I will show that the Amplituhedron picture suggests an interesting expansion of this quantity in terms of "loops of loops", unrelated to the usual small coupling expansion. In the leading order, I will provide an exact result to all orders in the coupling, and show the relation to the cusp anomalous dimension. Finally, I will discuss a certain interesting deformation of the geometric construction, and then conclude with future directions.
Cosmo: Wednesday, 14/06/2023, 11:00, Main Lecture Hall
Mehrdad Mirbabayi (ICTP, Italy)
A lower bound on dark matter mass
We argue that there is a lower bound of order 10-18 eV on dark matter mass if it is produced after inflation via a process with finite correlation length. Based on 2211.09775.
Cosmo: Wednesday, 07/06/2023, 11:00, Main Lecture Hall
Jiří Podolský (Charles University, Prague)
A new metric form of the whole class of type D black holes
Recently, we have found an improved metric form of the complete family of exact black hole spacetimes of algebraic type D, including any cosmological constant. This class was found by Debever in 1971, Plebanski and Demianski in 1976, and conveniently reformulated by Griffiths and Podolsky in 2005. In our new form of this metric the key functions are simplified, partially factorized, and fully explicit. They depend on seven parameters with direct physical meanings, namely m, a, l, alpha, e, g, Lambda which characterize mass, Kerr-like rotation, NUT parameter, acceleration, electric and magnetic charges of the black hole, and the cosmological constant, respectively. Moreover, this general metric reduces directly to the familiar forms of Kerr-Newman-AdS spacetime, charged Taub-NUT-AdS solution, or rotating and charged AdS C-metric. It also enables us to study various physical and geometrical properties, namely the character of singularities, two black-hole and two cosmo-acceleration horizons (in a generic situation), the related ergoregions, global structure including the Penrose conformal diagrams, parameters of cosmic strings causing the acceleration of the black holes, their rotation, pathological regions with closed timelike curves, or thermodynamic quantities.
Cosmo: Wednesday, 31/05/2023, 11:00, room 117
Adolfo Cisterna (Universidad de Tarapacá, Chile)
Ehlers Transformations as a Tool for Constructing Accelerating NUT Black Holes
This paper investigates the integrability properties of Einstein's theory of gravity in the context of accelerating NUT spacetimes by utilizing Ernst's description of stationary and axially symmetric electro-vacuum solutions. We employ Ehlers transformations, Lie point symmetries of the Einstein field equations, to efficiently endorse accelerating metrics with a nontrivial NUT charge. Under this context, we begin by re-deriving the known C-metric NUT spacetime described by Chng, Mann, and Stelea in a straightforward manner, and in the new form of the solution introduced by Podolský and Vrátný. Next, we construct for the first time an accelerating NUT black hole dressed with a conformally coupled scalar field. These solutions belong to the general class of type I spacetimes, therefore cannot be obtained from any limit of the Plebanśki-Demiański family whatsoever and their integration needs to be carried out independently. Including Maxwell fields is certainly permitted, however, the use of Ehlers transformations is subtle and requires further modifications. Ehlers transformations do not only partially rotate the mass parameter such that its magnetic component appears, but also rotate the corresponding gauge fields. The alignment of the electromagnetic potentials can be performed via an enhanced Ehlers transformation, but its use is proven to be limited by the presence of acceleration. We present a Reissner-Nordström-C-metric NUT-like black hole that correctly reproduces the Reissner-Nordström-C-metric and Reissner-Nordström-NUT line elements in the corresponding limiting cases but with a misaligned electromagnetic potential. We describe the main geometric features of these solutions and discuss possible embeddings of our geometries in external electromagnetic and rotating backgrounds.
Cosmo: Wednesday, 24/05/2023, 11:00, main lecture hall
Jorge Ovalle (Silesian University, Opava)
Black holes without Cauchy horizons and integrable singularities
Although we cannot understand the true nature of singularities in the framework of GR, it is possible to evade them by following a fairly simple strategy: generate regular BHs by filling the spacetime around the central singularity with some physically reasonable source of matter (which could be consequence of some new gravitational sector). This has produced a plethora of new regular BH solutions in recent years, mainly because the matter source used to evade the central singularity can be interpreted in terms of nonlinear electrodynamics. However, all these regular BH solutions contain a Cauchy horizon, a null hyper-surface beyond which predictability breaks down, and also leads to mass inflation at the perturbative level, a pathology which occurs even in loop quantum gravity inspired models. Even though the strong cosmic censorship conjecture establish the impossibility of extending spacetime beyond this region, in this talk we show how far we can go, without invoking this conjecture, in the building of a physically reasonable black hole without a Cauchy hyper-surface. Following this reasoning, we find a black hole lacking of Cauchy horizon, asymptotically flat and satisfying either the strong or dominant energy condition. The above is possible by demanding integrable singularity for the Ricci scalar, whose direct consequence is the appearance of finite tidal forces.
Cosmo: Wednesday, 10/05/2023, 11:00, main lecture hall
Arttu Rajantie (Imperial College London)
Stochastic effective theory for scalar fields in de Sitter space
The stochastic Starobinsky-Yokoyama approach is a powerful non-perturbative effective description of long-wavelength dynamics of a scalar field in de Sitter spacetime. It is based on the observation that on superhorizon distances the field behaves classically, with a noise term produced by subhorizon quantum modes. The approach has been mostly used to calculate the one-point probability distribution of the field, but its real power lies in describing the asymptotic long-distance behaviour of correlation functions through a spectral expansion. I demonstrate this by calculating isocurvature constraints for scalar dark matter models and decay rates of metastable vacua. I also show how to extend the stochastic theory beyond the overdamped approximation used by Starobinsky and Yokoyama. The parameters of this effective theory are determined at one-loop order in perturbation theory, and do not suffer from the same infrared problems as a direct perturbative computation of observables. Therefore the stochastic theory provides a powerful and accurate way of computing cosmological observables.
Cosmo: Wednesday, 03/05/2023, 11:00, main lecture hall
Luca Santoni (APC, Paris)
Dissipative Inflation with Scalar Particle Production
I will describe a new mechanism that gives rise to dissipation during cosmic inflation. In the simplest implementation, the mechanism requires the presence of a massive scalar field with a softly-broken global U(1) symmetry, along with the inflaton field. Compared to the scenario involving dissipation into gauge fields, particle production in this scenario takes place on parametrically sub-horizon scales. Consequently, the backreaction of the produced particles on the inflationary dynamics can be treated in a local manner, allowing us to compute their effects analytically. I will show the parametric dependence of the power spectrum and its deviation from the usual slow-roll expression, and evaluate the strengths of non-Gaussianity.
Strings: Tuesday, 02/05/2023, 14:00, room 226
Ido Ben-Dayan (Ariel University, Israel)
New themes in Cosmology
I will discuss two novel themes in Cosmology. First, the Swampland conjectures cast doubt on the scalar field cosmology paradigm, i.e. that Inflation and Dark Energy are driven by scalar field(s) with a potential. I will discard scalar fields altogether and consider "unparticles" - a slightly broken conformal theory on a cosmological background. The model has a limiting temperature due to cosmological dynamics. The result is a perfect fluid with a time-dependent equation of state that acts as radiation at early times and asymptotes to a cosmological constant at late times giving rise to a macroscopic emergent Dark Energy model. The model ameliorates the Hubble tension and the S8 tension and avoids the Swampland conjectures. Second, recent developments in the black hole information paradox suggest the existence of "Islands". These islands contribute to the entropy calculation and cause it to follow the Page curve restoring unitarity of the evaporation, partially resolving the paradox. An open question is whether such islands exist on cosmological backgrounds, and whether they play any role in the evolution of the Universe or Multiverse. I will answer the first question by showing that islands generically exist in Cosmology.
Cosmo: Wednesday, 26/04/2023, 11:00, main lecture hall
Juan Manuel Armaleo (Universidad de Buenos Aires, Argentina)
Probing spin-2 Ultra-Light Dark Matter (ULDM) with pulsars and gravitational waves
The energy content of the Universe is made up of approximately 95% by dark components: in particular, ~25% corresponds to dark matter. Several models have emerged through the years trying to explain the nature of such matter; recently, the so-called "Ultra-Light Dark Matter" (ULDM) model — named like this given its extremely small mass — has gained great interest in the community. In this talk I will present this model, highlighting its main characteristics. In particular I will focus on the spin-2 ULDM model and how we can use pulsars and gravitational waves interferometers to constrain the model.
Cosmo: Wednesday, 19/04/2023, 11:00, room 117
Stéphane Ilić (IJCLab, France)
More than the sum of its parts: joint analysis of LSS and CMB experiments
Through weak lensing and galaxy clustering measurements, future large-scale galaxy surveys will provide unprecedented constraints on the late Universe. On the other hand, high-quality CMB observations (Planck and future CMB experiments) can — and already do — put tight constraints on the early Universe. In this talk, I will show that combining these two sources of cosmological information can yield a significant lever arm and improve tremendously the constraints on our cosmological model. Moreover, I will also address the cross-correlation of those two types of signals, which can yield additional and significant constraints especially on extensions to the standard cosmological model. As a part of my talk, I will present in particular forecasts of the future Euclid x CMB cross-correlation constraints, performed by the CMB-cross correlations Science Working Group of the Euclid Collaboration.
Strings: Monday, 17/04/2023, 14:00, room 226
Alfredo Iorio (Charles University, Prague)
The three 'layers' of graphene monolayer and their analog GUPs (continued)
Analogs deal, quite successfully, with quantum field theory on curved spacetime. In this talk I shall point to the use of Dirac materials as analogs of high energy physics beyond the usual reach of analogs. In particular, the focus will be on the reproduction of generalized commutation relations, that are known to lead to generalized uncertainty principles (GUPs), a very active field of research in the phenomenology of quantum gravity.
Cosmo: Wednesday, 12/04/2023, 11:00, lecture hall
Nicolas Lecoeur (IJCLab, France)
Selecting Horndeski theories without apparent symmetries and their black hole solutions
Since the no-scalar-hair theorems of the 1970s, it had long been thought that four-dimensional, asymptotically flat black holes could not support any kind of non-minimally coupled real scalar hair, if not for the controversial Bocharova-Bronnikov-Melnikov-Bekenstein (BBMB) black hole. However, the 2010s have seen renewed interest in the healthy, higher-order scalar-tensor theories which were described by Horndeski in 1974, and easily escape the assumptions of the no-hair arguments. Up to now, all analytic, asymptotically Newtonian black hole solutions in these higher-order theories assumed some symmetry for the scalar-tensor theory. In the present seminar, we start from a class of Horndeski actions which do not possess any symmetry, and which include typical potentials arising from Kaluza-Klein dimensional reduction of higher-dimensional theories. For spherical spacetime, we establish integrability and compatibility conditions bearing on these generalized potentials. For such actions respecting these compatibility conditions, black holes with secondary hair are then obtained, including black holes with Schwarzschild-like asymptotics, whose important properties are discussed. The selected scalar-tensor theories do not present any symmetry. Rather, they appear to be the sum of two Lagrangian densities: one which is conformally invariant in five dimensions (but not in four), the other which is explicitly related to a Kaluza-Klein reduction of Einstein-Gauss-Bonnet theory.
Strings: Monday, 03/04/2023, 14:00, room 226
Igor Khavkine (Institute of Mathematics, Prague)
A look at the geometry of the 5-dimensional charged rotating black hole
The Einstein-Maxwell equations for a charged rotating black hole in 5 dimensions with two equal-magnitude angular momenta has been reduced to a system of non-linear ODEs and studied numerically by Kuntz et al. We study some geometric aspects of these solutions, confirming that they have regular horizon and null infinity. In addition, we rule out the possibility that the Weyl tensor of these spacetimes has special algebraic type in the bulk, while verifying the "geometric horizon" conjecture by showing that the Weyl tensor reduces to Type II on the horizon and to Type D on the bifurcation sphere. Based on arXiv:2112.13266.
Cosmo: Wednesday, 29/03/2023, 11:00, lecture hall
Justin C. Feng (IST Lisbon, Portugal)
The Weiss variation in gravitation theory
The Weiss variation formalism in mechanics and classical field theory concerns action variations that include explicit endpoint/boundary displacements. The Weiss variation is useful in field theory because it can be used to obtain expressions for the conjugate momenta and Hamiltonian without performing an explicit 3+1 decomposition beforehand. In this talk, I review the Weiss variation in mechanics, classical field theory, and in gravitation. In general relativity, one can recover (by way of an auxiliary connection formalism) a tensorial generalization of the Einstein canonical energy-momentum pseudotensor, as well as a reference-dependent Hamiltonian. After discussing the Weiss variation in general relativity, I discuss some applications of the Weiss variation formalism for modified gravity theories.
Strings: Monday, 27/03/2023
Aaron Wall (Cambridge, UK)
The Off-shell String Effective Action
Taking string theory off shell requires breaking Weyl invariance on the worldsheet, i.e. the worldsheet theory is now a QFT instead of a CFT. I will explain Tseytlin's approach to taking the worldsheet theory off-shell in a consistent manner, with a particular emphasis on the subtleties involved in calculating the sphere amplitude. This approach allows for the derivation of a classical string action which gives rise to the correct equations of motion and S-matrix, to all orders in perturbation theory. I will also briefly describe the Susskind-Uglum derivation of the black hole entropy S = A/4G from the off-shell string worldsheet. Based on arXiv:2211.08607 and 2211.16448.
Cosmo: Wednesday, 22/03/2023, 11:00, lecture hall
Oleg Lebedev (University of Helsinki, Finland)
Particle overproduction in the Early Universe
I discuss particle production during and after inflation. Even if particles have only gravitational interactions, they can be produced copiously leading to overabundance of dark relics. I discuss the resulting constraints on stable particles and their implications for quantum gravity.
Cosmo: Wednesday, 15/03/2023, 11:00, lecture hall
Sante Carloni (Charles University, Prague, Czechia)
Generalizing the coupling between spacetime and matter
We construct a new class of extension of General Relativity starting from the idea that the coupling between spacetime and matter is the key to understand dark phenomenology. We then focus on one of the simplest examples of such a class of theories which contains only one additional parameter dubbed "MEMe model". We show that this theory is able to offer a unified framework for dark energy and inflation. The MEMe model requires an extension of the classical Post-Newtonian expansion to be analyzed in the weak field, slow-motion approximation. We find that in the context of the MEMe model the structure of compact objects can be different from the ones of GR, whereas, if one considers only one type of fluid, the circular orbits of the fluid particles differ significantly from the usual ones only in the center of the matter distribution.
Strings: Monday, 13/03/2023, room 226
Alfredo Iorio (Charles University, Czech republic)
The three 'layers' of graphene monolayer and their analog GUPs
Analogs deal, quite successfully, with quantum field theory on curved spacetime. In this talk I shall point to the use of Dirac materials as analogs of high energy physics beyond the usual reach of analogs. In particular, the focus will be on the reproduction of generalized commutation relations, that are known to lead to generalized uncertainty principles (GUPs), a very active field of research in the phenomenology of quantum gravity.
Cosmo: Wednesday, 08/03/2023, 11:00, room 117
Christian Käding (Technische Universität Wien, Austria)
Directly computing reduced density matrices with influence functionals
The field theory of open quantum systems has ample applications in areas like particle or nuclear physics, cosmology and quantum gravity. Open quantum systems are commonly described by reduced density matrices, which are obtained by tracing out the environmental degrees of freedom, and whose evolution is given by quantum master equations. Solving such equations often poses an intricate or even analytically impossible task. As a way to circumvent such problems, we will present a first principle-based and practicable formalism which allows for the direct computation of reduced density matrix elements without having to consider a master equation. It is based on techniques from non-equilibrium quantum field theory like thermo field dynamics, the Schwinger-Keldysh formalism and the Feynman-Vernon influence functional. As a simple example, we will focus on a scalar field system interacting with an environment comprised of another scalar field.
Cosmo: Wednesday, 01/03/2023, 11:00, lecture hall
Nuno M. Santos (Universidade de Aveiro, Portugal)
Synchronized bosonic hair: equilibrium solutions
Bosonic fields can spin down rotating black holes (BHs) via superradiance. If massive, they may remain trapped in the vicinity of a BH and endow it with hair co-rotating in synchrony with the event horizon. An illustrative example of this mechanism is the family of BHs with synchronized hair, that can co-exist with Kerr BHs and emerge dynamically from them at some scales. In this talk, I will first explore the features of BHs with vanishingly little (scalar and vector) hair, drawing their similarity to the atomic orbitals of the electron in a hydrogen atom. Then, I will discuss how hairy such BHs can become from the growth and saturation of superradiant instabilities. Finally, I will address the thermodynamic stability of BHs with synchronized hair in the canonical ensemble.
Strings: Monday, 27/02/2023
Karapet Mkrtchyan (Imperial College, UK)
A novel Lagrangian formulation for (chiral) p-forms and their interactions
I will review the recent work on a (democratic) Lagrangian formulation for arbitrary p-forms that treats electric and magnetic degrees of freedom on an equal footing. This formulation allows simple generalization to arbitrary abelian interactions. In particular, I will demonstrate the complete solution for a Lorentz covariant Lagrangian problem for arbitrary abelian interactions of chiral p-forms in d=2p+2 dimensions and nonlinear electrodynamics in 3+1 dimensions.
Cosmo: Wednesday, 22/02/2023, 11:00, lecture hall
Daniela Doneva (University of Tübingen, Germany)
Gravitational waves as a test of fundamental physics
Gravitational waves promise to give answers to a number of fundamental questions like the nature of gravity in the regime of strong gravitational fields, the Kerr hypothesis, the dark matter and dark energy puzzle, etc. The next generation of gravitational wave detectors will observe hundreds of thousands of events per year starting the era of high-precision gravitational wave astrophysics. In the present talk, I will discuss what are our present and future capabilities to test fundamental physics through gravitational waves. I will focus primarily on merging compact binaries, as well as extreme mass ratio inspirals and binary pulsars. Especially interesting are the combinations of theories and astrophysical scenarios that give not only small cumulative deviations with respect to general relativity but also clear qualitatively different gravitational wave signatures that are smoking guns for beyond-GR physics.
Strings: Monday, 20/02/2023
Thomas Mertens (UGhent, Belgium)
From JT to 3d pure gravity
Taking inspiration from our understanding of 2d JT gravity, we develop aspects of 3d pure gravity. In particular, we propose an effective model of 3d pure gravity and discuss its factorization across entangling surfaces. Finally, we highlight some differences between gravity in its metric formulation and its first order gauge theoretic formulation, focussing on the underlying algebraic structure. Largely based on arXiv:2210.14196 and the earlier JT story in arXiv:1812.00918.
Cosmo: Wednesday, 15/02/2023, 11:00, lecture hall
Vitor Cardoso (IST Lisbon, Portugal)
Testing the existence of black hole horizons
One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes. Yet, true agnostic tests of the black hole nature of dark compact objects are hard to devise. I would like to describe possible tests of the geometry describing observations, and how one can quantify the presence of horizons in the spacetime.
Strings: Monday, 13/02/2023
Athanasios Chatzistavrakidis (Rudjer Boskovic Institute, Croatia)
Geometry on higher structures and the BV formalism
Higher structures such as Lie, Courant and L-infinity algebroids appear in the BV formalism, notably in the AKSZ construction of solutions to the classical master equation. In this talk, I will discuss the interplay between higher geometry on such structures (and on their description as dg-manifolds) and the gauge symmetries of topological sigma models and higher gauge theories. Emphasizing that the AKSZ construction does not apply in presence of Wess-Zumino terms, I will present a class of topological field theories in arbitrary dimensions and discuss the role of higher geometry in developing their BV formulation. Moreover I will argue that in the graded-geometric description of higher gauge theories, the structure of gauge transformations is governed by a Kapranov L-infinity[1] algebra.
Cosmo: Wednesday, 08/02/2023
Armando di Matteo (INFN Torino, Italy)
Ultra-high-energy cosmic rays: what we know and what we don't, and possible "new physics" implications
Over 60 years after their discovery, the sources of ultra-high-energy cosmic rays (UHECRs), atomic nuclei from outer space with energies over 1 EeV, are still unknown. Deflections by intergalactic and Galactic magnetic fields prevent us from straightforwardly inferring the position of their sources from their arrival directions as can be done with neutral messengers, and interactions with extragalactic background photons alter their energy spectrum and mass composition making it nontrivial to infer properties of their sources from terrestrial observations. Nevertheless, the large numbers of events detected by the last-generation UHECR detectors have allowed us to answer a few of the longstanding questions about these particles, though many others remain. Furthermore, their extreme energies allow us to probe certain hypotheses about physics beyond the Standard Model in regimes not accessible via artificial particle accelerators. In this seminar, I will provide an overview of the field, with a special emphasis on relatively recent developments and on frequently misunderstood issues, and an outlook for the near- and medium-term future.
Strings: Thursday, 02/02/2023
René Meyer (University of Wurzburg, Germany)
Approaches to Discrete Holography
I will review two recent attempts that aim at formulating a discrete holographic duality. Both are based on hyperbolic lattices, which are tilings of the hyperbolic plane. After an introduction to these tilings, I will present results that show that the Breitenlohner-Freedman bound for a scalar field on such hyperbolic tilings is unaffected, independent of the type of tiling. It gets only corrected by finite size effects. I will also present a realization of the BF bound in a hyperbolic electric circuit, and corresponding results. In the second part of the talk, I will discuss aperiodic XXZ type spin chains, i.e. spin chains defined on the aperiodic space obtained by cutting off a hyperbolic tiling at a finite radial distance. I will discuss the emergence of a non-trivial disordered fixed point, and present results for the logarithmic scaling of the entanglement entropy and the effective central charge of that fixed point. I will also discuss a hyperbolic tensor network representation that reproduces the ground state, as well as a large N version of the spin chain with SO(N) spins. I will end with an overview over interesting relations to other research areas, and open questions.
Cosmo: Wednesday, 01/02/2023, 11:00, lecture hall
Stefano Savastano (Potsdam Max Planck Institute, Germany)
Observing Continuous Gravitational Waves lensed by Sgr A*
With the rising of Gravitational Wave astronomy, observing lensed Gravitational Waves (GWs) signals in the future is a concrete possibility. The gravitational lensing of GWs provides a rich phenomenology to devise new probes of the matter distribution in the Universe. I will discuss how Continuous Gravitational Waves emitted by isolated neutron stars can be lensed by the super-massive black hole at the center of our galaxy, Sgr A*, and potentially observed by next-generation gravitational wave detectors. In particular, I will show that, with the motion of the lens taken into account, future detectors can accurately distinguish and measure parameters of lensed continuous waves from sources located behind Sgr A* within a broader area than previously considered, increasing the probability of observing such events.
Cosmo: Wednesday, 25/01/2023, 11:00, lecture hall
Thomas Sotiriou (University of Nottingham, UK)
Strong Gravity and Fundamental Physics
Strong gravity observations offer a new way to search for new fundamental fields. Scalar fields have been studied extensively in this context. Using them as a case study, I will discuss the following questions: how can new fields leave an imprint on black holes? What can theory tell us about which observations would be more sensitive to this new physics? And are all black holes the same?
Strings: Monday, 23/01/2023
Ivano Basile (LMU, Germany)
Infinite distances in multicritical CFTs and higher-spin holography
I will present the first study of the swampland in higher-spin gravity. In particular, holographic vector models offer a playground to study tensionless-like large-N limits from the point of view of the distance/duality conjecture. I will describe a notion of (information) distance in this discrete landscape and study the decay of higher-spin masses/anomalous dimensions along the limit. In striking contrast to the expected exponential decay, these models lead to a power-like decay. This suggests that stringy exponential decays are characteristic of matrix-like gauge theories, rather than vector models. Further evidence for this arises studying the information distance along coupling variations in Chern-Simons-matter CFTs, where matrix-like degrees of freedom dominate over vector-like ones and the decay is once again exponentially fast.
Cosmo: Wednesday, 11/01/2023, 11:00, lecture hall
David Kubiznak (Charles University, Prague, Czechia)
Remarkable symmetries of rotating black holes
It is well known that the Kerr geometry admits a non-trivial Killing tensor and its `square root' known as the Killing-Yano tensor. These two objects stand behind the Carter's constant of geodesic motion as well as allow for separability of test field equations in this background. The situation is even more remarkable in higher dimensions, where a single object -- the principal Killing-Yano tensor -- generates a tower of explicit and hidden symmetries responsible for integrability of geodesics and separability of test fields around higher-dimensional rotating black holes. Interestingly, similar yet different structure is already present for the slowly rotating black holes described by the `magic square' version of the Lense-Thirring solution, giving rise to a geometrically preferred spacetime that can be cast in the Painleve-Gullstrand form and admits a tower of exact rank-2 and higher rank Killing tensors whose number rapidly grows with the number of spacetime dimensions.
Strings: Monday, 12/12/2022, 14:00, room 226
Dieter van den Bleeken (Bogazici University, Turkey)
Bosonic supersymmetry
I will discuss how on a Poisson manifold with involution the space of functions is naturally equipped with a superalgebra structure. I will then illustrate that this explains the appearance of a superalgebra of conserved charges for simple purely bosonic (quantum/classical) mechanical systems, such as the free particle and harmonic oscillator, that were previously observed in the literature
Cosmo: Thursday, 08/12/2022, 16:00, SOLID building lecture hall
Marco Chianese (Università degli Studi di Napoli, Italy)
Novel probes of sub-GeV dark matter
The direct detection of sub-GeV dark matter interacting with nucleons and electrons is hampered by the low recoil energies induced by scatterings in the detectors. Novel ideas are therefore needed to circumvent this experimental limitation. For instance, higher recoil energies in the detector can be achieved in the case of boosted dark matter, where a component of dark matter particles is endowed with large kinetic energies. Furthermore, the scatterings with light dark matter can affect the cosmic-ray transport in astrophysical environments altering the primary and secondary particle spectra observed at the Earth. In this talk, I will review the current status of light dark matter probes and present two interesting scenarios. Firstly, I will show that the current evaporation of primordial black holes (alternative dark matter candidates) with masses from 1014 to 1018 grams is an efficient source of boosted light dark matter. Then, I will investigate the effects of the DM-proton scatterings in star-forming and starburst galaxies, which are well-known cosmic-ray "reservoirs" and well-motivated astrophysical emitters of high-energy neutrinos and gamma-rays through hadronic collisions. For both the two scenarios, I will explore the phenomenological implications and discuss new constraints on the dark matter parameter space.
Cosmo: Thursday, 01/12/2022, 16:00, lecture hall
Luc Blanchet (Université Paris-Saclay, France)
Dark matter at galactic scales & MOND
We review the phenomenology of dark matter at galactic scales and the intriguing MOND (MOdified Newtonian Dynamics) formula for the rotation curves of galaxies and the Tully-Fisher relation. We show that the MOND formula can be tested by the dynamics of planets in the Solar System. We present a particular model dubbed Dipolar Dark Matter which recovers the MOND formula at galactic scales and the standard cosmological model Lambda-CDM at cosmological scales.
Strings: Monday, 14/11/2022
Pranjal Nayak (CERN, Switzerland)
Random Matrices in the Quantum Mechanical Description of Black Holes
In this talk I'll describe the emergence of random matrix theory-like behavior in physical quantum theories, often referred to as quantum ergodicity. I'll describe an effective field theory description of this phenomenon, and how we believe it can be applied to higher dimensions. Finally, in low dimensional examples of AdS/CFT I'll discuss what quantum ergodicity tells us about holographic CFTs, gravitational physics and black holes.
Cosmo: Thursday, 10/11/2022, 16:00, lecture hall
Andrew Miller (Université Catholique de Louvain, Belgium)
Probes of dark matter with gravitational-wave detectors
Gravitational-wave interferometers such as LIGO, Virgo and KAGRA can be used to test the existence of dark matter. While most efforts have focused on finding gravitational waves from heavy dark matter, e.g. primordial black holes mergers, or quasi-monochromatic signals from depleting ultralight boson clouds around black holes, the interferometers can also be used to directly detect dark matter that interacts with various components, e.g. the mirrors or the beam splitter. In this sense, the interferometers act like particle physics experiments: the ultralight dark matter particles, of masses 1e-14 to 1e-11 eV, may interact with light, baryons or baryon-leptons in the mirrors and cause a quasi-sinusoidal force on them, or alter the values of the fundamental constants in the interferometer components. Even though these signals are not resulting from gravitational waves, both effects will manifest themselves as differential length changes, which can be precisely measured with LIGO, Virgo and KAGRA. We give an overview of the physics of such dark matter interaction signals, and present the results of recent searches for scalar and vector dark matter particles. While no signal has been found, the constraints that come from analyses of LIGO, Virgo and KAGRA data surpass those of other experiments that were designed to specifically search for dark matter (e.g. MICROSCOPE and the Eöt-Wash torsion balance), and represent a bridge between particle physics and gravitational-wave experiments.
Strings: Monday, 07/11/2022, room 226 14:00
Blagoje Oblak (l'Ecole Polytechnique, France)
Flat JT Gravity and the Schwarzian of BMS2
This talk is devoted to Jackiw-Teitelboim (JT) gravity in Bondi gauge, with a vanishing cosmological constant. The asymptotic symmetries of the theory span an infinite-dimensional group commonly dubbed `BMS2' (for Bondi-Metzner-Sachs in two dimensions), but most of the existing literature reduces this group to its warped Virasoro subgroup. I shall argue that one can avoid this reduction and use the BMS2 group throughout. In particular, the boundary action of the system is a BMS-Schwarzian with an extra zero-mode, and its partition function is one-loop exact with respect to the Haar measure on (centrally extended) BMS2. The peculiarities of BMS2 are pointed out, including the fact that it has a single coadjoint orbit at fixed (real) central charges. Allowing for a natural complexification affects this feature sharply, suggesting that more work is required to fully understand the phase space of asymptotically flat data in JT gravity. [Based on arXiv:2112.14609.]
Cosmo: Thursday, 03/11/2022, 16:00, lecture hall
Giorgio Torrieri (Universidade Estadual de Campinas, Brazil)
The equivalence principle and inertial-gravitational decoherence
Since the earliest paper on the topic by Matvei Bronstein [1] it was clear that the equivalence principle is incompatible with the usual separation between a "quantum system" and a "classical detector", namely the fact that the charge/mass ratio is "small". A modern treatment, based on open quantum systems and path integrals, can however directly address this issue, and systematically calculate corrections both in the case of a light recoiling detector and in the case of a heavy gravitating one. We illustrate this for an interferometric setup of the type of [2] and show that for all parameters a "semiclassical limit", where one can measure a phase shift due to gravitational attraction between quantum objects, is unlikely.
Based on [3].
[1] M.Bronstein, Gen.Rel.Grav. 44 (2012) 267-283 Original: Matvei Bronstein, Quantentheorie schwacher Gravitationsfelder, Physikalische Zeitschrift der Sowjetunion, Band 9, Heft 2–3, pp. 140–157 (1936).
[2] S.Bose et. al., arXiv:1707.06050 (PRL) C.Marletto, V.Vedral, arXiv:1707.06036 (PRL)
[3] G. Torrieri, arXiv: 2210.08586
Strings: Thursday, 03/11/2022, room 226 15:00
Jordan Francois (U. Mons, Belgium)
Presymplectic structure of gauge theories: boundaries, edge modes, variational connections and all that…
The boundary problem is the failure to associate a symplectic structure to a gauge theory over a bounded region of spacetime. Two strategies to circumvent this problem have been much discussed in the recent literature: the edge mode approach by Donnelly & Freidel, and the connection approach by Gomes & Riello. Relying on the bundle geometry of field space, we attempt to make them more systematic, so as to facilitate comparison and to shed some light on conceptual aspects. We illustrate the general results with the standard examples of Yang-Mills theory, the Cartan formulation of General Relativity, and Chern-Simons theory -- thereby reproducing several results of the literature.
Strings: Monday, 31/10/2022, 11:00, room 226
Eugenia Boffo (Charles University, Czech republic)
Spin field for the N=1 particle in the worldline
In this talk I will address the problem of Ramond-Ramond backgrounds in string theory, from the simplified viewpoint of the N=1 spinning particle. These fields arise as 2-particles excitations of the ground state. BRST cohomology of the worldline model leads to the right equations for the R-R fields. Deformations or twistings of the BRST differential by the latter can also be implemented consistently, yielding target space geometries that support the R-R fields. Based on joint work with Ivo Sachs, arXiv:2206.03243.
Cosmo: Thursday, 27/10/2022, 16:00, lecture hall
Enrico Barausse (SISSA, Trieste, Italy)
Gravitational wave generation in effective field theories of dark energy
I will review how non-linearities can allow for screening solar-system scales from non-tensorial gravitational polarizations, focusing on the case of scalar-tensor theories with derivative self-interactions (K-essence). I will then present fully relativistic simulations in these theories in 1+1 dimensions (stellar oscillations and collapse) and 3+1 dimensions (binary neutron stars), showing how to avoid breakdowns of the Cauchy problem that have affected similar attempts in the past. I will show that screening tends to suppress the (subdominant) dipole scalar emission in binary neutron star systems, but that it fails to quench monopole scalar emission in gravitational collapse, and quadrupole scalar emission in binaries.
Strings: Monday, 24/10/2022, 14:00, room 226
Swapnamay Mondal (Dublin Institute of Advanced Studies, Ireland)
Supersymmetric black holes and TTbar deformation
The entropy of supersymmetric black holes in string theory compactifications can be related to that of a D- or M-brane system, which in many cases can be further reduced to a two-dimensional conformal field theory (2d CFT). For black holes in M-theory, this relation involves a decoupling limit where the black hole mass diverges. We suggest that moving away from this limit corresponds to a specific irrelevant perturbation of the 2d CFT, namely the supersymmetric completion of the T Tbar deformation. It is demonstrated that the black hole mass matches precisely with the TTbar deformed energy levels, upon identifying the TTbar deformation parameter with the inverse of the leading term of the black hole mass. I will discuss various implications of this novel realization of the TTbar deformation, including a Hagedorn temperature for wrapped M5-branes, and potential change of degeneracies in the deformed theory.
Cosmo: Thursday, 20/10/2022, 16:00, lecture hall
Katy Clough (Queen Mary University, London, UK)
Black holes in fundamental field environments - the impact of initial data
There are several well-motivated scenarios in which fundamental fields could be present around black holes at a sufficient level to impact on the gravitational waveform of a merger. However, developing templates for the impact of such fields is challenging - in particular one issue that requires more attention is how to select and impose appropriate initial conditions for the field that represent their state at the late, dynamical, strong field phase of the merger. A correct specification will be crucial in obtaining sufficiently accurate waveforms and avoiding degeneracies with other effects.
Strings: Monday, 17/10/2022, 14:00, room 226
Chrysoula Markou (UMons, Belgium)
Advances in spin-2 physics
General relativity (GR) can be thought of as the unique theory of interacting massless spin-2 fields, the gravitons; states with similar properties are also present in string theory spectra. In this talk, we will be probing the previously largely unexplored interactions of the graviton with massive spin-2 string states. In particular, we will review recent results on their scattering and formulate a massive realisation of the double copy in string theory for the first time. We will further argue that, unlike open string spectra, closed string spectra may be able to accommodate the "dark graviton", namely the massive spin-2 state that appears in a GR extension known as ghost-free bimetric theory and which has been put forward as a viable dark matter candidate.
Cosmo: Thursday, 13/10/2022, 16:00, room 117
Rafael Porto (DESY, Hamburg, Germany)
Precision Gravity: From the LHC to LISA and ET
The era of gravitational wave science began in spectacular fashion with several detections already reported by the LIGO-Virgo-KAGRA collaboration, and many more yet to come with the future planned observatories such as LISA and the Einstein Telescope. Motivated by these initial experimental breakthroughs and the expected scientific output, a community effort has been established toward constructing high-accurate waveform models for the emission of gravitational waves from binary systems. In this talk I review how ideas and techniques from particle physics — such as effective field theory methods and modern integration techniques from collider physics — have impacted the state-of-the-art in our analytic understanding of the two-body problem in general relativity.
Cosmo: Thursday, 06/10/2022, 16:00, room 117
Yuichi Miyashita (Tokyo Institute of Technology, Japan)
Topological defects in nonlocal field theories
We consider the topological defects in the context of nonlocal field theories in which Lagrangians contain infinite-order differential operators. In particular, we analyze domain walls. We first determine the asymptotic behavior of the nonlocal domain wall close to the vacua. For the specific domain wall solution under investigation, we derive a theoretical constraint on the nonlocality energy scale, which must be larger than the corresponding symmetry-breaking scale. Subsequently, we find that nonlocality makes the width of the domain wall thinner and the energy per unit area smaller compared to the local case. This talk is based on arXiv:2203.04942.
Strings: Thursday, 03/10/2022, 14:00, room 226
Shlomo Razamat (Technion, Israel)
On IR dualities across dimensions
In this talk we will overview recent progress on understanding some aspects of IR dualities between Lagrangian constructions of 4d SCFTs and their engineering starting from 6d. We will illustrate these understandings mainly with the compactifications of D-type conformal matter and higher rank E-string theories down to four dimensions.
08/09/2022, 16:00, lecture hall
Diana López Nacir (University of Buenos Aires, Argentina)
Cosmological perturbations for Self-Interacting Warm Dark Matter scenario, numerical implementation and some observational constraints
The standard cosmological model assumes the existence of dark contributions to the energy content of the Universe. In particular, it assumes a mysterious component known as Dark Matter, which in the standard scenario is considered to be “Cold”. In this talk I will consider Self-Interacting Warm Dark Matter (SI-WDM) models as alternative candidates. After introducing the main motivations and the basic phenomenology, I will present a general framework for computing the evolution of cosmological perturbations at linear level, which derives from a Boltzmann hierarchy based on a parametrization of the scattering amplitude that allows one to retain certain model independence on the particular interaction Lagrangian. I will show some results we obtained using a numerical implementation of the framework in an extended version of CLASS code, and in particular some observational constraints we obtained using Milky Way satellites Counts and Lyman-alpha forest from a phenomenological approach. As a result, I will show that sufficient self-interaction could turn the lower bounds on the mass of the WDM particles less restrictive than without self-interaction and in particular relax constraints on the traditional νMSM model.
23/06/2022, 16:00, lecture hall
Shun-Pei Miao (National Cheng Kung University (NCKU), Tainan, Taiwan)
Gauge Independent Effective Field Equations
I describe a technique for removing gauge dependence from graviton loop corrections to the effective field equations. I present explicit results on flat space background for a massless, minimally coupled scalar and for electromagnetism. I then describe how the procedure generalizes to de Sitter background. This talk is based on arXiv:1708.06239
22/06/2022, 16:00, lecture hall
Richard P. Woodard (University of Florida at Gainesville, USA)
Summing Large Logarithms from Loops of Inflationary Gravitons
Quantum gravitational corrections on flat space background do not affect particle kinematics at all, and only make fractional changes of order G/r^2 to long range forces. The situation during inflation is very different because (1) the Hubble parameter H allows fractional corrections of the form G H^2 and (2) the continuous production of inflationary gravitons introduces a secular element. As a result, corrections to both particle kinematics and long range forces typically grow like logarithms of the scale factor and/or the spatial separation. If inflation persists long enough, this growth must eventually cause perturbation theory to break down, begging the question of what happens next. I report on recent progress in summing the very similar large logarithms which occur in nonlinear sigma models by combining a variant of Starobinsky's stochastic formalism with a variant of the renormalization group. I discuss how this technique can be generalized to quantum gravity. This talk is based on arXiv:2110.08715 with Shun-Pei Miao and Nick Tsamis.
16/06/2022, 16:00, room 117
Alexander Zhuk (Odessa I. I. Mechnikov National University, Ukraine & Center for Advanced Systems Understanding (CASUS), Görlitz, Germany)
Relativistic approach to the large-scale structure formation: cosmic screening vs. gevolution
Due to the modern telescopes, we found that the Universe is filled with a cosmic web which is composed of interconnected filaments of galaxies separated by giant voids. The emergence of this large-scale structure is one of the major challenges of modern cosmology. We study this phenomenon with the help of relativistic N-body cosmological simulation based on General Relativity. It is well known that gravity is the main force responsible for the structure formation in the Universe. In the first part of my talk, I demonstrate that in the cosmological setting gravitational interaction undergoes an exponential cutoff at large cosmological scales. This effect is called cosmic screening. It arises due to the interaction of the gravitational field with the background matter. Then, I compare two competing relativistic approaches to the N-body simulation of the Universe large-scale structure: “gevolution” vs. “screening”. To this end, employing the corresponding alternative computer codes, I demonstrate that the corresponding power spectra are in very good agreement between the compared schemes. However, since the perturbed Einstein equations have much simpler form in the “screening” approach, the simulation with this code consumes less computational time, saving almost 40% of CPU (central processing unit) hours.
13/06/2022, 14:00, room 226
Souvik Banerjee (Wurzburg, Germany)
Wormholes, Berry phases and factorization
I shall start by reviewing the connection between the wormholes and entanglement in the context of AdS/CFT. I shall then show how the Berry phase, a geometrical phase encoding information about the topology may be used to reveal similarities between the Hilbert space structure on both sides of the correspondence. This correspondence might open up an exciting new avenue to understanding the factorization puzzle in AdS/CFT. Furthermore, I shall argue how this concept unifies the role of entanglement in "creating" a generic quantum system, ranging from simple quantum mechanical models to entangled CFTs.
09/06/2022, 16:00, lecture hall
Matteo Fasiello (IFT, Madrid, Spain)
Probing the Early Universe with Gravitational Waves
Some of our best ideas on early universe physics are about to be put to the test by an unprecedented array of cosmological probes. The data these will collect span a vast range of scales, from the CMB to large scale structure, from pulsar timing arrays all the way to laser interferometers. This combined wealth of new information holds the potential to transform not just our understanding of cosmology, but also particle physics. Probing the earliest accessible epoch, the accelerated expansion known as inflation, is crucial: inflation can provide a cosmological portal to otherwise inaccessible energy scales. The spectacular success of the inflationary paradigm in explaining the origin of cosmic structure demands that we tackle a number of compelling questions still in need of an answer: what is the energy scale of inflation? What fields were active during inflation? In this talk I will review recent progress on the inflationary field content. I will survey different approaches to address the most pressing challenges and provide specific examples. I will then focus on the key observables, starting with primordial gravitational waves, and discuss their prospects for detection.
06/06/2022, 14:00, room 226
Matthijs Hogervorst (EPFL Lausanne, France)
Cosmology meets Conformal Bootstrap
Holography dictates that information about bulk physics is encoded by a conformally invariant quantum field theory that lives on the boundary of spacetime. It is tantalizing to apply this principle to de Sitter space, which roughly describes the cosmology of the actual universe. Even for the case of matter living on a stationary dS background, the above correspondence is confusing: the boundary theory, which describes cosmological correlators at late times, is a violently non-unitary CFT. This is unfortunate, as unitarity is a key ingredient in proving theorems about the landscape of possible theories. In this talk I will argue that unitarity is nonetheless recovered. In particular, I will give a first example that conformal bootstrap methods can be used to constrain the space of de Sitter theories.
02/06/2022, 16:00, lecture hall
Gianmassimo Tasinato (Swansea University, UK)
Probing the Physics of Inflation with Gravitational Wave Experiments
Cosmological inflation predicts the existence of a stochastic background of gravitational waves (GW), whose features depend on the model of inflation under consideration. There exist well motivated frameworks leading to an enhancement of the primordial GW spectrum at frequency scales testable with GW experiments, with specific features as parity violation, anisotropies, and non- Gaussianity. I will explain the properties of such scenarios, and their distinctive predictions for what respect GW observables. I will then discuss perspectives for testing these predictions with future GW experiments.
30/05/2022, 14:00, room 226
Max Guillen (Upsalla, Sweden)
Pure Spinor Field Theory Description of 10D super-Yang-Mills: Scattering Amplitudes and Color-Kinematics Duality
In this talk I will review the basic ingredients which allows one to formulate 10D super-Yang-Mills on pure spinor superspace. The respective pure spinor master action in the gauge b_{0}V = QΞ, will then be used to show that tree-level scattering amplitudes calculated via perturbiner methods, match those obtained from pure spinor CFT techniques. I will also discuss how to compute pure spinor kinematic numerators through the use of standard Feynman rules, and show these are described by compact expressions involving the b-ghost operator. Remarkably, it will be shown how color-kinematics duality immediately emerges in this pure spinor framework after imposing the Siegel gauge condition b_{0}V = 0.
26/05/2022, 16:00, lecture hall
Lavinia Heisenberg (Heidelberg University, Germany)
Tensions in LCDM and how to solve them
After introducing the standard model of cosmology and its parameters, I will discuss two important tensions between early and late-time measurements, namely the H0 tension and the sigma8 tension. Considering a small late-time deviation of the standard model, I will derive fully analytical conditions that any late-time dark energy model has to satisfy in order to solve both tensions simultaneously (see arxiv:2201.11623).
23/05/2022, 14:00, room 226
Claire Zukowski (UvA, Netherlands)
Virasoro Entanglement Berry Phases
I will describe the parallel transport of modular Hamiltonians encoding entanglement properties of a state. The Berry curvature associated to state-changing parallel transport is the Kirillov-Kostant symplectic form on an associated coadjoint orbit, one which differs appreciably from known Virasoro orbits. I will show that the boundary parallel transport process computes a bulk symplectic form for a Euclidean geometry obtained from the backreaction of a cosmic brane, with Dirichlet boundary conditions at the location of the brane. This construction gives a definition for the symplectic form on an entanglement wedge.
19/05/2022, 16:00, lecture hall
Michal Artymowski (Wyszynski University, Warsaw, Poland)
New applications of unparticles: Inflation, dark energy, bouncing cosmologies, and the Hubble tension
Unparticles are a hypothetical new form of matter created from fermions in an SU(N) gauge theory. Unparticles provide a wide spectrum of new cosmological applications. In my talk (based on arxiv:2010.02998 and arxiv:1912.10532), I will show that they can display a cosmological-constant-like behavior, and since then they can be used to generate cosmic inflation or dark energy. I will show realistic bouncing and cyclic Universes filled with unparticles and perfect fluid. I will also discuss constraints on unparticles energy density and their possible role in relaxing the Hubble tension.
16/05/2022, 14:00, room 226
Lorenz Eberhardt (IAS Princeton, USA)
Off-shell Partition Functions in 3d Gravity
I will discuss partition functions in three-dimensional quantum gravity with negative cosmological constant in canonical quantization. I will review the phase space and its quantization in detail, which leads to the computation of the gravity partition functions on 3d manifolds which do not support semiclassical saddles. It is often simpler to consider chiral gravity that only captures the left-movers, since ordinary gravity gives divergent answers. I finally explain a dual description in terms of topological recursion of a certain part of the partition function that is the uplift of the random matrix model for JT-gravity. Based on 2204.09789.
12/05/2022, 16:00, lecture hall
Earl Bellinger (Max Planck Institute, Garching, Germany)
Asteroseismic probes of stellar evolution and fundamental physics
Asteroseismology allows us to determine the properties of stars through the observation of their global oscillations, giving information about the star's mass, radius and age. Beyond being interesting in their own right, these measurements are essential for a variety of endeavours throughout astrophysics, such as galactic archaeology and the characterization of exoplanets. For the stars with the very best observations, it is possible to additionally measure some aspects of the internal stellar structure, such as the density and sound speed profile throughout the stellar core. This in turn presents the exciting opportunity to test the physics of stellar evolution. These asteroseismic tests can range from assessing mixing mechanisms in stellar interiors, to measuring cosmological effects such as a time-variable gravitational constant. In this seminar, I will give an overview of the asteroseismology of low-mass stars, and highlight the progress that is being made toward mapping out their interior structures.
04/05/2022, 16:00, room 117
Caner Unal (Ben-Gurion University of the Negev, Israel)
Spin in Active BHs and properties of ultralight particles
I will discuss the spin modifications to the fundamental plane of BH activity, an empirical correlation between X-ray, radio luminosity and the mass of active BHs. I will further focus on how to extract spin info from those multiwavelength signals and its implications/bounds on ultralight particle properties such as mass, self-interaction and energy density and their possibility to be some fraction of dark matter.
28/04/2022, 16:00, lecture hall
Jose Beltran Jimenez (University of Salamanca, Spain)
Quicksand in Affinesia
The equivalence principle naturally provides gravity with a geometrical character. However, the precise geometry we employ to describe it admits a certain flexibility. In particular, within a metric-affine framework, Einstein's gravity can be equivalently ascribed to the three independent objects that characterise a connection, i.e., curvature, torsion and non-metricity. After reviewing these three alternative descriptions of gravity, I will uncover a general teleparallel description of GR and how pathologies generally arise beyond the GR equivalents in the landscape of metric-affine theories.
25/04/2022, 14:00, room 226
Alex Belin (CERN, Switzerland)
Quantum chaos, OPE coefficients and wormholes
In this talk, I will discuss the statistical distribution of OPE coefficients in chaotic conformal field theories. I will present the OPE Randomness Hypothesis (ORH), a generalization of ETH to CFTs which treats any OPE coefficient involving a heavy operator as a pseudo-random variable with an approximate Gaussian distribution. I will then present some evidence for this conjecture, based on the size of the non-Gaussianities and on insights from random matrix theory. Turning to the bulk, I will argue that semi-classical gravity geometrizes these statistical correlations by wormhole geometries. I will show that the non-Gaussianities of the OPE coefficients predict a new connected wormhole geometry that dominates over the genus-2 wormhole.
21/04/2022, 16:00, lecture hall
Marek Lewicki (University of Warsaw, Poland)
Search for new physics through primordial gravitational waves
We are currently witnessing the dawn of a new era in astrophysics and cosmology, started by the LIGO/Virgo observations of gravitational waves. These signals also open a new window into processes taking place in the first moments of our Universe. This is due to the fact that GWs propagate freely from the moment of their production unlike like photon based signals which can only propagate freely since the Universe became transparent due to recombination.I will discuss prospects for GW detection with the next generation of experiments. Including the problems connected with observation of a primordial signal in the presence of a foreground produced much more recently by astrophysical objects. The specific early Universe sources I will focus on are cosmological first order phase transitions and cosmic string networks. I will also discuss to what extent we can probe the expansion of the Universe using these primordial GW signals.
13/04/2022, 14:00, lecture hall
Pavel K. Kovtun (University of Victoria, Canada)
Hydrodynamics beyond hydrodynamics
In this talk, I will discuss two questions. First, do the equations of relativistic hydrodynamics make sense? And second, how universal are the long-distance, late-time predictions of classical hydrodynamics?
07/04/2022, 16:00, lecture hall
Dong-Gang Wang (University of Cambridge, United Kingdom)
Bootstrapping Inflation: cosmological correlators with broken boosts
Correlation functions of primordial fluctuations provide us an exciting avenue into the physics with extremely high energy in the very early Universe. Recently the bootstrap approach has offered new perspectives and powerful tools to study these cosmological correlators. In this talk, by incorporating the latest developments, I will “bootstrap” two types of correlators generated by boost-breaking interactions during inflation, which are most relevant for the next-generation observations. The first one is the contact correlators arising from higher-derivative self-interactions of the inflaton. The second is the cosmological collider physics, where the masses and spins of heavy particles leave unique imprints in the scalar bispectra. Since the boost symmetries are broken in our consideration, the signals of non-Gaussianity are boosted to be detectable for near-future experiments. Furthermore through the bootstrap approach, we derive for the first time not only a complete set of these correlators systematically, but also their full shape information analytically.
31/03/2022, 16:00, lecture hall
Christos Charmousis (IJCLab Orsay, France)
Compact objects in scalar tensor theories
We will review some of the black hole solutions in scalar tensor theories focusing on the different symmetries of the underlying theories and principal properties. We will discuss one particular theory whose origin lies in higher dimensional Lovelock theory and construct a regular traversable wormhole and neutron star metrics.
24/03/2022, 16:00, lecture hall
Sergey Ketov (Leibniz University Hannover, Germany & Tokyo Metropolitan University, Japan)
Formation of primordial black holes after Starobinsky inflation in supergravity
The Starobinsky model of cosmological inflation is reviewed as the theoretical probe of a more fundamental theory of gravity for the very early Universe. The modified Starobinsky supergravity is introduced, and its observational predictions are derived and compared to the current astrophysical and cosmological observations. The specific mechanism of the primordial black holes production in supergravity is proposed, and its physical predictions for dark matter and induced gravitational waves are discussed in detail.
10/03/2022, 17:00, online
Daniel Green (UC, San Diego, USA)
A Tail of Eternal Inflation
Recent developments in our understanding of quantum field theory in de Sitter space have revealed how to derive the equations Stochastic Inflation and included corrections to them systematically. In this talk, I will review the Soft de Sitter Effective Theory and how it enables to calculate these equations. I will then apply the results to massless λΦ4 theory in de Sitter and calculate the next-to-next-to leading order corrections to the equations and relaxation eigenvalues. We will then apply the same techniques to primordial non-Gaussianity in single-field inflation where we will find the onset of eternal inflation becomes incalculable in some surprising circumstances.
17/03/2022, 16:00, lecture hall
Dmitri Semikoz (APC Paris, France)
Measurements of cosmological magnetic fields in the voids of large scale structure
In this talk I’ll review recent developments in the measurements of intergalactic magnetic fields in the voids of large scale structure with gamma-ray telescopes and ultra-high energy cosmic ray detectors. In particular, I'll show that the gamma-ray measurement method can be used to detect the primordial magnetic field with a strength of up to 10-11 G, values interesting for reducing H0 tension. Same magnetic field, if produced at QCD phase transition can be responsible for NANOGrav gravitational wave signal. Also I’ll discuss how one can distinguish magnetic field produced at inflation by simultaneous measurements of extended emission around several nearby TeV blazars. Finally, I'll show first upper limit on magnetic field in the void of large scale structure from ultra-high energy cosmic ray measurement in the direction of Perseus-Pisces supercluster.
24/02/2022, 16:00, lecture hall
Matthias Bartelmann (ITP, Heidelberg University, Germany)
On the track of universality in cosmic structures
Our cosmic neighbourhood is richly structured by galaxies, clusters, and larger objects which are mainly composed of dark matter. Gravitationally bound objects dominated by dark matter exhibit density profiles which are self-similar over many orders of magnitude in mass. Why is this so? I will use kinetic field theory to address this question, and show that cosmic structures develop universal correlations for wide classes of initial conditions.
17/02/2022, 16:00, lecture hall
Sk Jahanur Hoque (Charles University, Prague, Czechia)
Mass loss law for weak gravitational fields: With a positive cosmological constant
Bondi's celebrated mass loss formula measures the rate of change of energy carried away from an isolated system (in asymptotically flat space-time) by gravitational radiation. In this talk, we generalize the Bondi-Sachs formalism for de Sitter space-time. We also discuss the mass loss formula for linearized gravitational fields in de Sitter setting.
10/02/2022, 16:00, online seminar
Ke-Pan Xie (Nebraska University, USA)
Primordial black holes from a cosmic phase transition: The collapse of Fermi-balls
In this talk I will introduce a novel mechanism of forming primordial black holes (PBHs) via a first-order phase transition (FOPT). If a fermion species gains a huge mass in the true vacuum, the corresponding particles get trapped in the false vacuum as they do not have sufficient energy to penetrate the bubble wall. After the FOPT, the fermions are compressed into the false vacuum remnants to form non-topological solitons called Fermi-balls, and then collapse to PBHs due to the interior Yukawa attractive force. After describing the general mechanism, I will discuss a concrete application to the electroweak phase transition and demonstrate that a PBH dark matter scenario is possible.
03/02/2022, 16:00, online seminar
Alexander Ganz (Jagiellonian University, Krakow, Poland)
Minimally modified gravity and its phenomenological properties
Minimally modified gravity models are a class of modified gravity models with just two local degrees of freedom as in general relativity. In this talk I want to discuss their general properties such as the existence of a preferred foliation and discuss their phenomenology in the case of inflation and the late universe.
27/01/2022, 16:00, lecture hall
Adolfo Cisterna (TIFPA-INFN, Trento, Italy)
Taub-NUT spacetimes at the service of exact black bounces: Black holes, wormholes and bouncing cosmologies with a self-interacting scalar field
We present a new family of exact four dimensional Taub-NUT spacetimes in Einstein-Λ theory supplemented with a conformally coupled scalar field exhibiting a power-counting super-renormalizable potential. Our configurations are constructed in the following manner: A solution of a conformally coupled theory with a conformal potential, henceforth the seed (gμν,Φ), is transformed by the action of a specific change of frame in addition with a simultaneous shift of the scalar seed. The conformal factor of the transformation and the shift are both affine functions of the original scalar Φ. The new configuration, (g'μν,Φ'), now solves the field equations of a conformally coupled theory with the extended aforementioned renormalizable potential, this under the presence of an effective cosmological constant. The new solution spectrum is notoriously enhanced with respect to the original seed containing regular black holes, wormholes and bouncing cosmologies. For a non-vanishing cosmological constant exact black hole to wormhole and black hole to bouncing cosmology transitions are observable, both smoothly controlled by the mass parameter.
20/01/2022, 16:00, Zoom seminar
Valerie Domcke (CERN, Geneva, Switzerland)
Cosmology with axion-like particles
Axion-like particles may play a key role in early universe cosmology. They are naturally equipped with the right properties to explain cosmic inflation, can dynamically explain the smallness of the electroweak scale, may be involved in the generation of the matter antimatter asymmetry and are promising dark matter candidates. In this talk I discuss a generic but previously overlooked particle particle production mechanism, resulting in the dual production of gauge fields and fermions induced by axion-like particles. I will discuss how this crucially impacts all of the cosmological scenarios mentioned above and may be probed with upcoming gravitational wave detectors.
13/01/2022, 16:00, Zoom seminar
Marco Crisostomi (SISSA, Trieste, Italy)
Gravitational wave generation in dark energy theories
The big challenge in describing dark energy as a dynamical field is that we do not see any sign of it in local tests of gravity. Moreover, all gravitational wave events detected so far are in very good agreement with General Relativity predictions. In this talk I will introduce ``kinetic screening'' as a way to overcome this dichotomy and I will present our recent results in testing it with black hole collapse, and the late inspiral and merger of binary neutron stars.
02/12/2021, 16:00, Lecture Hall
Ville Vaskonen (IFAE, Barcelona, Spain)
Probing dark matter through gravitational waves
The LIGO-Virgo observations have already demonstrated the power of gravitational wave astronomy. In near future various experiments will probe gravitational wave signals across a broad range of frequencies providing invaluable insight into astrophysics, cosmology and fundamental physics. In this talk I will discuss how we can use these observations to test dark matter properties. I will focus on two signatures of compact dark matter objects: gravitational waves produced by their mergers, and lensing of gravitational waves. I will show that the LIGO-Virgo observations already provide constraints on compact dark matter and I will discuss the future prospects of gravitational wave probes of dark matter.
25/11/2021, 16:00, Lecture Hall
Anamaria Hell (LMU, Munich, Germany)
Exploring the dualities of massive gauge theories: Aμ vs. Bμν
We compare the massive Kalb-Ramond and Proca fields with a quartic self-interaction and show that the same strong coupling scale is present in both theories. In the Proca theory, the longitudinal mode enters the strongly coupled regime beyond this scale, while the two transverse modes propagate further and survive in the massless limit. In contrast, in case of the massive Kalb-Ramond field, the two transverse modes become strongly coupled beyond the Vainshtein scale, while the pseudo-scalar mode remains in the weak coupling regime and survives in the massless limit. This contradicts the numerous claims in the literature that these theories are dual to each other, which is shown to be false. We show that the difference between the theories can be traced already to the free theories without a self-interaction by studying the behavior of quantum fluctuations for different modes.
18/11/2021, 16:00, Zoom
Graham White (University of Tokyo, Japan)
Archaeology on the origin of matter
One of the most convincing reasons to expect physics beyond the Standard model is the inbalance between matter and anti-matter. Some fantastic paradigms exist that can be probed at a low scale including electroweak baryogenesis, mesogenesis and resonant leptogenesis. While these paradigms or worthy of dedicated attention, the elephant in the room is that there are two paradigms that are very minimal and involve physics at scales we cannot possibly reach with Earth based colliders in our life time. I will first discuss the nightmare scenario of thermal leptogenesis implemented with no BSM particle content beyond sterile neutrinos and an inflaton. In this case, measurements of the top and Higgs mass along with inflationary observables can shed some light on the plausibility, or lack thereof, of vanilla leptogenesis. I will then discuss the GUT leptogenesis and Affleck Dine baryogenesis. I argue in both these cases there are generic predictions of a primordial gravitational wave background that can be measured today. The presence of such a signal would lend plausibility to one of these scenarios. Finally I discuss the discriminating power of GWs in discerning the symmetry breaking path through the variable signals of hybrid defects.
11/11/2021, 16:00, Lecture Hall
Tomislav Prokopec (Utrecht University, Netherlands)
Quantum origin of dark energy and the Hubble tension
Local measurements of the Hubble parameter obtained from the distance ladder at low redshift are in tension with global values inferred from cosmological standard rulers. A key role in the tension is played by the assumptions on the cosmological history, in particular on the origin of dark energy. Here we consider a scenario where dark energy originates from the amplification of quantum fluctuations of a light field in inflation. We show that spatial correlations inherited from inflationary quantum fluctuations can reduce the Hubble tension down to one standard deviation, thus relieving the problem with respect to the standard cosmological model. Upcoming missions, like Euclid, will be able to test the predictions of models in this class.
04/11/2021, 16:30, Lecture Hall
Camilo Garcia Cely (DESY, Germany)
The CMB as a detector of gravitational waves
In complete analogy with axion dark matter, gravitational waves are converted into electromagnetic radiation when they propagate in magnetic fields. I will explain how this effect can be used to detect gravitational waves. With this in mind, I will examine gamma-ray observations by Fermi-LAT and HESS strongly suggesting the existence of a non-vanishing cosmic magnetic field. Then, I will show how the consequent conversion of gravitational waves into radio waves might distort the CMB, leading to bounds that exceed those from current terrestrial experiments. I will discuss prospects from the future gamma-ray observatory CTA and argue that future advances in 21 cm astronomy might push these bounds below the Neff constraint on the radiation density present during the CMB formation.
27/10/2021, 16:00, Lecture Hall
Stian Hartman (University of Oslo, Norway)
Self-interacting Bose-Einstein condensed dark matter; cosmological constraints and simulations
In this seminar I will talk about a particular kind of light scalar field dark matter, namely those with self-interactions, which I will call self-interacting Bose-Einstein condensed (SIBEC) dark matter. These have been found to possibly solve some of the small-scale issues of LCDM, such as the core-cusp problem, by providing an interaction pressure that supports hydrostatic halo cores on the order of kpc. Unlike their non-interacting ultra-light counterparts (fuzzy dark matter), there is not yet a large body of work dedicated to providing constraints on the strength of the SIBEC-DM self-interaction using large-scale observables. I will present such constraints, which weakly rules out the self-interactions generally thought to be needed to solve the cusp-core problem in the simplest scenario of SIBEC-DM. I will also talk about ongoing efforts to study structure formation in a SIBEC-DM universe using cosmological simulations.
21/10/2021, 16:00, Lecture Hall
Farbod Hassani (University of Oslo, Norway)
Characterizing the non-linear evolution of dark energy and modified gravity models
Understanding the reason behind the observed accelerating expansion of the Universe is one of the most notable puzzles in modern cosmology, and conceivably in fundamental physics. In the upcoming years, near future surveys will probe structure formation with unprecedented precision and will put firm constraints on the cosmological parameters, including those that describe properties of dark energy. In light of this, in the first part of my talk, I'm going to show a systematic extension of the Effective Field Theory of Dark Energy framework to non-linear clustering. As a first step, we have studied the k-essence model and have developed a relativistic N-body code, k-evolution. I'm going to talk about the k-evolution results, including the effect of k-essence perturbations on the matter and gravitational potential power spectra and the k-essence structures formed around the dark matter halos. In the second part of my talk, I'm going to show that for some choice of parameters the k-essence non-linearities suffer from a new instability and blow up in finite time. This talk will be based on the following publications and an ongoing work: arXiv:2107.14215, arXiv:2007.04968, arXiv:1910.01105, arXiv:1910.01104, arXiv:1906.04748.
14/10/2021, 16:00, Lecture Hall
Roman Konoplya (Peoples' Friendship U., Moscow, Russia)
Traversable wormholes in General Relativity without exotic matter
I will start by briefly reviewing wormholes - exotic compact objects the interest to which has been recently largely revived. In [J. Blazquez-Salcedo, C. Knoll, E. Radu, Phys. Rev. Lett. 126 (2021) no.10, 101102] asymptotically flat traversable wormhole solutions were obtained in the Einstein-Dirac-Maxwell theory without using exotic matter. The normalizable numerical solutions found in this work above require a peculiar behavior at the throat: the mirror symmetry relatively the throat leads to the nonsmoothness of gravitational and matter fields. In particular, one must postulate the changing of the sign of the fermionic charge density at the throat requiring coexistence of particle and antiparticles without annihilation and posing a membrane of matter at the throat with specific properties. Apparently this kind of configurations could not exist in nature. We show that there are wormhole solutions, which are asymmetric relative to the throat and endowed by smooth gravitational and matter fields, being, thereby, free from all the above problems. This indicates that such wormhole configurations could also be supported in a realistic scenario.
30/09/2021, 16:00, Zoom
Anton Baleato Lizancos (UC Berkeley, USA)
Fundamental Physics with CMB Lensing and Delensing
Gravitational lensing of CMB photons by the matter distribution of the Universe can be both a blessing and nuisance. It's a blessing because of the way it can be harnessed to map the structures responsible for the deflections, and from this, constrain any physics affecting the growth of cosmic structure, such as the sum of the neutrino masses or dark matter. But lensing is also a nuisance because it generates B-mode polarization which obscures the highly-sought-after primordial signal associated with gravitational waves generated during cosmic inflation, our most accessible portal to physics near the GUT scale. In this talk, I will focus on key systematic effects that need to be controlled in order to harness the full potential of the Simons Observatory (SO), CMB-S4, and other upcoming experiments to make progress in these exciting areas. In the first part of my talk, I will briefly review the ways in which emission from galaxies and clusters can bias power spectra and cross-correlations of CMB lensing reconstructions, and describe our ongoing efforts to understand these biases analytically. Then, in the second part, I will explain how the lensing contamination to CMB B-modes can be removed — what is known as delensing — and discuss our recent findings regarding the performance of different delensing methods. I will also summarize preparatory work to delens SO data, and highlight biases to watch out for (and how to mitigate them) when the matter proxy used for delensing is either the cosmic infrared background or a lensing reconstruction derived from the CMB itself.
23/09/2021, 16:00, Lecture Hall
Jan Burger (University of Iceland, Reykjavik, Iceland)
Conservation of radial actions in time-dependent spherical potentials
The Hamiltonian of a particle orbiting in a static, spherically symmetric potential can be written as a function of only two actions, the angular momentum and the radial action, which is a conserved quantity in adiabatically evolving potentials (a so-called adiabatic invariant). For that reason, canonical action-angle coordinates are frequently used as a basis for perturbative calculations, for instance in Hamiltonian perturbation theory, when considering the long-term evolution of near-equilibrium systems. In impulsively (fast) evolving potentials, conservation of radial actions breaks down and actions change discontinuously by a non-deterministic amount. Here, I focus on the transition between adiabatically and impulsively evolution. I show that the evolution of radial actions in mildly time-dependent potentials is given by an oscillation around a constant value, the amplitude of which is set by the rate at which the potential changes. As a consequence, the evolution of a distribution of radial actions is governed by a diffusion equation. Based on the derived drift and diffusion coefficients, I qualitatively discuss the non-linear regime and demonstrate that the non-linear evolution of radial action distributions is given by an asymmetric drift towards lower actions. I illustrate the relevance of these results on two astrophysical examples, accretion onto a cold dark matter (CDM) halo and the cusp-core transformation in a dwarf-sized self-interacting dark matter (SIDM) halo.
Academic year 2000/2001
17/06/2021 -- Marco Astorino (INFN, Milano, Italy)
Time: 16:00
Place: Zoom seminar
Analytical and regular solutions in four-dimensional General Relativity representing multiblack hole systems immersed in external gravitational fields are discussed. The external fields background is composed by an infinite multipolar expansion, which allows to regularise the conical singularities of an array of collinear static black holes. Charged, Rotating, NUT and accelerating generalisations are presented. Limits to the binary Majumdar–Papapetrou, Bonnor–Swaminarayan and the Bičák–Hoenselaers–Schmidt metrics are recovered.
10/06/2021 -- Francis-Yan Cyr-Racine (University of New Mexico)
Time: 16:00
Place: Zoom seminar
Observations of dark matter structure at the smallest scales can tell us about physical processes taking place in the dark sector at very early times. Here, we point out that the presence of light degrees of freedom coupling to dark matter in the early Universe introduces a localized feature in the halo mass function. This leads to a mass function that is distinct in shape than either warm dark matter or cold dark matter, hence distinguishing these models from other leading classes of dark matter theories. We present analytical calculations of these mass functions and show that they closely match N-body simulations results. We also discuss the impact of these mass functions at high-redshift on the 21-cm signal from cosmic dawn. We briefly discuss how current constraints on the abundance of small-scale dark matter structure do not directly apply to these models due to the multi-scale nature of their mass function.
27/05/2021 -- Daniel Litim (University of Sussex, UK)
Time: 16:00
Place: Zoom seminar
Fixed points under the renormalisation group are key for a fundamental definition of quantum field theory. They can be free such as in asymptotic freedom of QCD, or interacting, such as in asymptotic safety. In this talk, I provide rigorous results for interacting UV and IR fixed points in general 4d QFTs with or without supersymmetry. I will also discuss the state of the art for fixed points in 4d quantum gravity with or without matter, including an overview of results and open challenges.
20/05/2021 -- Macarena Lagos (Columbia University, NY, USA)
Time: 16:00
Place: Zoom seminar
Gravitational waves (GWs) allow us to probe the content of the Universe and the behaviour of gravity on cosmological scales, through information contained in their propagation. For instance, the presence of dynamical fields interacting non-minimally with gravity may induce a non-trivial propagation of GWs, changing their propagation speed, dispersion relation, or detected amplitude, among others. In this talk, I will discuss particular cosmological scenarios where GWs interact with another tensor field, such as in the theory of massive bigravity. I will illustrate explicitly how the GW signal from a coalescence of black holes gets distorted during propagation, generating specific features such as echoes of the GW signal emitted. These strong features suggest that stringent constraints on interacting GWs can be placed with current and future GW detectors.
13/05/2021 -- Swetha Bhagwat (Sapienza, University of Rome, Italy)
Time: 16:00
Place: Zoom seminar
In this talk I will talk about a recent work where we propose a new test of GR. The gravitational waves emitted during the coalescence of binary black holes offers an excellent probe to test the behaviour of strong gravity at different length scales. In this work, we propose a test called the merger-ringdown consistency test that focuses on probing horizon-scale dynamics of strong-gravity using the binary black hole ringdowns. This test is a modification of the more traditional inspiral-merger-ringdown consistency test. I will present a proof-of-concept study of this test using simulated binary black hole ringdowns embedded in the Einstein Telescope-like noise. Furthermore, we use a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.
06/05/2021 -- Viviana Niro (APC Paris, France)
Time: 16:00
Place: Zoom seminar
The HAWC telescopes has recently revealed new spectra for gamma-ray sources in the Galactic plane. In this talk I will review the possibility of detecting these sources at KM3 detectors. I will consider, with particular emphasis, the 2HWC J1825-134 source. Amongst the HAWC sources, it is indeed the most luminous in the multi-TeV domain and therefore is one of the first that should be searched for with a neutrino telescope in the northern hemisphere. I will show the prospects to detect this source at the KM3NeT detector and comment on the possibilities for others neutrino telescopes. I will consider, moreover, the gamma-ray sources eHWC J1907+063, eHWC J2019+368 and 2HWC J1857+027. For these sources, I will show the prediction for neutrinos at the IceCube detector, presenting the calculation of the statistical significance, considering 10 and 20 years of running time, and I will comment on the current results reported by the collaboration.
29/04/2021 -- Emel Altas (Karamanoglu Mehmetbey University, Turkey)
Time: 16:00
Place: Zoom seminar
Using the time evolution equations of (cosmological) general relativity in the first order Fischer-Marsden form, we construct an integral that measures the amount of nonstationary energy on a given spacelike hypersurface in D dimensions. We also construct analytical initial data for a slowly moving and rotating black hole for generic orientations of the linear momentum and the spin. We solve the Hamiltonian constraint approximately and work out the properties of the apparent horizon and show the dependence of its shape on the angle between the spin and the linear momentum. In particular, a dimple, whose location depends on the mentioned angle, arises on the two-sphere geometry of the apparent horizon. We exclusively work in the case of conformally flat initial metrics.
22/04/2021 -- Teodor Borislavov Vasilev (Universidad Complutense de Madrid, Spain)
Time: 16:00
Place: Zoom seminar
The big rip, the little rip and the little sibling of the big rip are cosmological doomsdays predicted by some phantom dark energy models that could describe the future evolution of our own Universe. When the Universe evolves towards either of these future cosmic events, all bounded structures and, ultimately, space-time itself are ripped apart. Nevertheless, it is commonly belief that quantum gravity effects may smooth or avoid these (classical) singularities. In this talk I will review the occurrence of these rip-like events in the scheme of alternative metric $f(R)$ theories of gravity from both classical and quantum points of view. The quantum analysis will be performed in the framework of $f(R)$ quantum geometrodynamics. This is a canonical quantization procedure based on the Wheeler-DeWitt equation for the case of $f(R)$ theories of gravity. In this context, I will discuss the avoidance of these (classical) singularities by means of the DeWitt criterion.
15/04/2021 -- Famaey Benoit (Observatoire astronomique de Strasbourg, France)
Time: 16:00
Place: Zoom seminar
In this talk I will summarize the intriguing phenomenology associated to galaxy scaling relations, a phenomenology which is still challenging to understand in the standard context. I will show how the MOND paradigm of Milgrom naturally solves most of these puzzles. I will also show that it however comes with new puzzles. I will focus the end of the talk on some of the main constraints that should be taken into account for MONDian model-building, especially on galaxy cluster scales.
25/03/2021 -- Johannes Noller (Cambridge University, DAMTP, UK)
Time: 16:00
Place: Zoom seminar
Recent years have seen great progress in probing gravitational physics on a vast range of scales, from the very largest cosmological scales to the microscopic ones associated with high energy particle physics. In this talk I will give a whistle stop tour of some of the different physical systems we can use to learn more about gravity in this way, with a focus on how we can use them synoptically to learn more about dark energy. Stops will include gravitational waves emitted by binary systems, the cosmic microwave background, large scale structure formation, and (theoretical) bounds on the behaviour of gravity on scales inaccessible to current experiments.
18/03/2021 -- Kazuya Koyama (Portsmouth U., ICG, UK)
Time: 16:00
Place: Zoom seminar
Future galaxy surveys such as Euclid, LSST and SKA will cover larger and larger scales where general relativistic effects become important. On the other hand, our study of large scale structure still relies on Newtonian N-body simulations. I show how standard Newtonian N-body simulations can be interpreted in terms of the weak-field limit of general relativity. Our framework allows the inclusion of radiation perturbations and the non-linear evolution of matter. I show how to construct the weak-field metric by combining Newtonian simulations with results from Einstein-Boltzmann codes. I discuss observational effects on weak lensing and ray tracing, identifying important relativistic corrections. Finally, I show that this framework can be extended to gravitational theories beyond general relativity.
11/03/2021 -- Raissa Mendes (Universidade Federal Fluminenense, Brazil)
Time: 16:00
Place: Zoom seminar
In this talk, I will discuss how extreme properties that may be present in the interior of some neutron stars can turn them into unique laboratories for tests of modified theories of gravity. In particular, I will focus on the case of scalar-tensor theories with screening mechanisms. These theories offer an interesting framework for cosmology, since the scalar degree of freedom could help to drive the accelerated expansion of the universe, while screening off its effects in solar system scales, where general relativity is very well tested. Although it is typically understood that screening becomes more effective in high density environments, we will show in a few interesting models how it can actually fail in the densest places in nature - the core of some neutron stars.
04/03/2021 -- Ali Seraj (Universite Libre de Bruxelles)
Time: 16:00
Place: Zoom seminar
In this talk, I will review the gravitational wave memory effect in Einstein general relativity and discuss its relation to BMS symmetries. Then I will consider Brans-Dicke theory containing an additional gravitational degree of freedom. This mode is associated with novel memory effects. However, being a scalar, it is not obvious which symmetry this memory corresponds to. I will show that the memories associated with the breathing mode correspond to the asymptotic symmetries of a dual 2 form representation of the scalar field.
18/02/2021 -- François Larrouturou (IAP Paris, France)
Time: 16:00
Place: Zoom seminar
In 2011, F. Hassan and R. Rosen achieved the construction of the first gosth-free theory of two interacting spin-2 fields. But despite its great elegance and interesting phenomenological implications, this theory of "bigravity" suffers from a gradient-type instability. Moreover, it postulates the existence of vectorial and scalar gravitational modes, the latter being severely constrained by observations of binary pulsars. This stimulated us to construct a "minimal" theory of bigravity, ie. a theory of two interacting spin-2 fields that propagates only four tensorial degrees of freedom. This talk will first introduce the motivations that led to this "minimal" theory of bigravity, and review its construction by a Hamiltonian procedure. I will then present its cosmological phenomenology, and show that it provides a stable nonlinear completion of the cosmology of the Hassan-Rosen bigravity. We will end by discussing interesting phenomenological features and possible ways to test the theory.
11/02/2021 -- Ondrej Hulik (Charles University, Prague)
Time: 16:00
Place: Zoom seminar
I will review the formulation of generalized and exceptional geometry as the underlying geometry of supergravity and M-theory. With suitable generalized framework one can treat these distict types of geometries as a special case of one object "G-algebroid". I will sketch main underlying idea and how is this formulation useful in treating T/U duality.
04/02/2021 -- Giulia Cusin (Universite de Geneve, Switzerland)
Time: 16:00
Place: Zoom seminar
There are two possible approaches to describe a population of astrophysical gravitational wave (GW) sources: one can focus on high signal-to-noise sources that can be detected individually, and build a catalogue. Alternatively, one can take a background-approach and study the incoherent superposition of GW signals emitted by the entire population (both resolved and unresolved sources) from the onset of stellar activity until today. A detailed description of signals from resolvable sources, and of the properties of a stochastic background, including propagation effects, is crucial to extract accurate information on the underlying source population. Moreover, these two observables contain complementary astrophysical information and, once combined, they can provide insight on the properties of a faint and distant sub-population that cannot be accessed with any other means of observation. In my talk I will outline the differences and the complementarity of these two approaches, from the point of view of observations and of theoretical modeling, and stress a few caveats to be kept in mind when deriving predictions to be compared with (present and future) datasets.
28/01/2021 -- Paolo Benincasa (IFT Madrid, Spain)
Time: 16:00
Place: Zoom seminar
QFT in nearly dS space-times and, more generally, in FRW backgrounds allows us to describe correlations at the end of inflation. However, how to extract fundamental physics out of them is still a challange: we do not even know how fundamental pillars such as causality and unitarity of the time evolution constrain them. In this talk I will present a recent program which aims to construct the wavefunction of the universe, which generates these correlations, directly from first principles without making any reference to time evolution: these observables naturally live at the boundary of the nearly dS/FRW space-times and the time evolution is integrated out. I will discuss two approaches: one approach aims to construct the wavefunction from the knowledge of its general analytic properties, in a similar fashion as scattering amplitudes in flat space-time (which can be formulated directly from on-shell data with no reference to fields whatsoever); the second approach aims to find new mathematical objects, with their own first principle definition, which has the very same properties we ascribe to the wavefunction of the universe, with all the basic physical principles such as causality and unitarity, emerging from their intrinsic definition.
21/01/2021 -- Timothy Anson (Universite Paris-Saclay, France)
Time: 16:00
Place: Zoom seminar
Starting from a recently constructed stealth Kerr solution of higher order scalar tensor theory, I will discuss disformal versions of the Kerr spacetime with a constant disformal factor and a regular scalar field. While the disformed metric has only a ring singularity and asymptotically is quite similar to Kerr, it is neither Ricci flat nor circular. Non-circularity has far reaching consequences on the structure of the solution. In particular, I will discuss the properties of important hypersurfaces in the disformed spacetime: ergosphere, stationary limit and event horizon, and highlight the differences with the Kerr metric.
14/01/2021 -- Siddharth Prabhu (Tata Institute of Fundamental Research, India)
Time: 16:00
Place: Zoom seminar
In the last couple of decades, we have learnt a great deal about quantum gravity and its holographic nature in asymptotically AdS spacetimes. Here, we explore this idea in asymptotically flat spacetimes with the following question: Can an observer on the boundary of the spacetime distinguish between two states that are deemed distinguishable by an observer in the bulk? We argue that semiclassical gravity is an effective tool to answer this question, and extrapolate its results to make a few reasonable assumptions regarding the low energy structure of any complete theory of quantum gravity. Using these, we argue that an asymptotic observer indeed has access to all the information in a theory with only massless bulk excitations, provided they are at the past boundary of future null infinity. We also show that information available in any cut of future null infinity is also available in any later cut, but the converse doesn't hold. Similar results hold for past null infinity. We will comment on several interesting questions that this line of investigation sheds light on.
17/12/2020 -- Oksana Iarygina (Leiden University, NL)
Time: 16:00
Place: Zoom seminar
The reheating era in the early universe, that connects inflation and big-bang nucleosynthesis, is still very weakly constrained. However, inefficient preheating can lead to a prolonged matter-dominated phase after inflation, changing the time during inflation when the Cosmic Microwave Background (CMB) modes exit the horizon. This shifts CMB predictions and thus can break the degeneracy of otherwise indistinguishable inflation models. Typically, models that allow a UV completion, include many distinct fields, often with curved field-space manifolds. Based on arXiv:2005.00528, arXiv:1810.02804, the present talk focuses on the physical mass scales that control the dynamics and observable predictions of all multi-field models with a non-zero field-space curvature: the Hessian of the potential, the turning rates of the trajectory and the field-space curvatures. We analyse how their interplay affects reheating and shifts inflationary predictions. We also demonstrate the existence of a region in parameter space, where the symmetric and asymmetric multi-field alpha-attractors, that are known by the universality of their single-field inflationary predictions, are explicitly not the same: one preheats and one does not. This leads to a different cosmic history for the two models, with one possibly exhibiting a long matter-dominated phase, and a shift in the observational predictions for ns and r.
10/12/2020 -- Horng Sheng Chia (Institute for Advanced Study, Princeton, USA)
Time: 16:00
Place: Zoom seminar
Black holes are never isolated in realistic astrophysical environments; instead, they are often perturbed by complicated external tidal fields. How does a black hole respond to these tidal perturbations? In this talk, I will discuss both the conservative and dissipative responses of the Kerr black hole to a weak and adiabatic gravitational field. The former describes how the black hole would change its shape due to these tidal interactions, and is quantified by the so-called “Love numbers”. On the other hand, the latter describes how energy and angular momentum are exchanged between the black hole and its tidal environment due to the absorptive nature of the event horizon. I will show that the Love numbers of the Kerr black hole vanish identically — in other words, you cannot stretch a black hole. I will also describe how the Kerr black hole's dissipative response implies that energy and angular momentum can either be lost to or extracted from the black hole, with the latter process commonly known as the black hole superradiance. I will end by discussing how these tidal responses leave distinct imprints on the gravitational waves emitted by binary black holes.
03/12/2020 -- Antony Lewis (University of Sussex, UK)
Time: 16:00
Place: Zoom seminar
Cosmological measurements from the cosmic microwave background, large-scale structure, lensing, supernovae and other data are now able to constrain multiple cosmological parameters to percent-level precision within in the context of the standard Lambda-CDM cosmology. Disagreements between these measurements assuming Lambda-CDM could provide strong evidence for beyond-Lambda-CDM physics. I review the status of current measurements and their agreement (or otherwise) within the standard cosmological model. I’ll mention some possible types of model extensions that could help to resolve H0 tensions and how new physics might be pinned down by forthcoming data. Galaxy and CMB lensing provide interesting complementary constraints; I’ll show the state-of-the-art results from CMB lensing, compare with galaxy lensing and discuss possible implications.
26/11/2020 -- Eiichiro Komatsu (Max-Planck-Institute for Astrophysics, Germany)
Time: 16:00
Place: Zoom seminar
Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics. In this presentation we report on a new measurement of parity violation from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.
19/11/2020 -- Mikhail Shaposhnikov (EPFL)
Time: 16:00
Place: Zoom seminar
It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it incorporates Higgs inflation at energies below the onset of the strong-coupling of the theory. Second, it contains a four-fermion interaction that originates from torsion associated with spin degrees of freedom. This interaction leads to a novel universal mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. Finally, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect.
12/11/2020 -- William Barker (Kavli Institute for Cosmology, Cambridge, UK)
Time: 16:00
Place: Zoom seminar
Several novel Poincare gauge theories of gravity (curvature and torsion) were recently found to be unitary/power-counting renormalizable in the weak regime, and pass solar system tests [1,2]. We show these theories contain LCDM as an attractor state, despite neither the Einstein-Hilbert or cosmological constant terms appearing the action: the only extra parameter (xLCDM) adds effective dark radiation to relieve the Hubble tension [3]. We show that the phenomenology of the general ten-parameter theory, including novel theories, can be easily understood through a non-canonical bi-scalar-tensor analogue [4]. We discuss ongoing Dirac-Bergmann analysis of the Hamiltonian in the strong regime.([1] arXiv:1812.02675, [2] arXiv:1910.14197, [3] arXiv:2003.02690, [4] arXiv:2006.03581)
05/11/2020 -- Fedor Bezrikov (University of Manchester, UK)
Time: 16:00
Place: Zoom seminar
I'll make a review of the attempts to understand a seemingly simple process of reheating in Higgs inflation. Although reheating can be readily expected to happen at rather high temperatures, its details leave imprint on the number of inflationary e-foldings and, thus, on predictions for CMB parameters. The quest for the understanding reheating took some time, starting form simple, but incomplete approach, evolving into realisation that careful study in the strong coupled regime is inevitable. The approach of perturbative UV completion of the model by $R^2$ inflation was hoped to provide immediate answer to preheating dynamics in a weakly coupled theory, but turned into an ongoing study of evolution in non-linear potentials. I will also mention that in some cases even pure non-regularised Higgs inflation can allow for calculable predictions for preheating.
22/10/2020 -- Eugene Lim (King's College London, UK)
Time: 16:00
Place: Zoom seminar
Inflation is now the paradigmatic theory of the Big Bang. But is it deserved? I will describe the conceptual and theoretical challenges that Inflation is still facing, argue that we should keep an open mind. In particular, I will argue that while it is a theory that claims to be a theory of initial conditions of the Universe, successful inflation actually depends on an intimate interplay between its own initial conditions and the inflationary model. I will show how one might go about probing this interplay by testing whether inflation can begin if its own initial conditions are not homogenous.
15/10/2020 -- Filippo Camilloni (University of Perugia, IT and Niels Bohr Institute, DK)
Time: 16:00
Place: Zoom seminar
Force-free electrodynamics is a non-linear regime of Maxwell’s equations often employed to provide a minimal non-trivial level of description for pulsar and black hole magnetospheres. For a solution of this system to be physically meaningful the field has to be magnetically dominated, Fˆ2=Bˆ2-Eˆ2>0, however no analytic solution is known to respect this requirement in the background of a highly-spinning black hole. In this talk I will show how the Near-Horizon Extreme Kerr (NHEK) region might play a crucial role for the construction of sensible models of extreme Kerr magnetospheres . Any stationary and axisymmetric force-free solution in the extreme Kerr background is observed to converge to an attractor in the NHEK region. We used this attractor as an universal starting point to develop a new perturbative approach, showing that at the second order in perturbation theory it is possible to find magnetically-dominated force-free fields. A similar attractor mechanism occurs in the Near-Horizon Near-Extreme Kerr (near-NHEK) region of a nearly-extreme Kerr black hole, thus providing a way to extend this formalism outside extremality.
08/10/2020 -- Anne Green (University of Nottingham, UK)
Time: 16:00
Place: Lecture Hall
Primordial Black Holes (PBHs) are black holes formed in the early Universe, for instance from the collapse of large density perturbations generated by inflation. The discovery of gravitational waves from mergers of ~10 Solar mass black hole binaries has led to increased interest in PBHs as a dark matter candidate. I will review the formation of PBHs and the limits on their abundance, with particular emphasis on microlensing constraints in the Solar mass region. I will also emphasise key open questions in the field (e.g. clustering, and methods for constraining asteroid mass PBHs).
12/03/2020 -- Keigo Shimada (Tokyo Institute of Techonology)
Time: 16:00
Place: 226
Scalar-tensor theories in metric-affine geometry are formulated. General Relativity is currently the most successful gravitational theory which has surpassed countless observations. However, in recent years, it has been noticed that GR cannot explain some cosmological phenomena such as inflation, dark energy and dark matter. To solve this, countless alternative gravitational theories beyond General Relativity has been proposed. However, most require the geometry to be Riemannian, just as in GR. In this talk, it will be shown how one could extend theories of gravity by 'deforming' Riemann Geometry into what is called metric-affine geometry, in which not only the metric but also that connection is an independent variable that is decided from the gravitational action. By applying metric-affine formalism to scalar-tensor theories, one notices that there are different and fruitful characteristics that appear when compared to the Riemann counterpart. Especially, through the novel symmetry of the connection called 'projective symmetry', one may find natural ways to eliminate ghosts that are caused by higher derivatives. Finally, some possible applications would be discussed. References: Phys.Rev. D98 (2018) no.4, 044038 Phys. Rev. D 100, 044037 (2019).
27/02/2020 -- Elias Kiritsis (APC, Paris & Crete University)
Time: 16:00
Place: 226
I will discuss ideas on how gravity can be an emergent interaction in QFT, what guarantees the emergent diffeomorphism invariance, what are its general features and properties and what could be the possible implications for realistic gravitational physics.
02/03/2020 -- Tarek Anous (Amsterdam University)
Time: 14:00
Place: 226
The BFSS matrix model provides an example of gauge-theory / gravity duality where the gauge theory is a model of ordinary quantum mechanics with no spatial subsystems. If there exists a connection between areas and entropies in this model similar to the Ryu-Takayanagi formula, the entropies must be more general than the usual subsystem entanglement entropies. I will give a brief overview of the BFSS/D0 brane geometry duality and describe general features of the extremal surfaces in the bulk. I will then discuss the possible entropic quantities in the matrix model that could be dual to the ‘regulated areas’ (which I will define) of these extremal surfaces.
20/02/2020 -- Georgios Loukes-Gerakopoulos (Astronomical Institute, Prague)
Time: 16:00
Place: 226
In this talk three different studies we have undertaken to address cosmological issues, like dark energy, will be presented and their results will be discussed. In these studies we have tried to remain model agnostic as much as possible. In particular, our first study (arxiv:1902.11051) concerns a cosmic fluid obeying rest-mass conservation of unspecified equation of state (EoS) in an unspecified background assuming just that the fluid's speed of sound is positive and less than the speed of light. Our second study (arXiv:2001.00825) performs a dynamical analysis of a barotropic fluid of unspecified EoS with positive energy density in spatially curved Friedmann-Robertson-Walker (FRW) spacetimes. While, the third study (arXiv:1905.08512) performs a dynamical analysis of a broad class of non-minimally coupled real scalar fields in spatially curved FRW spacetime with unspecified positive potential.
13/02/2020 -- Paolo Creminelli (ICTP, Trieste)
Time: 16:00
Place: 226
Cosmic inflation makes the universe flat and homogeneous, but under which conditions inflation will start? I will discuss some analytical results that show, with very weak assumptions, that inflation starts somewhere and some (partial) results about a de Sitter ho-hair theorem.
30/01/2020 -- Tomas Ledvinka (Charles University, Prague)
Time: 16:00
Place: 226
Despite the tremendous success of mathematical general relativity which revealed among others surprising features of the geometry of rotating (Kerr) black holes and developed approximation techniques to study early stages of their inspiral, the necessity to describe completely the merger of two black holes lead to a substantial progress of numerical relativity. This field necessarily uses techniques of modern computer science to amass and command number crunching capabilities of current computers as well as numerical methods for partial differential equations, but the successful computer simulations also required new type of answers to questions "what is the black hole" and "what kind of equations are the Einstein ones". From this perspective I will also mention some results on hyperbolicity analysis of 3+1 reductions of Einstein equations and coordinate choice and horizon formation for collapse of gravitational waves into a black hole.
29/01/2020 -- Harold Erbin (University of Turin)
Time: 14:00
Place: 226
Machine learning has revolutionized most fields it has penetrated, and the range of its applications is growing rapidly. The last years has seen efforts towards bringing the tools of machine learning to lattice QFT. After giving a general idea of what is machine learning, I will present two recent results on lattice QFT: 1) computing the Casimir energy for a 3d QFT with arbitrary Dirichlet boundary conditions, 2) predicting the critical temperature of the confinement phase transition in 2+1 QED at different lattice sizes.
23/01/2020 -- Chris Clarkson (Queen Mary University of London)
Time: 16:00
Place: 226
Over the coming decade new surveys will map the cosmos over huge volumes. This will allow us to probe general relativity on unprecedented scales. I shall discuss some of the new relativistic effects that may be significant on these scales. Though corrections to the Newtonian picture of observations of structure formation are small, they should be detectable, and offer new insights into gravity on scales approaching the horizon.
16/12/2019 -- Pavel Motoloch (CITA, Toronto)
Time: 16:00
Place: 226
Measurements of galaxy angular momenta can, at least in principle, be used to probe fundamental physics such as primordial gravitational waves and non-Gaussianity. In my talk I explain how galaxy spins arise from the initial density perturbations, describe how they are sensitive to various physical parameters of interest and finally detail our related observational effort.
12/12/2019 -- Filippo Vernizzi (Universite Paris Saclay)
Time: 16:00
Place: 226
The observed accelerated expansion of the Universe opens up the possibility that general relativity is modified on cosmological scales. While this has motivated the theoretical study of many alternative theories that will be tested by the next generation of cosmic large-scale structure surveys, I will show that the recent observations of gravitational waves by LIGO/Virgo has dramatic consequences on these theories.
28/11/2019 -- Guillermo Ballesteros (Autonoma University, Madrid)
Time: 16:00
Place: 226
I will discuss the idea that black holes may constitute a large fraction of the Universe’s dark matter, focusing mostly on their formation from large primordial fluctuations generated during inflation. I will summarize the ups and downs of this mechanism and explain some ideas that help to alleviate its main shortcomings.
21/11/2019 -- Giovanni Acquaviva (Charles University, Prague, CZ)
Time: 16:00
Place: 226
It is known that the entropy of a system contained in a certain volume is bounded from above by the entropy of a black hole with corresponding surface area. We relate such universal bound to the existence of fundamental degrees of freedom and provide model-independent considerations about their features. In particular, both geometry and fields propagating on it are seen as phenomena emergent from the more fundamental dynamics, in analogy with many examples in condensed matter physics. An immediate consequence is that, even though the fundamental evolution is considered unitary, the fields develop an entanglement with the spacetime geometry, hence leading to an effective non-unitary evolution on the emergent level. We exemplify some consequences of this scenario by providing a toy-model of black hole evaporation: the entanglement between geometry and fields is interpreted at our low-energy scales as an effective loss of information in Hawking radiation. A question currently under scrutiny is how can unitary and continuum quantum field theory emerge from such fundamental picture.
04/11/2019 -- Dionysios Anninos (King's College London, UK)
Time: 16:00
Place: 226
We discuss quantum fields on a Euclidean sphere and their relation to de Sitter space. Various cases are considered, including particles of different spins, and masses. Some emphasis will be placed on the three-dimensional case. Time permitting, we will consider higher spin theories.
17/10/2019 -- Sk Jahanur Hoque (Chennai Mathematical Institute, India)
Time: 16:00
Place: 226
We discuss different notion of charges for asymptotically de Sitter space-time. We present a covariant phase space construction of hamiltonian generators of asymptotic symmetries with `Dirichlet' boundary conditions in de Sitter spacetime, extending a previous study of J\"ager. We show that the de Sitter charges so defined are identical to those of Ashtekar, Bonga, and Kesavan (ABK). We then present a comparison of ABK charges with other notions of de Sitter charges. We compare ABK charges with counterterm charges, showing that they differ only by a constant offset, which is determined in terms of the boundary metric alone. We also compare ABK charges with charges defined by Kelly and Marolf at spatial infinity of de Sitter spacetime. When the formalisms can be compared, we show that the two definitions agree.
10/10/2019 -- Cosimo Bambi (Fudan University)
Time: 16:00
Place: 226
Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. However, the strong field regime is still largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations only when gravity becomes strong. X-ray reflection spectroscopy is potentially a powerful tool for testing the strong gravity region around astrophysical black holes with electromagnetic radiation. In this talk, I will present the reflection model RELXILL_NK designed for testing the metric around black holes and the current constraints on possible new physics from the analysis of a few sources.
03/10/2019 -- Dalimil Mazac (Simons Center for Geometry and Physics, Stony Brook University)
Time: 14:00
Place: 226
Ultraviolet consistency of quantum gravitational theories requires the presence of new states at or below the Planck scale. In the setting of AdS3/CFT2, this statement follows from the modular bootstrap. It has been a long-standing problem to improve the best upper bound on the mass of the lightest non-graviton state in this context. I will explain how this can be done using the "analytic extremal functionals", which were originally developed for the four-point bootstrap in 1D. The new analytic upper bound on the dimension of the lightest nontrivial primary is c/8.503... at large c (central charge) -- an improvement over the previous best bound c/6 due to Hellerman. I will also explain that the sphere packing problem of Euclidean geometry can be studied using a version of the modular bootstrap. The analytic functionals apply also in this context. They lead directly to the recent solution of the sphere-packing problem in 8 and 24 dimensions due to Viazovska and Cohn+Kumar+Miller+Radchenko+Viazovska.
The talk will be based on https://arxiv.org/pdf/1905.01319.pdf
02/10/2019 -- Thales Azevedo (Institute of Physics - UFRJ)
Time: 11:00
Place: 226
Recently, a gauge theory built out of dimension-six operators such as (DF)^2 appeared in the double-copy construction of conformal supergravity amplitudes. In this talk, I will show how theories of that kind are related to conventional, sectorized and ambitwistor string theories.
01/10/2019 -- Poulami Nandi (Indian Institute of Technology Kanpur, India)
Time: 14:00
Place: 117
Conformal Carrollian groups are known to be isomorphic to Bondi-Metzner-Sachs (BMS) groups that arise as the asymptotic symmetries at the null boundary of Minkowski spacetime. The Carrollian algebra is obtained from the Poincare algebra by taking the speed of light to zero, and the conformal version similarly follows. In this paper, we construct explicit examples of Conformal Carrollian field theories as limits of relativistic conformal theories, which include Carrollian versions of scalars, fermions, electromagnetism, Yang-Mills theory and general gauge theories coupled to matter fields. Due to the isomorphism with BMS symmetries, these field theories form prototypical examples of holographic duals to gravitational theories in asymptotically flat spacetimes. The intricacies of the limiting procedure lead to a plethora of different Carrollian sectors in the gauge theories we consider. Concentrating on the equations of motion of these theories, we show that even in dimensions d = 4, there is an infinite enhancement of the underlying symmetry structure. Our analysis is general enough to suggest that this infinite enhancement is a generic feature of the ultra-relativistic limit that we consider.
30/09/2019 -- Antoine Bourget (Imperial College London, UK)
Time: 14:00
Place: 226
I will explore the geometrical structure of Higgs branches of quantum field theories with 8 supercharges in 3, 4, 5 and 6 dimensions. They are symplectic singularities, and as such admit a decomposition (or foliation) into so-called symplectic leaves, which are related to each other by transverse slices. We identify this foliation with the pattern of partial Higgs mechanism of the theory and, using brane systems and recently introduced notions of magnetic quivers and quiver subtraction, we formalise the rules to obtain the Hasse diagram which encodes the structure of the foliation.
19/09/2019 -- Gizem Sengor (Czech Academy of Sciences, CZ)
Time: 16:00
Place: 226
The symmetry group of de Sitter, can accommodate fields of various mass and spin among its unitary irreducible representations. These unitary representations are labeled by the spin and scaling dimension. The scaling dimension depends on the mass and spin of the field and can have purely imaginary values. This talk focuses on scalar fields on de Sitter and aims to show that even the purely imaginary weights correspond to unitary operators on de Sitter, which seems contrary to the case on Anti de Sitter.
By studying the late time limit of scalar field solutions with different masses (conformally coupled, heavy and light fields); we identify the unitary representations they correspond to with respect to their scaling dimension and recognize them as late time boundary operators. The definition for a positive definite inner product on de Sitter is subtle. For operators with real scaling dimension it involves a so called intertwining operator. By carefully accounting for the presence or the absence of the intertwining operator we show that all of the identified boundary operators have positive definite norm and are thus unitary representations.
12/08/2019 -- Camilo Garcia-Cely (DESY, Hamburg)
Time: 14:00
Place: 226
In this talk, I will discuss MeV spin-2 dark matter. In particular, I will show that such a particle typically self-interacts and undergoes self-annihilations via 3-to-2 processes. I will discuss its production mechanisms and also identify the regions of the parameter space where self-interactions can alleviate the discrepancies at small scales between the predictions of the collisionless dark matter paradigm and cosmological N-body simulations.
01/08/2019 -- Andrei Frolov (Simon Fraser University)
Time: 16:00
Place: 226
The simple story of primordial gravitational waves produced by inflation sourcing B-modes of Cosmic Microwave Background polarization in reality is complicated by the fact that we are looking through the coloured glass of astrophysical foregrounds originating much closer to home. I will talk about amplitude of primordial B-modes we expect from inflation, characterization of polarized dust foregrounds from Planck Legacy data, and where do we go from here. In particular, I will show new reconstruction of the large-scale galactic magnetic field responsible for the patterns we see in the dust polarization, and explain how it allows accurate modelling of the polarized dust emission for design and analysis of future CMB experiments.
22/07/2019 -- Agnes Ferte (Jet Propulsion Laboratory, Pasadena, USA)
Time: 14:00
Place: 226
The universe has been going through a phase of accelerated expansion for the last 6 billion years. Understanding the origin of this cosmic acceleration is one of the main goals of observational cosmology: is it caused by a cosmological constant or a dynamical dark energy? Or is it a sign that we don’t understand the laws of gravity on cosmological scales? In this talk I will first describe weak lensing, a powerful observable that helps addressing these fundamental questions. I will then give an overview of the current experimental context for weak lensing. In the main part of the talk, I will present my results on tests of gravity on large scales through weak lensing and end by presenting the Precision Projector Laboratory, which goal is to characterize the new generation of detectors that will be used in future galaxy surveys.
19/07/2019 -- Xingang Chen (Harvard University)
Time: 14:00
Place: 226
How to model-independently distinguish the inflation scenario from alternatives to inflation, as the origin of the Big Bang Cosmology, is an important challenge in modern cosmology. In this talk, we show that massive fields in the primordial universe function as standard clocks and imprint clock signals in the density perturbations, which directly record the scale factor of the universe as a function of time, a(t). This function is the defining property of any primordial universe scenario, so can be used to identify the inflation scenario, or one of its alternatives, in a model-independent fashion. The signals also encode the mass and spin spectra of the particle physics at the energy scale of the primordial universe.
16/07/2019 -- Leonardo Modesto (SUSTech, Shenzhen, China )
Time: 14:00
Place: 117
In order to have a unitary and finite quantum gravity, we propose a weakly nonlocal completion of the Einstein-Hilbert's action compatible with causality (a Shapiro's time advance never occurs in Nonlocal Gravity). As a consequence of finiteness, there is no Weyl anomaly and the theory turns out to be conformal invariant at classical as well at quantum level. Therefore, finite nonlocal quantum gravity is a conformal invariant theory in the spontaneously broken phase of the Weyl symmetry. The coupling to matter enjoy the same properties with and without supersymmetry. As an application, Weyl conformal symmetry solves the black hole's singularity issue and the cosmological singularity problem, otherwise unavoidable in a generally covariant local or non-local gravitational theory. Following and extending the seminal paper by Narlikar and Kembhavi, we are able to provide explicit examples of singularity free black hole exact solutions. The absence of divergences is based on the finiteness of the curvature invariants and on the geodesic completion. Indeed, no massive or massless particles can reach the former singularity in a finite amount of proper time or of affine parameter.
15/07/2019 -- Mirek Rapcak (Perimeter Institute, Canada)
Time: 11:00
Place: 226
I will discuss generalizations of the $\mathcal{W}_{1+\infty}$ algebra denoted as $\mathcal{W}_{m|n\times \infty}$ generated by a super-matrix of fields for each integral spin $i=1,2,3,\dots$. Truncations of the algebra are in correspondence with holomorphic functions on singular Calabi-Yau three-folds given by the zero locus of $xy=z^mw^n$. I propose a free-field realization of such truncations generalizing the Miura transformation for $\mathcal{W}_N$ algebras. Relations in the ring of holomorphic functions lead to bosonisation-like relations between different free-field realizations. The algebras are expected to be AGT dual to gauge theories supported on divisors corresponding to the zero locii of such holomorphic functions. The discussion uncovers many non-trivial relations between vertex operator algebras, algebraic geometry and gauge theory.
11/07/2019 -- David Svoboda (Perimeter Institute, Canada)
Time: 14:00
Place: 226
We present a para-complex analogy of the Generalized Kähler (GK) geometry, generalized para-Kähler (GpK) geometry. We show that similarly to GK geometry describing targets of 2D (2,2) supersymmetric sigma models, the GpK geometry describes the targets of (2,2) twisted supersymmetric sigma models. We then discuss topological twists of such sigma models. Because the involved geometries are para-complex, they provide new examples -- in particular of topological theories -- on manifolds that are not complex, contrary to the usual (2,2) case.
08/07/2019 -- Adolfo Cisterna (Chile University)
Time: 14:00
Place: 226
In this talk a new method for the construction of homogenous black strings is shown. The method, which is based on a particular scalar-dressing of the extra dimensions of the spacetime under consideration, allow us to construct the black string generalization of the AdS Schwarzschild black hole in any dimension in General Relativity. Furthermore the method can be generalized to provide the black string extension of the Boulware-Deser black hole, or the black string extension of any black hole contained in the Lovelock theory. It will be also discussed how to construct black string with non-trivial matter fields.
04/07/2019 -- Cesar Arias (Riemann Center for Geometry and Physics (Leibniz University of Hannover))
Time: 14:00
Place: 226
It has been proposed that Vasiliev’s nonlinear equations can be extracted from a cubic action principle of the Chern–Simons type, built up from a set of differential forms, a trace operation and a star product inherited from the associative algebra, and a nilpotent differential containing the (gauged) de Rham differential. In this talk, we argue that all of these algebraic structures can be naturally modeled by a class of two-dimensional topological models, referred to as differential Poisson sigma models, which we analyse in some detail.
24/06/2019 -- Andrea Fontanella (Instituto de Fisica Teorica UAM/CSIC, Madrid)
Time: 14:00
Place: 226
I will present how we found the hidden relativistic symmetry in the context of AdS2 and AdS3 integrable superstring theories (arXiv:1903.10759). Then I shall discuss how such symmetry can be used in AdS3 to write down the Thermodynamic Bethe Ansatz for massless non-relativistic modes from the one available in literature for massless relativistic modes.
10/06/2019 -- Dmitry Gorbunov (Institute for Nuclear Research, Moscow)
Time: 14:00
Place: 226
Inflation can explain why the Universe is flat and homogeneous at large scales. However, it is not falsifiable unless also responsible for the matter perturbations sourcing the cosmic structure formation and anisotropy of cosmic microwave background. Moreover, even in that case different models often give (almost) the same predictions for the cosmological spectra, and it would be nice to test these inflationary models in other ways. The Higgs inflation is one of the examples naturally providing with such independent tests. A recently suggested modification with $R^2$-term solves the strong coupling problem in the original Higgs inflation allowing for perturbative matching of high-energy and low-energy model coupling constants, which is required to perform such direct tests. A remarkable feature of the model is instant preheating due to tachyonic instabilities in Higgs and vector boson sectors, which ask for a special study.
07/06/2019 -- Mairi Sakellariadou (King's College London)
Time: 14:00
Place: 226
The direct detections of Gravitational Waves (GWs) by the Advanced LIGO and Advanced Virgo interferometers have opened a new era of astronomy. Aside the current detections associated with individual loud events, one expects a superposition of coincident unresolved events leading to a stochastic GW background (SGWB). After reviewing briefly the SGWB, I will discuss how the anisotropic distribution of sources and the inhomogeneous geometry of the intervening spacetime can induce anisotropies. I will consider a SGWB produced by (1) cosmic strings and (2) by compact binary coalescences. I will show that while the SGWB monopole is sensitive to the particular model one uses, the anisotropic angular power spectrum is basically insensitive to the cosmic string model or the nature of binary black holes population. I will then discuss the noise in the anisotropies of the astrophysical GW background sourced by the finite sampling of both the galaxy distribution and the compact binary coalescence event rate.
04/06/2019 -- Tanmay Vachaspati (Arizona State University)
Time: 13:30
Place: FZU Lecture Hall
We show that certain quantum systems in non-trivial classical backgrounds can be mapped into entirely classical systems in higher dimensions. The evolution of the classical system can be used to obtain the particle production rate as well as the quantum backreaction on the classical background. The technique has many potential applications, including breather/oscillon dynamics, Hawking radiation and black hole evaporation, and particle production during inflation.
21/05/2019 -- Julian Adamek (Queen Mary University of London)
Time: 16:00
Place: 117
I will give a brief overview of the latest code release v1.2 of gevolution, with particular attention to the new features that facilitate the analysis on observers' past light cones. This provides a framework to include all interesting relativistic contributions in the prediction of large-scale structure observables.
13/05/2019 -- Tomislav Prokopec (Utrecht University)
Time: 14:00
Place: 226
In this talk I will give an introduction on how to compute quantum corrections in inflation. I will review quantum effects in interacting scalar theories, scalar quantum electrodynamics and summarize on the quantum one loop corrections to dynmical gravitons and scalar gravitational potentials. Understanding these type of corrections is of crucial importance for our understanding of how large can be the quantum corrections to cosmological perturbations during inflation.
10/04/2019 -- Ogan Ozsoy (Swansea University)
Time: 14:00
Place: 117
Observations of Cosmic Microwave Background ( CMB ) radiation appear to be consistent with the simplest realizations of the inflationary paradigm: single field slow-roll inflation. However, in practice, CMB probes can provide us information about the inflationary dynamics only for a limited range of scales that correspond to a small portion of the dynamics compared to required time span of inflation in solving the standard problems of Hot Big Bang cosmology. This leaves us with a large portion of the dynamics together with a vast range of scales that are pretty much uncharted and yet to be explored. In this talk, I will focus on two possible observational windows together with a simple primordial mechanism that can provide us the opportunity to probe the inflationary dynamics on small scales compared to the CMB. In this context, I will show two exemplary scenarios that has potential to accomplish this goal through enhanced scalar and tensor fluctuations during inflation.
25/03/2019 -- James Bonifacio (Case Western Reserve University)
Time: 14:00
Place: 226
A free massless scalar in flat space has an infinite number of shift symmetries. In (A)dS, each of these symmetries is preserved only for particles with particular discrete masses. I will show how these shift symmetries generalize to massive higher-spin particles and explain how these are related to partially massless symmetries. For the case of scalar fields, I discuss deformations of the underlying symmetry algebras and whether there exist invariant interactions. This leads to a ghost-free theory in (A)dS that is invariant under a deformed quadratic shift symmetry and which reduces in flat space to the special Galileon. This theory has a rich structure of interactions that are completely fixed by the nonlinear symmetry, including a nontrivial potential. Lastly, I will speculate on possible generalizations to interacting massive higher-spin particles.
18/03/2019 -- Masahide Yamaguchi (Tokyo Institute of Technology)
Time: 14:00
Place: 226
We propose a new class of higher derivative scalar-tensor theories without the Ostrogradsky's ghost instabilities. The construction of our theory is originally motivated by a scalar field with spacelike gradient, which enables us to fix a gauge in which the scalar field appears to be non-dynamical. We dub such a gauge as the spatial gauge. Though the scalar field loses its dynamics, the spatial gauge fixing breaks the time diffeomorphism invariance and thus excites a scalar mode in the gravity sector. We generalize this idea and construct a general class of scalar-tensor theories through a non-dynamical scalar field, which preserves only spatial covariance. We perform a Hamiltonian analysis and confirm that there are at most three (two tensors and one scalar) dynamical degrees of freedom, which ensures the absence of a degree of freedom due to higher derivatives. Our construction opens a new branch of scalar-tensor theories with higher derivatives.
14/03/2019 -- Stefano Camera (University of Turin)
Time: 16:00
Place: 226
'Synergy' means 'the interaction of two or more agents to obtain a combined effect greater than the sum of their separate effects'. With this in mind, in this talk I shall present my current lines of research, all focussed on developing novel combinations of astrophysical and cosmological observables to the aim of testing the foundations of the concordance cosmological model. Specifically, I shall discuss how innovative cross-correlations can mitigate the impact of systematic effects, noise and cosmic variance, to the end of studying dark energy and modified gravity models, detecting particle dark matter signatures, and testing gravity and inflation on the largest cosmic scales. All, with a view on the current and oncoming generation of cosmological experiments and large-scale surveys.
28/02/2019 -- Gizem Sengor (FZU)
Time: 16:00
Place: 226
Cosmological backgrounds in general posses time dependence. On these backgrounds scalar degrees of freedom that transform nonlinearly under time diffeomorphisms arise to guarantee the time diffeomorphism invariance of the action. In the early universe these time dependent backgrounds can be attributed to the presence of time dependent scalar fields that dominate the energy momentum density of the universe. Then the species of the scalar degree of freedom that transforms nonlinearly under time diffeomorphisms correspond to perturbations of the scalar field that gives rise to the time dependence of the cosmological background at a given era. Effective field theories (EFT) of cosmological perturbations generalize the interactions between cosmological perturbations of different species based on their transformation properties under diffeomorphisms. Preheating refers to the stage at the end of inflation where the inflaton field continues to dominate the energy momentum density but transfers its energy to other fields through resonance, as opposed to perturbative decays. The aim of this talk is to consider general interactions between the perturbations of the inflaton and a second scalar field during Preheating, to understand the scales these interactions introduce and explore which species propagate as effective degrees of freedom at different scales.
27/02/2019 -- Antonio Racioppi (NICPB, Tallinn, Estonia)
Time: 14:00
Place: 117
We study models of chaotic inflation where the inflaton field $\phi$ is coupled non-minimally to gravity via $\xi \phi^n R$, a.k.a. $\xi$-attractors. We focus on the Palatini theory of gravity and we show that in this case Starobinsky inflation is not any more a universal attractor. On the other hand we prove that, once quantum corrections are taken into account, the strong coupling limit of (a certain class of) $\xi$-attractor models will move into linear inflation regardless of the adopted gravity formulation metric or Palatini).
25/02/2019 -- Jakub Vicha (NICPB, Tallinn, Estonia)
Time: 14:00
Place: 226
The Pierre Auger Observatory is the currently largest cosmic-ray detector covering ultra-high energies from 10^18 eV to 10^20 eV. The size of exposure accumulated since 2004 granted measurements of unprecedented precisions on energy spectrum, mass composition and anisotropy searches. These measurements guide us slowly to the sources of ultra-high energy cosmic rays, which is a tantalizing mystery of physics. A brief introduction to the field of ultra-high energy cosmic rays will be given together with a description of the Pierre Auger Observatory and its detection techniques. Then, an overview of the most interesting results will follow.
18/02/2019 -- Vojtech Witzany (Astronomical Institute of the Czech Academy of Sciences)
Time: 14:00
Place: 226
A compact stellar mass object inspiralling onto a massive black hole deviates from geodesic motion due to radiation-reaction forces as well as finite-size effects. Such deviations need to be included with sufficient precision into wave-form models for the upcoming space-based gravitational-wave detector LISA. I will present the formulation and solution of the Hamilton-Jacobi equation of a generic geodesic in Kerr space-time perturbed by the spin-curvature coupling, the leading order finite-size effect. In return, this solution allows to compute a number of observables such as the turning points of the orbits as well as the fundamental frequencies of motion. These results essentially solve the question of conservative finite-size effects in extreme mass ratio inspirals.
10/12/2018 -- Shinji Mukohyama (Kyoto University and Tokyo University)
Time: 14:00
Place: 226
It is generally believed that modification of general relativity inevitably introduce extra physical degree(s) of freedom. In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology.
26/11/2018 -- Ondrej Pejcha (Charles University, Prague)
Time: 14:00
Place: 226
Interest in the transient astronomical sky has increased tremendously thanks to modern time-domain surveys, which have discovered unexpected diversity in previously known phenomena and identified many new classes of transients. I will focus on two types of transients that are important for the nucleosynthesis in the Universe and the evolution of gravitational wave sources. I will argue that the deaths of massive stars marked by core-collapse supernovae are highly sensitive to initial conditions, which leads to a complex pattern of neutron star and black hole formation. Many stars are members of binary systems and their evolution can be significantly affected by a catastrophic interaction, which results in the rapid loss of mass, energy and angular momentum, and sometimes even merger of the binary star. This phase was recently connected to a newly identified group of red transients. I will present surprising findings from the theoretical interpretation of observations of these red transients.
23/11/2018 -- Rachel Houtz (IFT Madrid)
Time: 14:00
Place: 117
In this talk I present a model with an enlarged color sector which solves the strong CP problem via new massless fermions. QCD color is unified with another non-Abelian group with a large confinement scale. The spontaneous breaking of the unified color group provides a source of naturally large axion mass due to small size instantons, and as a result no very light axions are present in the low-energy spectrum. The axion scale may be around a few TeV which translates to observable signals at colliders. This model naturally enlarges the parameter space for axions which solve the strong CP problem well beyond that of invisible axion models.
22/11/2018 -- Antonino Marciano (Fudan University, Shanghai)
Time: 16:00
Place: 226
We discuss how inflation and bounce cosmology can emerge from a four-fermion interaction induced by torsion. Inflation can arise from coupling torsion to Standard Model fermions, without any need of introducing new scalar particles beyond the Standard Model. Within this picture, the inflaton field can be a composite field of the SM-particles and arises from a Nambu-Jona-Lasinio mechanism in curved space-time, non-minimally coupled with the Ricci scalar. The model we specify predicts small value of the r-parameter, namely r ~ 10-3 - 10-2, which nonetheless would be detectable by the next generation of experiments, including BICEP 3 and the ALI-CMB projects. On the other hand, bouncing cosmology can be also accounted for in terms of fermion condensates, with the remarking appearance of an ekpyrotic phenomenon, which is solely due to the quantum corrections to the fermion potential. We finally argue about the richness of the phenomenological perspective encoded in both the schemes.
20/11/2018 -- Andrea Addazi (Fudan University, Shanghai)
Time: 14:00
Place: 117 or 226
We will discuss dark matter models which are related to dark first order phase transitions (D.F.O.P.T.) in the early Universe. D.F.O.P.T. sources an efficient materialization of dark bubbles. Dark bubbles can scatter each others producing a gravitational radiation background, detectable in next generation of experiments. We will discuss the specific framework of Majoron dark matter, as a neutrino mass and a Warm Dark Matter genesis model. Majorons may also be detected in next laboratory experiments: i) electron-positron high luminosity colliders; ii) neutrino-less-double-beta decays processes; iii) experiments searching for baryon violating Neutron-Antineutron transitions.
05/11/2018 -- Subodh Patil (Niels Bohr Institute, Copenhagen)
Time: 14:00
Place: 226
In this talk, we present an amusing observation that primordial gravitational waves, if ever observed, can be used to bound the hidden field content of the universe. This is because a large number of hidden fields can resum to potentially observable logarithmic runnings for the graviton two-point function in the context of single field inflation, courtesy of a `large N' expansion. This allows one to translate ever more precise bounds on the tensor to scalar consistency relation into bounds on the hidden field content of the universe, with potential implications for phenomenological constructions that address naturalness with a large number of species. Along the way, we'll review how the cutoff for an EFT that includes gravity changes as we incorporate matter, identifying two distinct scales for gravity. We'll also need to address certain subtleties regarding loop corrections on cosmological backgrounds, especially with regards to the correct implementation of dimensional regularization.
25/10/2018 -- Alexey Golovnev (St.-Petersburg State University)
Time: 16:00
Place: 117 or 226
I will review the basic construction of teleparallel gravity and its modifications, with a special emphasis on local Lorentz transformations in the tetrad space. One of these modified models, the f(T), is widely used for cosmological model building. I will explain how (linear) cosmological perturbations should be treated in f(T) and in similar models. Finally I will discuss the problem of dynamical structure and Hamiltonian analysis for modified teleparallel gravity.
16/10/2018 -- Lasha Berezhiani (Max Planck Institute of Physics, Munich)
Time: 14:00
Place: 117
After a brief review of some of the empirical correlations between dark and baryonic sectors within galaxies, I will discuss a novel theory of dark matter superfluidity as a potential explanation of this observations. I will argue that, depending on the mass and self-interaction cross section of dark matter particles, the superfluid may in principle be formed in the central regions of galactic halos. After this, I will discuss the criteria that need to be met by superfluid properties in order to account for the above-mentioned empirical correlations.
8/10/2018 -- Katherine Freese (Nordita, Sweden)
Time: 14:00
Place: 226
Inflation, a period of accelerated expansion at the beginning of the Universe, seeks to explain the (otherwise mysterious) large scale smoothness, isotropy, and “oldness” of the Universe. An important product of this inflationary epoch is the origin of density perturbations that are the seeds of galaxies and other large structures today. The density perturbations and gravitational waves produced by inflation provide sensitive tests of both the inflationary paradigm and of individual inflationary models. In the past decade predictions of inflation have been tested by Cosmic Microwave Background data, most recently with the Planck satellite observations. The basic idea of inflation matches the data and sensitive tests have been made of individual models. Planck data have ruled out most inflation models. I will discuss the status of Natural Inflation, a model that my collaborators and I originally proposed in 1990, as well as modern variants. Natural inflation uses “axions” as the inflaton, where the term “axion” is used generically for a field with a flat potential as a result of a shift symmetry. The successes of inflation as well as the potential discoveries in upcoming data will be emphasized.
17/09/2018 -- Elena De Paoli (Marseille, CPT)
Time: 14:00
Place: 117
We identify a symplectic potential for general relativity in tetrad and connection variables that is fully gauge-invariant, using the freedom to add surface terms. When torsion vanishes, it does not lead to surface charges associated with the internal Lorentz transformations, and reduces exactly to the symplectic potential given by the Einstein-Hilbert action. In particular, it reproduces the Komar form when the variation is a Lie derivative, and the geometric expression in terms of extrinsic curvature and 2d corner data for a general variation. As a direct application of this analysis we prove that the first law of black hole mechanics follows from the Noether identity associated with the covariant Lie derivative, and that it is independent of the ambiguities in the symplectic potential provided one takes into account the presence of non-trivial Lorentz charges that these ambiguities can introduce.
30/07/2018 Shun-Pei Miao (National Cheng Kung University, Taiwan)
Time: 1:30pm
Place: 226
We consider an additional fine-tuning problem which afflicts scalar-driven models of inflation. The problem is that successful reheating requires the inflaton be coupled to ordinary matter, and quantum fluctuations of this matter induces Coleman-Weinberg potentials which are not Planck-suppressed. Unlike the flat space case, these potentials depend upon a still-unknown nonlocal functional of the metric which agrees with the Hubble parameter for de Sitter. Such a potential cannot be completely subtracted off by any local action. We numerically consider the effect of subtracting it off at the beginning of inflation in a simple model. For fermions the effect is to prevent inflation from ending unless the Yukawa coupling to the inflaton is so small as to endanger reheating. For gauge bosons the effect is to make inflation end almost instantly, again unless the gauge charge is unacceptably small.
27/07/2018 Richard Woodard (University of Florida, USA)
Time: 2pm
Place: 226
MOND is a phenomenological model which modifies the extreme weak field regime of Newtonian gravity so as to explain galactic rotation curves without dark matter. If correct, it must be the non-relativistic, static limit of some relativistic modified gravity theory. I show how the only possible metric-based modification of gravity is nonlocal, and I construct the action using the Tully-Fisher relation and weak lensing. Then I explore the consequences of this model for cosmology. This talk is based on four arXiv papers: 1106.4984, 1405.0393,1608.07858 and 1804.01669.
26/07/2018 Dam Thanh Son (Kadanoff Center for Theoretical Physics, University of Chicago, USA)
Time: 2pm
Place: FZU lecture hall
25/07/2018 Oleg Teryaev (Joint Institute for Nuclear Research, Dubna, Russia)
Time: 2pm
Place: FZU lecture hall
The energy-momentum tensor matrix elements describe the particle coupling to gravitational field. They are responsible for gravity action on particle spin which may result, in particular, in neutrino spin-flip in anisotropic Universe. One of the proton's formfactors, related to the pressure of quarks, was recently experimentally extracted from the data obtained in Jefferson Lab. (Nature, V. 557, p. 396, May 17, 2018). The pressure is extremely large with the distribution analogous to that in macroscopic stable object, like star.
24/07/2018 Renato Costa (University of Cape Town, South Africa)
Time: 2pm
Place: 117
The singularity problem is one of the hints that the \LambdaCDM models has to be extended at very high energies. We use the guiding principle of symmetries to extend the FLRW background to an explicitly T-dual one which is well described by double field theory (DFT). We show that, at the level of the background, one can have a singularity-free cosmology once the dual time coordinate introduced by DFT is inversely related to the standard time coordinate of general relativity. We also show that introducing matter in DFT cosmology naturally leads to the correct equation of state for the winding modes and to a more clear interpretation of the connection between the two time coordinates.
18/07/2018 Massimiliano Rinaldi (Trento University, Italy)
Time: 2pm
Place: 117
In this talk I will present a scalar-tensor model of modified gravity that is globally scale-invariant. Such a symmetry spontaneously breaks to give rise to a mass scale, and an inflationary scenario naturally emerges. The same model will be presented both in the Jordan and in the Einstein frame and the compatibility with current observations will be discussed.
17/07/2018 Alessandro Drago (Ferrara University, Italy)
Time: 2pm
Place: room 117
I will discuss what we have learnt from the first merger of two neutron stars observed in gravitational waves and in E.M. waves. My discussion will include information coming from new theoretical analyses and also from x-ray data collected by satellites.
12/07/2018 Jarah Evslin (Institute of Modern Physics, Lanzhou, China)
Time: 14:00
Place: room 226
There are at least two 3 sigma anomalies in the cosmic expansion rate. One is the discrepancy between the local Universe measurement of the Hubble constant by Riess et al and also using strong lensing time delays vs the best fit Planck result assuming LCDM. The other is the Lyman alpha forest Baryon Acoustic Oscillation (BAO) measurement, which disagrees with LCDM when combined with other BAO measurements or Planck. We note that unanchored BAO provides a robust geometric probe, free of all but the most basic cosmological assumptions. Using it, we find that if these anomalies are confirmed, the first necessarily implies a change in pre-recombination cosmology while the second implies dynamical dark energy between the redshifts z=2 and z=0.6.
03/07/2018 Yi-Zen Chu (National Central University, Taiwan)
Time: 14:00
Place: Room 226
Despite being associated with massless particles, electromagnetic and gravitational waves do not propagate strictly on the null cone in curved spacetimes. They also develop tails, traveling inside the light cone. This tail effect, in particular, provides a contribution to the self-force of compact bodies orbiting super-massive black holes, which in turn are believed to be important sources of gravitational waves for future space based detectors like LISA, TianQin and Taiji. For the first portion of my talk I will describe my efforts to explore novel methods to understand the tail effect in curved geometries -- primarily in cosmological spacetimes. Some of the spin-offs include the (small) discovery of new type of gravitational wave memory effect induced by tails. If time permits, for the second part of my talk, I will address a seemingly basic aspect of gravitational wave theory that -- as far as I am aware -- has not received proper clarification in the literature to date. Specifically, the "transverse-traceless" gravitational wave (GW) is usually touted as the gauge-invariant observable; while practical computations actually do not strictly yield this "TT" GW. Furthermore, the gauge-invariant TT GW is actually acausally related to its matter source, as can be seen by simply computing its associated Green's function. I will clarify the situation for the spin-1 photon, as an analogy to the gravitational case.
28/06/2018 Andreas Albrecht (University of California at Davis, USA)
Time: 14:00
Place: FZU Lecture Hall
I review the current status of cosmic inflation, including successes and open questions. I also scrutinize the question of the famous cosmological "tuning puzzles" and analyze the extent to which inflation does and does not resolve these. I explain why I think the open questions about inflation are deeply scientifically exciting. They should not be regarded as "failures" of inflation, nor should they be swept under the rug.
27/06/2018 Andreas Albrecht (University of California at Davis, USA)
Time: 14:00
Place: room 117
Decoherence and "einselection" have important roles in quantum physics, and are understood to be important in the emergence of classical behavior. Traditional discussions of einselection all assume an arrow of time. The extent to which einselection (and thus the emergence of classicality) is tied to an arrow of time has possibly deep implications for cosmology. In this talk I present some early results on this topic based on calculations in a toy model related to the classic Caldeira Leggett model, which I solve unitarily in all regimes. This talk will include introductory material, and will not assume prior familiarity with decoherence, einselection or cosmology.
26/06/2018 Eugeny Babichev (LPT, Orsay, France)
Time: 14:00
Place: room 117
A Hamiltonian density bounded from below implies that the lowest-energy state is stable. I will discuss that, contrary to common lore, an unbounded Hamiltonian density does not necessarily imply an instability: this is a coordinate-dependent statement. I will give the correct stability criterion, using the relative orientation of the causal cones for all propagating degrees of freedom. I will then apply this criterion to an exact Schwarzschild-de Sitter solution of a beyond-Horndeski theory, while taking into account the recent experimental constraint regarding the speed of gravitational waves coming from GW170817.
25/06/2018 Wojciech Hellwing (Warsaw, Poland)
Time: 14:00
Place: room 226
While the Earth-base laboratories keep trying very hard to elucidate on the nature of the elusive dark matter particles the other very promising avenue to test and/or falsify potential dark matter candidates resides in astrophysical observations. In this context our own Galaxy - the Milky Way - with its unique set of satellites shows potential to serve as a extraterrestrial laboratory for dark matter. The very physical nature of dark matter particles and especially the differences between the main candidate, the neutralino of Cold Dark Matter (CDM), and its currently strongest competitor, the sterile neutrino of Warm Dark Matter candidate, may lead to significant differences in the properties of dwarf galaxies. Such objects are dominated (by mass) by their host DM haloes and therefore provide an unique view on the physical properties of DM. I shall discuss our recent efforts to use the state-of-the-art galaxy formation hydrodynamical simulation scheme of the EAGLE project as well as high-resolution Copernicus Complexio N-body simulations to study the galaxy formation of Milky Way like systems in CDM and WDM scenarios. Our results render new insights on potential ways to use astronomical observations for falsifying the CDM paradigm and testing its competitors.
15/06/2018 -- Emre Kahya (Istanbul)
Time: 11:00
Place: 226
I will discuss quantum gravitational loop effects to observable quantities such as curvature power spectrum and primordial non-gaussianity of Cosmic Microwave Background (CMB) radiation. We first review the previously shown case where one gets a time dependence for zeta-zeta correlator due to loop corrections. Then we investigate the effect of these loop corrections to primordial non-gaussianity of CMB.
14/06/2018 -- Emre Kahya (Istanbul)
Time: 16:00
Place: 226
The gravitational wave (GW) signal (GW170817) from the coalescence of binary neutron stars was simultaneously seen throughout the electromagnetic (EM) spectrum from radio waves to gamma rays. We point out that this simultaneous detection rules out a class of modified gravity theories, and provides another indirect evidence for the existence of dark matter.
06/06/2018 -- Sébastien Clesse (University of Namur)
Time: 14:00
Place: 226
I will present the current status of primordial black holes as a Dark Matter candidate, a scenario that has recently seen a strong revival of interest. Formation models, astrophysical and cosmological constraints, as well as observations pointing towards the possible existence of primordial black holes, with abundances comparable to the one of dark matter, will be reviewed and discussed, including the gravitational waves from massive black hole mergers detected by LIGO/VIRGO. Finding evidence of even a single primordial black hole could have groundbreaking consequences for our understanding of the early Universe and of High Energy physics.
21/05/2018 -- Maksym Ovchynnikov (Leiden University)
Time: 14:00
Place: 226
It is well-known that the Standard Model of particle physics does not explain dark matter, neutrino masses and matter-antimatter asymmetry of the Universe and therefore has to be extended. This means that there should exist some new particles that are either too heavy to be found before (the “energy frontier”) or interact too feebly (the “intensity frontier”). In the absence of a good guiding principle predicting where we should look for new physics, we consider the so-called “portals” — renormalizable interactions between new particles and the Standard Model. We review these portals and their phenomenology at the “intensity frontier” (in particular at SHiP). We pay special attention to the searches of dark matter particle through these portals and discuss the cosmological status of “light dark matter”.
17/05/2018 -- Lorenzo Pizzuti (Trieste)
Time: 16:00
Place: 226
I will provide a brief overview on my work concerning constraints on modified gravity models obtained using galaxy cluster mass profile determinations. In particular, I will present the results of a paper in which we combined the information given by the kinematic of galaxies in cluster with the information provided by lensing analyses for 2 galaxy clusters of the CLASH-CLASH\VLT collaboration to get constraints on f(R) models. In order to discuss the applicability of the proposed method in view of future imaging and spectroscopic surveys, I will further introduce my current study of cosmological simulations, aiming at estimating and calibrating the impact of systematics.
15/05/2018 -- Santiago Casas (CEA Paris-Saclay)
Time: 15:00
Place: 117
The large freedom in the free functions affecting linear perturbations in theories of modified gravity and dark energy leads to the burden of parametrization, which means that the observational constraints depend strongly on the way these free functions are parametrized. Using a model-independent test of gravity, alleviates this problem and it even frees us from assumptions about initial conditions, galaxy bias or the nature of dark matter. In this talk I will present the first model-independent reconstruction of the gravitational slip as a function of redshift, using present data on large scale structure and the Hubble function. For future data I will show how we can use these tests to rule out entire classes of modified gravity models and how we have to handle, in a Bayesian way, the constraints from models which are very close to LCDM and might not even be clearly distinguishable, even with next generation surveys.
07/05/2018 -- Diego Blas (King's College London)
Time: 14:00
Place: 226
The high quality of the data from pulsar timing makes of it a fantastic resource to understand gravitational phenomena. Traditionally this has been used to test general relativity. In this talk I will describe a less explored possibility: using pulsar timing to understand dark matter properties. I will focus on (possibly) detectable modifications of binary orbits due to the interaction with dark matter in different scenarios.
04/05/2018 -- Peter Tinyakov (University Of Brussels)
Time: 14:00
Place: 226
Compact stars - neutron stars and white dwarfs - can capture and accumulate dark matter. Even though only a tiny fraction of the star mass can be accumulated in realistic conditions, this may lead to dramatic consequences such as the star collapse into a black hole. Thus, mere existence of neutron stars and white dwarfs sets constraints on DM models where this phenomenon occurs. Alternatively, if only a fraction of NS is converted into black holes, these may be identified with the gravitational wave detectors: the masses of such BH are around one solar mass, while stellar evolution does not lead to BH lighter than ~2 solar masses. We will discuss in detail two examples: the DM composed of primordial black holes, and asymmetric DM with self-interactions.
03/05/2018 -- Jan Novák (Technical University of Liberec)
Time: 14:00
Place: 226
We investigate the Universe at the late stage of its evolution and inside the cell of uniformity 150 - 370 MPc. We consider the Universe to be filled at these scales with dust like matter, a minimally coupled Galileon field and radiation as matter sources. We will use the mechanical approach and therefore the peculiar velocities of the inhomogeneities as well as fluctuations of other perfect fluids are nonrelativistic. Such fluids are said to be coupled, because they are concentrated around inhomogeneities. We investigate the conditions under which the Galileon field can become coupled. We know from previous work that at background level coupled scalar field behave as a two-component perfect fluid: a network of frustrated cosmic string and cosmological constant. We found a correction for the Galileon field, which behaves like matter. We investigate a similar task for K-essence models and we try to find the conditions under which the K-essence scalar field with the most general form for its action can become coupled. We investigate at the background level three particular examples of the K-essence models: (1) the pure kinetic K-essence field, (2) a K-essence with a constant speed of sound and (3) the K-essence model with the Lagrangian bX+cX2−V (φ). We demonstrate that if the K-essence is coupled, all these K-essence models take the form of multicomponent perfect fluids where one of the component is the cosmological constant. Therefore, they can provide the late-time cosmic acceleration and be simultaneously compatible with the mechanical approach.
10/04/2018 -- Luca Marzola (National Institute of Chemical Physics and Biophysics, Tallinn)
Time: 14:00
Place: 117
In this talk I review the origin of the 21-cm line and explain why the particle physics community is making such a big deal out of it. I will show two possible ways to use the new results of the EDGES experiment and try to convince you that, maybe, you should have a look into the matter too.
12/04/2018 -- Tomi Koivisto (NORDITA, Stockholm)
Time: 14:00
Place: 226
Teleparallel gravity is formulated in terms of a flat spacetime affine connection. In the symmetric teleparallelism, the affine connection is further torsion-free. These simplifications may improve the theory of gravity both technically (only first derivatives and no boundary term in the action) and conceptually (resolution of the gravitational energy, separation of the inertial effects). In the talk we will review these formulations and discuss some recent developments in the symmetric teleparallel geometry that were reported in the pre-print arXiv:1803.10185.
27/03/2018 -- Julian Adamek (Queen Mary University London)
Time: 14:00
Place: 117
I present a general (relativistic) framework for numerical simulations of cosmic large-scale structure in the context of generic metric theories of gravity. The full spacetime metric is evolved within a weak-field description, while cold dark matter is represented as an N-body ensemble that follows timelike geodesics. The framework allows one to study phenomena that lead to generic modifications of the metric perturbations, either by introducing new relativistic sources or by modifying the theory of gravity.
20/03/2018 -- Pat Stengel (University of Stockholm)
Time: 14:00
Place: 117
The Standard Model Higgs boson, which has previously been shown to develop an effective vacuum expectation value during inflation, can give rise to large particle masses during inflation and reheating, leading to temporary blocking of the reheating process and a lower reheat temperature after inflation. We study the effects on the multiple stages of reheating: resonant particle production (preheating) as well as perturbative decays from coherent oscillations of the inflaton field. Specifically, we study both the cases of the inflaton coupling to Standard Model fermions through Yukawa interactions as well as to Abelian gauge fields through a Chern-Simons term. We find that, in the case of perturbative inflaton decay to SM fermions, reheating can be delayed due to Higgs blocking and the reheat temperature can decrease by up to an order of magnitude. In the case of gauge-reheating, Higgs-generated masses of the gauge fields can suppress preheating even for large inflaton-gauge couplings. In extreme cases, preheating can be shut down completely and must be substituted by perturbative decay as the dominant reheating channel. Finally, we discuss the distribution of reheating temperatures in different Hubble patches, arising from the stochastic nature of the Higgs VEV during inflation and its implications for the generation of both adiabatic and isocurvature fluctuations.
15/03/2018 -- Ilidio Lopes (University of Lisbon)
Time: 14:00
Place: 226
For the past decade asteroseismology has opened a new window into studying the physics inside stars. Today, it is well known that more than ten thousand stars have been found to exhibit solar-like oscillations. This large amount of high-quality data for stars of different masses and sizes is having a profound impact in our understanding of the structure of stars in the main and the post-main sequence, on the formation and evolution of stellar clusters in our Galaxy. Moreover, it can be used to test new fundamental laws of nature including the existence of dark matter. While many particle candidates have been proposed as the main constituents of dark matter, the impact of such candidates in the evolution of stars has been sparsely addressed. In this talk, I will focus on the impact that dark matter has in the evolution of stars, and how stellar oscillations have been used to constrain the properties of dark matter. I will discuss the potential of the next generation of asteroseismic missions helping us to address this problem.
20/02/2018 -- Eric A. Bergshoeff (University of Groningen)
Time: 14:00
Place: 226
A Schroedinger equation proposed for the GMP gapped spin-2 mode of fractional Quantum Hall states is found from a novel non-relativistic limit, applicable only in 2+1 dimensions, of the massive spin-2 Fierz-Pauli field equations. It is also found from a novel null reduction of the linearized Einstein field equations in 3+1 dimensions, and in this context a uniform distribution of spin-2 particles implies, via a Brinkmann-wave solution of the non-linear Einstein equations, a confining harmonic oscillator potential for the individual particles.
11/12/2017 -- Martin Roček (SUNY, Stony Brook)
Time: 16:00
Place: auditorium
WZW models and generalized geometry
I'll review (2,2) superspace and explore how to describe the generalized kahler structure of (2,2) supersymmetric WZW models, presenting surprising new results for SU(3).
8/11/2017 -- Pierre Fleury (University of Geneva)
Time: 14:00
Place: 117
Weak lensing with finite beams
The standard theory of weak gravitational lensing relies on the infinitesimal light beam approximation. In this context, images are distorted by convergence and shear, the respective sources of which unphysically depend on the resolution of the distribution of matter—the so-called Ricci-Weyl problem. In this talk, I will discuss a strong-lensing-inspired formalism designed to deal with finite light beams. I will show that it solves the Ricci-Weyl problem. Furthermore, finite-size effects systematically enhance the beam’s distortions, which could affect the interpretation of cosmic shear data.
6/11/2017 -- Andrei Gruzinov (New York University)
Time: 14:00
Place: 226
Particle production by real (astrophysical) black holes
The rate of production of light bosons (if they exist) by astrophysical black holes is calculated. Observability of this effect is discussed.
23/10/2017 -- George Pappas (Lisbon Centre for Astrophysics)
Time: 14:00
Place: 226
Neutron stars as matter and gravity laboratories
Compact objects in general and neutron stars (NSs) in particular open a window to some of the most extreme physics we can find in nature. On the one hand in the interior of NSs we can find matter in very extreme densities, exceeding nuclear densities and anything we can probe in the laboratory, while on the other hand NSs are related to the strongest gravitational fields next only to those found in black holes. Therefore studying NSs gives us access to both supranuclear densities as well as strong gravity and can be used to get information and test our theories of matter (equation of state) and gravity. The relevant properties of the structure of NSs are encoded on the spacetime around them and by studying the astrophysical processes that take place around NSs we can map that spacetime and extract these properties (i.e., the multipole moments, the equation of state, etc). In this talk we will discuss these properties of NSs and how they are related to the properties of the spacetime around them both in GR and in one of the proposed alternative theories of gravity. We will also talk about the relation of these properties to astrophysical observables and how one could tell these theories apart.
16/10/2017 -- Tessa Baker (Oxford University)
Time: 14:00
Place: 226
Tests of Beyond-Einstein Gravity
Corrections to General Relativity on large distance scales are under consideration as an explanation of cosmic acceleration. However, studying extended gravity models on an individual basis is a labour-intensive way of testing these ideas. I will explain how instead EFT-inspired parameterised methods can be used as a powerful and efficient way of testing for deviations from GR. I will outline the theoretical foundations of these techniques, and describe the current status of their observational constraints.
8/9/2017 -- Dani Figueroa (CERN)
Time: 14:00
Place: 117
Higgs Cosmology: implications of the Higgs for the early Universe.
I will discuss some of the consequences arising when we take into account the existence, and hence the presence, of the Standard Model Higgs during Inflation. In particular, I will derive stringent constraints on the couplings of the Higgs to the inflationary and gravitational sectors. I will also discuss the circumstances under which the Higgs can be resposible for the origin of the Standard Model species required by the 'hot Big Bang' paradigm. If there is enough time, I will also discuss the implications of all this for primordial gravitational waves.
6/9/2017 -- Sergey Ketov (Tokyo Metropolitan University)
Time: 14:00
Place: 117
Starobinsky inflation in supergravity
I begin with an introduction to Starobinsky inflation based on (R+R^2) gravity, in light of Planck data about CMB. Next, I introduce the supergravity extensions of Starobinsky inflation, review their problems and possible solutions. I conclude with a discussion of reheating after Starobinsky inflation in the context of supergravity.
29/6/2017 -- Bruce Bassett (University of Cape Town)
Time: 14:00
Place: Room 117
Rise of the Machine: AI and Fundamental Science
With the recent spectacular advances in machine learning we are naturally confronted with the question of the limits of Artificial Intelligence (AI). Here we will review how AI is being used in astronomy, discuss the future role of AI in fundamental science and finally discuss whether AI will ever be able to undertake its own original research.
28/6/2017 -- Dmitry Semikoz (APC, Paris)
Time: 14:00
Place: Lecture hall
Signatures of a two million year old nearby supernova in antimatter data.
In this talk I will show how one can explain multiple anomalies in the cosmic ray data by adding the effects of a 2 million year old nearby supernova to static model of galactic cosmic rays. In particular, this supernova can explain the excess of positrons and antiprotons above 20 GeV found by PAMELA and AMS-02, the discrepancy in the slopes of the spectra of cosmic ray protons and heavier nuclei in the TeV-PeV energy range and the plateau in cosmic ray dipole anisotropy in the 2-50 TeV energy range. Same supernova was responsible for Fe60 measured in ocean crust.
2/6/2017 -- David Alonso (University of Oxford)
Time: 14:00
Place: Room 117
Science with future ground-based CMB experiments.
After the findings of Planck, the immediate future of CMB observations lies with the next generation of ground-based experiments. In this talk I will first introduce the most compelling science objectives for these experiments in combination with future large-scale-structure surveys. Then I will describe a number of novel observational methods to tackle these objectives enabled by the enhanced angular resolution and reduced noise levels of Stage-3 and Stage-4 observatories, as well as the main challenges they will face.
22/5/2017 -- Mathieu Langer (Université Paris-Sud)
Time: 14:00
Place: Room 117
Magnetizing the intergalactic medium during reionization.
An increasing amount of evidence indicates that cosmological sheets, filaments and voids may be substantially magnetised. The origin of magnetic fields in the the Intergalactic Medium is currently uncertain. It seems now well known that non-standard extensions to the physics of the Standard Model are capable of providing mechanisms susceptible of magnetising the Universe at large. Much less well known is the fact that standard, classical physics of matter-radiation interactions possesses actually the same potential. After reviewing briefly our current knowledge about magnetic fields on the largest scales, I will discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous Intergalactic Medium. Operating in the neighbourhood of ionising sources during the Epoch of Reionization, this mechanism is capable of generating magnetic seeds of relevant strengths on scales comparable to the distance between ionising sources. In addition, summing up the contributions of all ionising sources and taking into account the distribution of gas inhomogeneities, I will show that this mechanism leaves the IGM, at the end of Reionization, with a level of magnetization that might account for the current magnetic fields strengths in the cosmic web.
--based on Durrive & Langer, MNRAS, 2015, and Durrive et al. MNRAS 2017 (submitted)--
16/5/2017 -- Sergey Sibiryakov (CERN, EPFL, INR RAS)
Time: 16:00
Place: Room 117
Counts-in-cells statistics of cosmic structure and non-perturbative methods of quantum field theory
I will show how the probability distribution for matter over/under- densities in spherical patches of the universe can be derived from first principles using the instanton technique borrowed from quantum field theory. The spherical collapse solution plays the role of an instanton, whereas deviations from sphericity are consistently accounted for by a Gaussian integral over small perturbations around the instanton. The method is valid even for large — by a factor ten — deviations from the mean density and provides a way to probe the dynamics of dark matter and statistics of initial fluctuations in the regime where perturbative treatment does not apply.
25/4/2017 -- Ippocratis Saltas (University of Lisbon)
Time: 11:00
Place: Room 117
What can unimodular gravity teach us about the cosmological constant?
Unimodular gravity became very popular over the last years as a theory that could shed light on the cosmological—constant problem. In this talk, I will explain the idea behind unimodular gravity, and discuss its (in)ability to bring a new perspective to the problem of the cosmological vacuum.
12/4/2017 -- Andrei Nomerotski (Brookhaven National Lab, USA)
Time: 14:00
Place: Lecture Theatre
Status and Plans for Large Synoptic Survey Telescope
Investigation of Dark Energy remains one of the most compelling tasks for modern cosmology. It can be studied with several probes which are accessible through precise and deep surveys of the Universe. In the talk I will review the status and plans for Large Synoptic Survey Telescope, which will precisely measure the positions and shapes of billions of galaxies along with estimates of their distances, providing an order-of-magnitude improvement relative to current experiments. LSST Camera employs thick, fully depleted CCDs with extended infrared sensitivity. The talk will provide more detail on the camera design and will discuss limitations on the achievable precision coming from the instrumentation.
6/4/2017 -- Alex Vikman (CEICO, Institute of Physics)
Time: 14:00
Place: Lecture Theatre
The Phantom of the Cosmological Time-Crystals
I will discuss a recently proposed new cosmological phase where a scalar field moves exactly periodically in an expanding spatially-flat Friedmann universe. On average this phase has a vacuum or de Sitter equation of state and can be interesting to model Inflation and Dark Energy in a novel way. This phase corresponds to a limiting cycle of the equations of motion and can be considered as a cosmological realization of a general idea of a "time-crystal" introduced by Wilczek et. all in 2012. Recently we showed that this cosmological phase is only possible, provided the Null Energy Condition is violated and the so-called Phantom divide is crossed. Using methods from the dynamical systems, we proved that in a rather general class of single scalar field models called k-essence: i) this crossing causes infinite growth of quantum perturbations on short length-scales, and ii) exactly periodic solutions are only possible, provided the limiting cycle encircles a singularity in the phase plane. The configurations neighboring this singular curve in the phase space are linearly unstable on one side of the curve and superluminal on the other side. Moreover, the increment of the instability is infinitely growing for each mode by approaching the singularity, while for the configurations on the other side, the sound speed is growing without limit. We illustrated our general results by analytical and numerical studies of particular models proposed by Wilczek and collaborators. Finally I will briefly discuss systems where this idea of time-crystals may be realized.
3/4/2017 -- Jnan Maharana (Institute of Physics, Sachivalaya Marg, India)
Time: 14:00
Place: Room 226.
Scattering of Stringy States and T-duality
First a brief overview of target space duality will be presented. Compactification of a closed bosonic string in its massless backgrounds wil be considered when it is compactified on a d-dimensional torus. The vertex operators associated with the moduli of the compactified closed string will be constructed. The Kawai-Llewellyn-Tye factorization technique will be utilized to show the T-duality transformation properties of S-matrix for the moduli.
27/3/2017 -- Michal Bilek (Astronomical Institute of the Czech Academy of Sciences)
Time: 14:00
Place: Room 117.
Galaxy interactions in MOdified Newtonian Dynamics (MOND)
MOdified Newtonian Dynamics (MOND) is a promising attempt to solve the missing mass problem by changing the standard laws of physics rather than by postulating the dark matter. MOND has been inspired by observations of isolated disk galaxies. It is thus important to test it also in other objects. First, I will give a short introduction to MOND and review the published simulations of interacting galaxies. I will then present my work on testing MOND in elliptical galaxies using remnants of accreted satellites and my simulation of the past close encounter of Milky Way and the Andromeda galaxy that MOND predicts. I will discuss observational evidence for this encounter.
27/2/2017 -- Misao Sasaki (Yukawa Institute for Theoretical Physics, Kyoto -- director)
Time: 16:00
Place: Room 117.
Signatures from inflationary massive gravity.
Inflation is a natural platform for modified gravity. In this talk, we consider
a theory that spontaneously violates the local SO(3) symmetry, which gives
rise to a preferred spatial frame during inflation.
As a result, the tensor modes become massive. We argue that this theory
leads to several interesting observational signatures.
23/2/2017 -- Misao Sasaki
Time: 14:00
Place: Lecture hall.
Colloquium: Inflation and Beyond.
There is strong observational evidence now that the Universe has
experienced an almost exponential expansion at its very early stage, called
inflation. In this talk I first review the inflationary universe and its observational
predictions. Then I discuss possible future directions beyond and behind theory
of inflation, and their observational signatures.
14/12/2016 -- Giovanni Acquaviva (Charles University, Prague)
Time: 14:00
Place: Room 226
Dark matter perturbations with causal bulk viscosity
We analyse the evolution of perturbations of cold dark matter endowed with bulk viscosity. Focusing on structure formation well within the Hubble radius, the perturbative analysis is carried out in the Newtonian approximation while the bulk viscosity is described by Israel-Stewart's causal theory of dissipation. Differently from previous analysis based on non-causal theories, we obtain a density contrast evolution governed by a third order equation. This framework can be employed to address some of the current inconsistencies in the observed clustering of galaxies.
9/12/2016 -- David Pirtskhalava (EPFL, Lausanne, Switzerland)
Time: 14:00
Place:Room 117.
Relaxing the Cosmological Constant
14/11/2016 --Glenn Barnich (Université Libre de Bruxelles & International Solvay Institutes)
Time: 14:00
Place: Room 117
Finite BMS transformations
18/10/2016 -- Eugeny Babichev (Laboratoire de Physique Théorique d'Orsay, Orsay, France)
Time: 14:00
Place: Room 117
Gravitational origin of dark matter