Central European Institute for Cosmology and Fundamental Physics
 

Past seminars

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)

15/10/2018, 11:00    Hidehiko Shimada,  TBA

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


  

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