Past seminars

17/06/2021 -- Marco Astorino (INFN, Milano, Italy)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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.

03/10/2019 -- Dalimil Mazac (Simons Center for Geometry and Physics, Stony Brook University)

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.

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)

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.