Central European Institute for Cosmology and Fundamental Physics

# Past seminars

12/03/2020, 16:00 Keigo Shimada (Tokyo Institute of Technology), Metric-affine Geometry and Ghost-free structure of Scalar-tensor Theories

02/03/2020, 14:00 Tarek Anous (University of Amsterdam), Areas and entropies in BFSS/gravity duality

27/02/2020, 16:00 Elias Kiritsis (APC, Paris & Crete University), Emergent gravity from Hidden sectors

20/02/2020, 16:00 Georgios Loukes-Gerakopoulos (Astronomical Institute, Prague), Model agnostic approaches to cosmology

13/02/2020, 16:00 Paolo Creminelli (ICTP, Trieste), Initial Conditions for Inflation

30/01/2020, 16:00 Tomas Ledvinka (Charles University, Prague), Dynamic vacuum spacetimes in a computer

29/01/2020, 14:00 Harold Erbin (University of Turin), Machine learning for QFT

23/01/2020, 16:00 Chris Clarkson (Queen Mary University of London) - CANCELLED , General relativity in the era of large scale surveys

16/12/2019, 16:00 Pavel Motloch (CITA, Toronto), Galaxy spins as probes of fundamental physics

12/12/2019, 16:00 Filippo Vernizzi (Universite Paris Saclay), Dark energy after gravitational wave observations

28/11/2019, 16:00 Guillermo Ballesteros (Autonoma University, Madrid), Primordial black hole dark matter from inflation

21/11/2019, 16:00 Giovanni Acquaviva (Charles University, Prague, CZ), Emergent gravity, Bekenstein bound and unitarity

04/11/2019, 16:00   Dionysios Anninos, de Sitter horizons and sphere partition functions

17/10/2019, 16:00   Sk Jahanur Hoque, Conserved charges in asymptotically de Sitter spacetimes

10/10/2019, 16:00   Cosimo Bambi, Testing general relativity using X-ray reflection spectroscopy

03/10/2019, 14:00   Dalimil Mazac, Sphere packing, quantum gravity and extremal functionals

02/10/2019, 11:00   Thales Azevedo, (DF)^2 gauge theories and strings

01/10/2019, 14:00   Poulami Nandi, Field Theories with Conformal Carrollian Symmetry

30/09/2019, 14:00   Antoine Bourget,  The Higgs Mechanism - Hasse Diagrams for Symplectic Singularities

19/09/2019, 16:00    Gizem Sengor,  Unitarity at the Late Time Boundary of de Sitter

16/08/2019, 14:00    Tomas Prochazka, Thermodynamics Bethe Ansatz

12/08/2019, 14:00    Camilo Garcia-Cely, Self-interacting Spin-2 Dark Matter

01/08/2019, 16:00    Andrei Frolov, Cosmic dust is everywhere

22/07/2019, 14:00    Agnes Ferte, Probing The Accelerated Universe Through Weak Lensing

19/07/2019, 14:00    Xingang Chen, Probing the Origin of the Big Bang Cosmology

16/07/2019, 14:00    Leonardo Modesto, Nonlocal Quantum Gravity

15/07/2019, 11:00    Mirek Rapcak, On Miura transformation for $W_{n|m \times \infty}$ and related geometry

11/07/2019, 14:00    David Svoboda, Paracomplex Geometry and Twisted Supersymmetry

08/07/2019, 14:00    Adolfo Cisterna, Homogenous AdS black strings in General Relativity and Lovelock theories

04/07/2019, 14:00    Cesar Arias,  Higher Spins and Topological Strings

24/06/2019, 14:00    Andrea Fontanella, Hidden relativistic symmetry in AdS/CFT

10/06/2019, 14:00    Dmitry Gorbunov, Higgs inflation in weak coupling regime

07/06/2019, 14:00    Mairi Sakellariadou, Stochastic gravitational waves background and its anisotropies

04/06/2019, 13:30    Tanmay Vachaspati, A Classical-Quantum Correspondence

21/05/2019, 16:00    Julian Adamek, Gevolution v1.2 - relativistic N-body simulations and light-cone analysis

13/05/2019, 14:00    Tomislav Prokopec, Quantum corrections during inflation

10/04/2019, 14:00    Ogan Ozsoy, Probing Early Universe on Small Scales

25/03/2019, 14:00    James Bonifacio, Shift Symmetries in (Anti) de Sitter Space

18/03/2019, 14:00    Masahide Yamaguchi, Higher derivative scalar-tensor theory through a non-dynamical scalar field

14/03/2019, 16:00    Stefano Camera,  Synergic cosmology across the spectrum

28/02/2019, 16:00    Gizem Sengor,  A look at Cosmological Perturbations during Preheating with Effective Field Theory Methods

27/02/2019, 14:00    Antonio Racioppi,  The Palatini side of inflationary attractors

25/02/2019, 14:00    Jakub Vicha,  Probing the Universe at the Highest Energies with the Pierre Auger Observatory

19/02/2019, 14:00    Yuji Satoh,  TBA (room 226)

18/02/2019, 14:00    Vojtech Witzany,  Spin-perturbed orbits near black holes

10/12/2018, 14:00    Shinji Mukohyama,  Minimalism in modified gravity

26/11/2018, 14:00    Ondrej Pejcha,  Explosive deaths of stars: core-collapse supernovae and stellar mergers

23/11/2018, 14:00    Rachel Houtz,  Color Unified Dynamical Axion

22/11/2018, 16:00    Antonino Marciano,  Non-dynamical torsion from fermions and CMBR phenomenology

20/11/2018, 14:00    Andrea Addazi,  Gravitational waves from Dark bubbles in the early Universe

05/11/2018, 14:00    Subodh Patil,  Tensor bounds on the hidden universe

25/10/2018, 16:00    Alexey Golovnev,  Modified teleparallel gravity

16/10/2018, 14:00    Lasha Berezhiani,  Superfluid Dark Matter

15/10/2018, 14:00    Hiroyuki Sagawa,  Recent results of the Telescope Array experiment on ultra-high energy cosmic rays (Division Seminar)

08/10/2018, 14:00    Katherine Freese,  Inflationary Cosmology in Light of Cosmic Microwave Background Data

17/09/2018, 14:00    Elena de Paoli,  A gauge-invariant symplectic potential for tetrad general relativity

30/07/2018, 13:30    Shun-Pei Miao,  A Cosmological Coleman Weinberg Potentials and Inflation

27/07/2018, 14:00    Richard Woodard,  A Nonlocal Metric Realization of MOND

26/07/2018, 14:00    Dam Thanh Son,  From fractional quantum Hall effect to field-theoretical dualities

25/07/2018, 14:00    Oleg Teryaev,  Graviitational formfactors and pressure in elementary particles

24/07/2018, 14:00    Renato Costa,  Singularity free Universe in double field theory

18/07/2018, 14:00    Massimiliano Rinaldi,  Scale-invariant inflation

17/07/2018, 14:00    Alessandro Drago,  What do mergers of neutron stars tell us about nuclear physics?

16/07/2018, 14:00    Miroslav Rapcak,  Representation Theory of Vertex Operator Algebras and Gukov-Witten defects

12/07/2018, 14:00    Jarah Evslin,  Cosmic Expansion Anomalies as Seen by Baryon Acoustic Oscillations

03/07/2018, 14:00    Yi-Zen Chu,  Theoretical Explorations in Gravitational Physics

28/06/2018, 14:00    Andreas Albrecht,  Perspectives on Cosmic Inflation

27/06/2018, 14:00    Andreas Albrecht,  Einselection and Equilibrium

26/06/2018, 14:00    Eugeny Babichev,  Hamiltonian vs stability and application to scalar-tensor theories

25/06/2018, 14:00    Wojciech Hellwing,  How to falsify CDM (and test its alternatives)?

15/06/2018, 11:00    Emre Kahya,  Loop Corrections to Primordial non-Gaussianties

14/06/2018, 16:00    Emre Kahya,  GW170817 Falsifies Dark Matter Emulators

12/06/2018, 14:00    Dam Thanh Son,  Quantum Hall effect and field-theoretic dualities

06/07/2018, 14:00    Sébastien Clesse,  Primordial Black Holes as the Dark Matter

21/05/2018, 14:00    Maksym Ovchynnikov ,  New physics at the intensity frontier

17/05/2018, 16:00    Lorenzo Pizzuti,  Modified gravity with galaxy cluster mass profiles: from data to simulations

15/05/2018, 15:00    Santiago Casas,  Model-independent tests of gravity with present data and future surveys

07/05/2018, 14:00    Diego Blas,  Probing dark matter properties with pulsar timing

04/05/2018, 14:00    Peter Tinyakov,  Compact stars as dark matter probes

03/05/2018, 14:00    Jan Novák,  Scalar perturbations of Galileon cosmologies in the mechanical approach in the late Universe

12/04/2018, 14:00    Tomi Koivisto,  Symmetric Teleparallelism

10/04/2018, 14:00    Luca Marzola,  The 21-cm Line

27/03/2018, 14:00    Julian Adamek,  Evolving The Metric - N-body simulations for relativistic cosmology

20/03/2018, 14:00     Patrick Stengel,  The Higgs boson can delay Reheating after Inflation

15/03/2018, 14:00    Ilidio Lopes,  Impact of dark matter in stellar oscillations

20/02/2018, 14:00     Eric Bergshoeff,  Gravity and the spin-2 planar Schroedinger equation

12/02/2018, 14:00     Roberto Oliveri,  Gravitational multipole moments from Noether charges

01/02/2018, 16:00     Luca Visinelli,  Axions in cosmology and astrophysics

30/01/2018, 14:00     Eleonora Villa,  Theoretical systematics in galaxy clustering in LCDM and beyond

23/01/2018, 14:00     Petr Satunin,  Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons

12/12/2017, 14:00     David Svoboda,  Twisted brackets, fluxes, and deformations of para-Kahler manifolds

11/12/2017, 16:00     Martin Roček,  WZW models and generalized geometry

05/12/2017, 14:00     Dimitris Skliros,  Coherent states in String Theory

04/12/2017, 14:00     Ed Copeland,  Screening mechanisms and testing for them in cosmology and the laboratory

30/11/2017, 14:00     Marc Gillioz,  Sum Rules for the "c" Anomaly in 4 Dimensions

28/11/2017, 14:00     Sugumi Kanno,  Decoherence of Bubble Universes

27/11/2017, 14:00     Rachel Houtz,  Little Conformal Symmetry and Neutral Naturalness

08/11/2017, 14:00     Pierre Fleury,  Weak lensing with finite beams

07/11/2017, 14:00     Frederik Lauf,  Classification of three-dimensional Chern-Simons-matter theories

06/11/2017, 14:00     Andrei Gruzinov,  Particle production by real (astrophysical) black holes

23/10/2017, 14:00     George Pappas,  Neutron stars as matter and gravity laboratories

16/10/2017, 14:00     Tessa Baker,  Tests of Beyond-Einstein Gravity

02/10/2017 14:00    Piotr Surowka  New developments in hydrodynamics
08/09/2017, 14:00   Dani Figueroa  Higgs Cosmology: implications of the Higgs for the early Universe
06/09/2017, 14:00    Sergey Ketov  Starobinsky inflation in supergravity
06/09/2017, 11:00   Dalimil Mazáč  Analytic conformal bootstrap and QFT in AdS2
29/06/2017, 14:00   Bruce Bassett  Rise of the Machine: AI and Fundamental Science
28/06/2017,  14:00   Dmitri Semikoz  Signatures of a two million year old nearby supernova in antimatter data
02/06/2017,  14:00   David Alonso  Science with future ground-based CMB experiments
22/05/2017,  14:00   Mathieu Langer  Magnetizing the intergalactic medium during reionization
16/05/2017,  16:00   Sergey Sibiryakov  Counts-in-cells statistics of cosmic structure
25/04/2017,  14:00    Ippocratis Saltas  What can unimodular gravity teach us about the cosmological constant?
12/04/2017,  14:00    Andrei Nomerotski  Status and Plans for Large Synoptic Survey Telescope
06/04/2017,  14:00    Alex Vikman  The Phantom of the Cosmological Time-Crystals
03/04/2017,  14:00    Jnan Maharana  Scattering of Stringy States and T-duality
27/03/2017,  14:00   Michal Bilek  Galaxy interactions in MOdified Newtonian Dynamics (MOND)
27/02/2017,  16:00   Misao Sasaki,  Signatures from inflationary massive gravity
23/02/2017,  14:00   Misao Sasaki, Inflation and Beyond
14/12/2016,  14:00   Giovanni Acquaviva,Dark matter perturbations with causal bulk viscosity
09/12/2016,  14:00   David Pirtskhalava  Relaxing the Cosmological Constant
14/11/2016,  14:00   Glenn Barnich,   Finite BMS transformations
18/10/2016   14:00   Eugeny Babichev, Gravitational origin of dark matter

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
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