Central European Institute for Cosmology and Fundamental Physics
 

Past seminars


08/09/2022
Diana López Nacir (University of Buenos Aires, Argentina)
Cosmological perturbations for Self-Interacting Warm Dark Matter scenario, numerical implementation and some observational constraints

23/06/2022
Shun-Pei Miao (National Cheng Kung University (NCKU), Tainan, Taiwan)
Gauge Independent Effective Field Equations

22/06/2022
Richard P. Woodard (University of Florida at Gainesville, USA)
Summing Large Logarithms from Loops of Inflationary Gravitons

16/06/2022
Alexander Zhuk (Odessa I. I. Mechnikov National University, Ukraine & Center for Advanced Systems Understanding (CASUS), Görlitz, Germany)
Relativistic approach to the large-scale structure formation: cosmic screening vs. gevolution

09/06/2022
Matteo Fasiello (IFT, Madrid, Spain)
Probing the Early Universe with Gravitational Waves

02/06/2022
Gianmassimo Tasinato (Swansea University, UK)
Probing the Physics of Inflation with Gravitational Wave Experiments

26/05/2022
Lavinia Heisenberg (Heidelberg University, Germany)
Tensions in LCDM and how to solve them

19/05/2022
Michal Artymowski (Wyszynski University, Warsaw, Poland)
New applications of unparticles: Inflation, dark energy, bouncing cosmologies, and the Hubble tension

12/05/2022
Earl Bellinger (Max Planck Institute, Garching, Germany)
Asteroseismic probes of stellar evolution and fundamental physics

04/05/2022
Caner Unal (Ben-Gurion University of the Negev, Israel)
Spin in Active BHs and properties of ultralight particles

28/04/2022
Jose Beltran Jimenez (University of Salamanca, Spain)
Quicksand in Affinesia

21/04/2022
Marek Lewicki (University of Warsaw, Poland)
Search for new physics through primordial gravitational waves

13/04/2022
Pavel K. Kovtun (University of Victoria, Canada)
Hydrodynamics beyond hydrodynamics

07/04/2022
Dong-Gang Wang (University of Cambridge, United Kingdom)
Bootstrapping Inflation: cosmological correlators with broken boosts

31/03/2022
Christos Charmousis (IJCLab Orsay, France)
Compact objects in scalar tensor theories

24/03/2022
Sergey Ketov (Leibniz University Hannover, Germany & Tokyo Metropolitan University, Japan)
Formation of primordial black holes after Starobinsky inflation in supergravity
10/03/2022
Daniel Green (UC, San Diego, USA)
A Tail of Eternal Inflation

17/03/2022
Dmitri Semikoz (APC Paris, France)
Measurements of cosmological magnetic fields in the voids of large scale structure

24/02/2022, 16:00, lecture hall
Matthias Bartelmann (ITP, Heidelberg University, Germany)

17/02/2022
Sk Jahanur Hoque (Charles University, Prague, Czechia)
Mass loss law for weak gravitational fields: With a positive cosmological constant

10/02/2022
Ke-Pan Xie (Nebraska University, USA)
Primordial black holes from a cosmic phase transition: The collapse of Fermi-balls

03/02/2022
Alexander Ganz (Jagiellonian University, Krakow, Poland)
Minimally modified gravity and its phenomenological properties

27/01/2022
Adolfo Cisterna (TIFPA-INFN, Trento, Italy)
Taub-NUT spacetimes at the service of exact black bounces: Black holes, wormholes and bouncing cosmologies with a self-interacting scalar field

20/01/2022
Valerie Domcke (CERN, Geneva, Switzerland)
Cosmology with axion-like particles

13/01/2022
Marco Crisostomi (SISSA, Trieste, Italy)
Gravitational wave generation in dark energy theories

02/12/2021
Ville Vaskonen (IFAE, Barcelona, Spain)
Probing dark matter through gravitational waves

25/11/2021
Anamaria Hell (LMU, Munich, Germany)
Exploring the dualities of massive gauge theories: Aμ vs. Bμν

18/11/2021
Graham White (University of Tokyo, Japan)
Archaeology on the origin of matter

11/11/2021
Tomislav Prokopec (Utrecht University, Netherlands)
Quantum origin of dark energy and the Hubble tension

04/11/2021
Camilo Garcia Cely (DESY, Germany)
The CMB as a detector of gravitational waves

27/10/2021
Stian Hartman (University of Oslo, Norway)
Self-interacting Bose-Einstein condensed dark matter; cosmological constraints and simulations

21/10/2021
Farbod Hassani (University of Oslo, Norway)
Characterizing the non-linear evolution of dark energy and modified gravity models

14/10/2021
Roman Konoplya (Peoples' Friendship U., Moscow, Russia)
Traversable wormholes in General Relativity without exotic matter

30/09/2021
Anton Baleato Lizancos (UC Berkeley, USA)
Fundamental Physics with CMB Lensing and Delensing

23/09/2021
Jan Burger (University of Iceland, Reykjavik, Iceland)
Conservation of radial actions in time-dependent spherical potentials

 

Academic year 2000/2001

 

17/06/2021 - 4pm, Marco Astorino (INFN, Milano, Italy), Multi-black holes at equilibrium in an external gravitational field

10/06/2021 - 4pm, Francis-Yan Cyr-Racine (University of New Mexico), A cosmological dark matter collider experiment

27/05/2021 - 4pm, Daniel Litim (University of Sussex, UK), Asymptotic safety - from particle physics to quantum gravity

20/05/2021 - 4pm, Macarena Lagos (Columbia University, NY, USA), Interacting Gravitational Waves

13/05/2021 - 4pm, Swetha Bhagwat (Sapienza, University of Rome, Italy), Merger-ringdown consistency test

06/05/2021 - 4pm, Viviana Niro (APC Paris, France), Neutrinos from galactic sources

29/04/2021 - 4pm, Emel Altas (Karamanoglu Mehmetbey University, Turkey), Nonstationary energy in general relativity and approximate analytical description of apparent horizons for initial data with momentum and spin

22/04/2021 - 4pm, Teodor Borislavov Vasilev (Universidad Complutense de Madrid, Spain), Classical and quantum f(R) cosmology: The big rip, the little rip and the little sibling of the big rip

15/04/2021 - 4pm, Famaey Benoit (Observatoire astronomique de Strasbourg, France), MOND: phenomenological review and constraints for model-building

25/03/2021 - 4pm, Johannes Noller (Cambridge University - DAMTP, UK), Testing gravity on all scales

18/03/2021 - 4pm, Kazuya Koyama (Portsmouth U., ICG, UK), General relativistic weak-field limit and Newtonian N-body simulations

11/03/2021 - 4pm, Raissa Mendes (Universidade Federal Fluminenense, Brazil), Probing general relativity with the most extreme neutron stars

04/03/2021 - 4pm, Ali Seraj (Universite Libre de Bruxelles, Belgium), Gravitational memory in modified gravity and two-form symmetries

18/02/2021 - 4pm, François Larrouturou (IAP Paris, France), Refocus on the essentials: introducing the Minimal Theory of Bigravity

11/02/2021 - 4pm, Ondrej Hulik (Charles University, Prague), Generalized and Exceptional geometry, A Unified Framework

04/02/2021 - 4pm, Giulia Cusin (Universite de Geneve, Switzerland), Catalogue and background description of a population of GW sources: features, complementarities and caveats.

28/01/2021 - 4pm, Paolo Benincasa (IFT Madrid, Spain), Towards a reformulation of QFT in expanding universes.

21/01/2021 - 4pm, Timothy Anson (Universite Paris-Saclay, France), Disforming the Kerr metric

14/01/2021 - 4pm, Siddharth Prabhu (Tata Institute of Fundamental Research, India), The holographic nature of quantum information storage in asymptotically flat spacetimes

17/12/2020 - 4pm, Oksana Iarygina (Leiden University, NL), The physical mass scales of multi-field preheating

10/12/2020 - 4pm, Horng Sheng Chia (Institute for Advanced Study, Princeton, USA), Tidal Deformation and Dissipation of Rotating Black Holes

03/12/2020 - 4pm, Antony Lewis (University of Sussex, UK), Concordance, tensions and gravitational lensing

26/11/2020 - 4pm, Eiichiro Komatsu (Max-Planck-Institut für Astrophysik, Germany), Hunting for parity-violating physics in polarisation of the cosmic microwave background


19/11/2020 - 4pm, Mikhail Shaposhnikov (EPFL), Einstein-Cartan gravity: Inflation, Dark Matter and Electroweak Symmetry Breaking

12/11/2020 - 4pm, William Barker (Kavli Institute for Cosmology, Cambridge, UK), Dark energy and radiation from the novel gauge gravity theories

05/11/2020 - 4pm, Fedor Bezrukov (University of Manchester, UK), What do we know about preheating in Higgs Inflation and its relatives?

22/10/2020 - 4pm, Eugene Lim (King's College London, UK), Challenging the Inflationary Paradigm

15/10/2020 - 4pm, Filippo Camilloni (University of Perugia, IT and Niels Bohr Institute, DK), Moving away from the Near-Horizon Attractor of the Extreme Kerr Force-Free Magnetospheres

08/10/2020 - 4pm, Anne Green (University of Nottingham, UK, Primordial Black Holes as a dark matter candidate

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)

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




08/09/2022, 16:00, lecture hall
Diana López Nacir (University of Buenos Aires, Argentina)
Cosmological perturbations for Self-Interacting Warm Dark Matter scenario, numerical implementation and some observational constraints
 
The standard cosmological model assumes the existence of dark contributions to the energy content of the Universe. In particular, it assumes a mysterious component known as Dark Matter, which in the standard scenario is considered to be “Cold”. In this talk I will consider Self-Interacting Warm Dark Matter (SI-WDM) models as alternative candidates. After introducing the main motivations and the basic phenomenology, I will present a general framework for computing the evolution of cosmological perturbations at linear level, which derives from a Boltzmann hierarchy based on a parametrization of the scattering amplitude that allows one to retain certain model independence on the particular interaction Lagrangian. I will show some results we obtained using a numerical implementation of the framework in an extended version of CLASS code, and in particular some observational constraints we obtained using Milky Way satellites Counts and Lyman-alpha forest from a phenomenological approach. As a result, I will show that sufficient self-interaction could turn the lower bounds on the mass of the WDM particles less restrictive than without self-interaction and in particular relax constraints on the traditional νMSM model.

23/06/2022, 16:00, lecture hall
Shun-Pei Miao (National Cheng Kung University (NCKU), Tainan, Taiwan)
Gauge Independent Effective Field Equations
 
I describe a technique for removing gauge dependence from graviton loop corrections to the effective field equations. I present explicit results on flat space background for a massless, minimally coupled scalar and for electromagnetism. I then describe how the procedure generalizes to de Sitter background. This talk is based on arXiv:1708.06239

22/06/2022, 16:00, lecture hall
Richard P. Woodard (University of Florida at Gainesville, USA)
Summing Large Logarithms from Loops of Inflationary Gravitons
 
Quantum gravitational corrections on flat space background do not affect particle kinematics at all, and only make fractional changes of order G/r^2 to long range forces. The situation during inflation is very different because (1) the Hubble parameter H allows fractional corrections of the form G H^2 and (2) the continuous production of inflationary gravitons introduces a secular element. As a result, corrections to both particle kinematics and long range forces typically grow like logarithms of the scale factor and/or the spatial separation. If inflation persists long enough, this growth must eventually cause perturbation theory to break down, begging the question of what happens next. I report on recent progress in summing the very similar large logarithms which occur in nonlinear sigma models by combining a variant of Starobinsky's stochastic formalism with a variant of the renormalization group. I discuss how this technique can be generalized to quantum gravity. This talk is based on arXiv:2110.08715 with Shun-Pei Miao and Nick Tsamis.

16/06/2022, 16:00, room 117
Alexander Zhuk (Odessa I. I. Mechnikov National University, Ukraine & Center for Advanced Systems Understanding (CASUS), Görlitz, Germany)
Relativistic approach to the large-scale structure formation: cosmic screening vs. gevolution
 
Due to the modern telescopes, we found that the Universe is filled with a cosmic web which is composed of interconnected filaments of galaxies separated by giant voids. The emergence of this large-scale structure is one of the major challenges of modern cosmology. We study this phenomenon with the help of relativistic N-body cosmological simulation based on General Relativity. It is well known that gravity is the main force responsible for the structure formation in the Universe. In the first part of my talk, I demonstrate that in the cosmological setting gravitational interaction undergoes an exponential cutoff at large cosmological scales. This effect is called cosmic screening. It arises due to the interaction of the gravitational field with the background matter. Then, I compare two competing relativistic approaches to the N-body simulation of the Universe large-scale structure: “gevolution” vs. “screening”. To this end, employing the corresponding alternative computer codes, I demonstrate that the corresponding power spectra are in very good agreement between the compared schemes. However, since the perturbed Einstein equations have much simpler form in the “screening” approach, the simulation with this code consumes less computational time, saving almost 40% of CPU (central processing unit) hours.

09/06/2022, 16:00, lecture hall
Matteo Fasiello (IFT, Madrid, Spain)
Probing the Early Universe with Gravitational Waves
 
Some of our best ideas on early universe physics are about to be put to the test by an unprecedented array of cosmological probes. The data these will collect span a vast range of scales, from the CMB to large scale structure, from pulsar timing arrays all the way to laser interferometers. This combined wealth of new information holds the potential to transform not just our understanding of cosmology, but also particle physics. Probing the earliest accessible epoch, the accelerated expansion known as inflation, is crucial: inflation can provide a cosmological portal to otherwise inaccessible energy scales. The spectacular success of the inflationary paradigm in explaining the origin of cosmic structure demands that we tackle a number of compelling questions still in need of an answer: what is the energy scale of inflation? What fields were active during inflation? In this talk I will review recent progress on the inflationary field content. I will survey different approaches to address the most pressing challenges and provide specific examples. I will then focus on the key observables, starting with primordial gravitational waves, and discuss their prospects for detection.

02/06/2022, 16:00, lecture hall
Gianmassimo Tasinato (Swansea University, UK)
Probing the Physics of Inflation with Gravitational Wave Experiments
 
Cosmological inflation predicts the existence of a stochastic background of gravitational waves (GW), whose features depend on the model of inflation under consideration. There exist well motivated frameworks leading to an enhancement of the primordial GW spectrum at frequency scales testable with GW experiments, with specific features as parity violation, anisotropies, and non- Gaussianity. I will explain the properties of such scenarios, and their distinctive predictions for what respect GW observables. I will then discuss perspectives for testing these predictions with future GW experiments.

26/05/2022, 16:00, lecture hall
Lavinia Heisenberg (Heidelberg University, Germany)
Tensions in LCDM and how to solve them
 
After introducing the standard model of cosmology and its parameters, I will discuss two important tensions between early and late-time measurements, namely the H0 tension and the sigma8 tension. Considering a small late-time deviation of the standard model, I will derive fully analytical conditions that any late-time dark energy model has to satisfy in order to solve both tensions simultaneously (see arxiv:2201.11623).

19/05/2022, 16:00, lecture hall
Michal Artymowski (Wyszynski University, Warsaw, Poland)
New applications of unparticles: Inflation, dark energy, bouncing cosmologies, and the Hubble tension
 
Unparticles are a hypothetical new form of matter created from fermions in an SU(N) gauge theory. Unparticles provide a wide spectrum of new cosmological applications. In my talk (based on arxiv:2010.02998 and arxiv:1912.10532), I will show that they can display a cosmological-constant-like behavior, and since then they can be used to generate cosmic inflation or dark energy. I will show realistic bouncing and cyclic Universes filled with unparticles and perfect fluid. I will also discuss constraints on unparticles energy density and their possible role in relaxing the Hubble tension.

12/05/2022, 16:00, lecture hall
Earl Bellinger (Max Planck Institute, Garching, Germany)
Asteroseismic probes of stellar evolution and fundamental physics
 
Asteroseismology allows us to determine the properties of stars through the observation of their global oscillations, giving information about the star's mass, radius and age. Beyond being interesting in their own right, these measurements are essential for a variety of endeavours throughout astrophysics, such as galactic archaeology and the characterization of exoplanets. For the stars with the very best observations, it is possible to additionally measure some aspects of the internal stellar structure, such as the density and sound speed profile throughout the stellar core. This in turn presents the exciting opportunity to test the physics of stellar evolution. These asteroseismic tests can range from assessing mixing mechanisms in stellar interiors, to measuring cosmological effects such as a time-variable gravitational constant. In this seminar, I will give an overview of the asteroseismology of low-mass stars, and highlight the progress that is being made toward mapping out their interior structures.

04/05/2022, 16:00, room 117
Caner Unal (Ben-Gurion University of the Negev, Israel)
Spin in Active BHs and properties of ultralight particles
 
I will discuss the spin modifications to the fundamental plane of BH activity, an empirical correlation between X-ray, radio luminosity and the mass of active BHs. I will further focus on how to extract spin info from those multiwavelength signals and its implications/bounds on ultralight particle properties such as mass, self-interaction and energy density and their possibility to be some fraction of dark matter.

28/04/2022, 16:00, lecture hall
Jose Beltran Jimenez (University of Salamanca, Spain)
Quicksand in Affinesia
 
The equivalence principle naturally provides gravity with a geometrical character. However, the precise geometry we employ to describe it admits a certain flexibility. In particular, within a metric-affine framework, Einstein's gravity can be equivalently ascribed to the three independent objects that characterise a connection, i.e., curvature, torsion and non-metricity. After reviewing these three alternative descriptions of gravity, I will uncover a general teleparallel description of GR and how pathologies generally arise beyond the GR equivalents in the landscape of metric-affine theories.

21/04/2022, 16:00, lecture hall
Marek Lewicki (University of Warsaw, Poland)
Search for new physics through primordial gravitational waves
 
We are currently witnessing the dawn of a new era in astrophysics and cosmology, started by the LIGO/Virgo observations of gravitational waves. These signals also open a new window into processes taking place in the first moments of our Universe. This is due to the fact that GWs propagate freely from the moment of their production unlike like photon based signals which can only propagate freely since the Universe became transparent due to recombination.I will discuss prospects for GW detection with the next generation of experiments. Including the problems connected with observation of a primordial signal in the presence of a foreground produced much more recently by astrophysical objects. The specific early Universe sources I will focus on are cosmological first order phase transitions and cosmic string networks. I will also discuss to what extent we can probe the expansion of the Universe using these primordial GW signals.

13/04/2022, 14:00, lecture hall
Pavel K. Kovtun (University of Victoria, Canada)
Hydrodynamics beyond hydrodynamics
 
In this talk, I will discuss two questions. First, do the equations of relativistic hydrodynamics make sense? And second, how universal are the long-distance, late-time predictions of classical hydrodynamics?

07/04/2022, 16:00, lecture hall
Dong-Gang Wang (University of Cambridge, United Kingdom)
Bootstrapping Inflation: cosmological correlators with broken boosts
 
Correlation functions of primordial fluctuations provide us an exciting avenue into the physics with extremely high energy in the very early Universe. Recently the bootstrap approach has offered new perspectives and powerful tools to study these cosmological correlators. In this talk, by incorporating the latest developments, I will “bootstrap” two types of correlators generated by boost-breaking interactions during inflation, which are most relevant for the next-generation observations. The first one is the contact correlators arising from higher-derivative self-interactions of the inflaton. The second is the cosmological collider physics, where the masses and spins of heavy particles leave unique imprints in the scalar bispectra. Since the boost symmetries are broken in our consideration, the signals of non-Gaussianity are boosted to be detectable for near-future experiments. Furthermore through the bootstrap approach, we derive for the first time not only a complete set of these correlators systematically, but also their full shape information analytically.

31/03/2022, 16:00, lecture hall
Christos Charmousis (IJCLab Orsay, France)
Compact objects in scalar tensor theories
 
We will review some of the black hole solutions in scalar tensor theories focusing on the different symmetries of the underlying theories and principal properties. We will discuss one particular theory whose origin lies in higher dimensional Lovelock theory and construct a regular traversable wormhole and neutron star metrics.

24/03/2022, 16:00, lecture hall
Sergey Ketov (Leibniz University Hannover, Germany & Tokyo Metropolitan University, Japan)
Formation of primordial black holes after Starobinsky inflation in supergravity
 
The Starobinsky model of cosmological inflation is reviewed as the theoretical probe of a more fundamental theory of gravity for the very early Universe. The modified Starobinsky supergravity is introduced, and its observational predictions are derived and compared to the current astrophysical and cosmological observations. The specific mechanism of the primordial black holes production in supergravity is proposed, and its physical predictions for dark matter and induced gravitational waves are discussed in detail.

10/03/2022, 17:00, online
Daniel Green (UC, San Diego, USA)
A Tail of Eternal Inflation
 
Recent developments in our understanding of quantum field theory in de Sitter space have revealed how to derive the equations Stochastic Inflation and included corrections to them systematically. In this talk, I will review the Soft de Sitter Effective Theory and how it enables to calculate these equations. I will then apply the results to massless λΦ4 theory in de Sitter and calculate the next-to-next-to leading order corrections to the equations and relaxation eigenvalues. We will then apply the same techniques to primordial non-Gaussianity in single-field inflation where we will find the onset of eternal inflation becomes incalculable in some surprising circumstances.

17/03/2022, 16:00, lecture hall
Dmitri Semikoz (APC Paris, France)
Measurements of cosmological magnetic fields in the voids of large scale structure
 
In this talk I’ll review recent developments in the measurements of intergalactic magnetic fields in the voids of large scale structure with gamma-ray telescopes and ultra-high energy cosmic ray detectors. In particular, I'll show that the gamma-ray measurement method can be used to detect the primordial magnetic field with a strength of up to 10-11 G, values interesting for reducing H0 tension. Same magnetic field, if produced at QCD phase transition can be responsible for NANOGrav gravitational wave signal. Also I’ll discuss how one can distinguish magnetic field produced at inflation by simultaneous measurements of extended emission around several nearby TeV blazars. Finally, I'll show first upper limit on magnetic field in the void of large scale structure from ultra-high energy cosmic ray measurement in the direction of Perseus-Pisces supercluster.

24/02/2022, 16:00, lecture hall
Matthias Bartelmann (ITP, Heidelberg University, Germany)
On the track of universality in cosmic structures
 
Our cosmic neighbourhood is richly structured by galaxies, clusters, and larger objects which are mainly composed of dark matter. Gravitationally bound objects dominated by dark matter exhibit density profiles which are self-similar over many orders of magnitude in mass. Why is this so? I will use kinetic field theory to address this question, and show that cosmic structures develop universal correlations for wide classes of initial conditions.

17/02/2022, 16:00, lecture hall
Sk Jahanur Hoque (Charles University, Prague, Czechia)
Mass loss law for weak gravitational fields: With a positive cosmological constant
 
Bondi's celebrated mass loss formula measures the rate of change of energy carried away from an isolated system (in asymptotically flat space-time) by gravitational radiation. In this talk, we generalize the Bondi-Sachs formalism for de Sitter space-time. We also discuss the mass loss formula for linearized gravitational fields in de Sitter setting.

10/02/2022, 16:00, online seminar
Ke-Pan Xie (Nebraska University, USA)
Primordial black holes from a cosmic phase transition: The collapse of Fermi-balls
 
In this talk I will introduce a novel mechanism of forming primordial black holes (PBHs) via a first-order phase transition (FOPT). If a fermion species gains a huge mass in the true vacuum, the corresponding particles get trapped in the false vacuum as they do not have sufficient energy to penetrate the bubble wall. After the FOPT, the fermions are compressed into the false vacuum remnants to form non-topological solitons called Fermi-balls, and then collapse to PBHs due to the interior Yukawa attractive force. After describing the general mechanism, I will discuss a concrete application to the electroweak phase transition and demonstrate that a PBH dark matter scenario is possible.

03/02/2022, 16:00, online seminar
Alexander Ganz (Jagiellonian University, Krakow, Poland)
Minimally modified gravity and its phenomenological properties
 
Minimally modified gravity models are a class of modified gravity models with just two local degrees of freedom as in general relativity. In this talk I want to discuss their general properties such as the existence of a preferred foliation and discuss their phenomenology in the case of inflation and the late universe.

27/01/2022, 16:00, lecture hall
Adolfo Cisterna (TIFPA-INFN, Trento, Italy)
Taub-NUT spacetimes at the service of exact black bounces: Black holes, wormholes and bouncing cosmologies with a self-interacting scalar field
 
We present a new family of exact four dimensional Taub-NUT spacetimes in Einstein-Λ theory supplemented with a conformally coupled scalar field exhibiting a power-counting super-renormalizable potential. Our configurations are constructed in the following manner: A solution of a conformally coupled theory with a conformal potential, henceforth the seed (gμν,Φ), is transformed by the action of a specific change of frame in addition with a simultaneous shift of the scalar seed. The conformal factor of the transformation and the shift are both affine functions of the original scalar Φ. The new configuration, (g'μν,Φ'), now solves the field equations of a conformally coupled theory with the extended aforementioned renormalizable potential, this under the presence of an effective cosmological constant. The new solution spectrum is notoriously enhanced with respect to the original seed containing regular black holes, wormholes and bouncing cosmologies. For a non-vanishing cosmological constant exact black hole to wormhole and black hole to bouncing cosmology transitions are observable, both smoothly controlled by the mass parameter.

20/01/2022, 16:00, Zoom seminar
Valerie Domcke (CERN, Geneva, Switzerland)
Cosmology with axion-like particles
Axion-like particles may play a key role in early universe cosmology. They are naturally equipped with the right properties to explain cosmic inflation, can dynamically explain the smallness of the electroweak scale, may be involved in the generation of the matter antimatter asymmetry and are promising dark matter candidates. In this talk I discuss a generic but previously overlooked particle particle production mechanism, resulting in the dual production of gauge fields and fermions induced by axion-like particles. I will discuss how this crucially impacts all of the cosmological scenarios mentioned above and may be probed with upcoming gravitational wave detectors.

13/01/2022, 16:00, Zoom seminar
Marco Crisostomi (SISSA, Trieste, Italy)
Gravitational wave generation in dark energy theories
The big challenge in describing dark energy as a dynamical field is that we do not see any sign of it in local tests of gravity. Moreover, all gravitational wave events detected so far are in very good agreement with General Relativity predictions. In this talk I will introduce ``kinetic screening'' as a way to overcome this dichotomy and I will present our recent results in testing it with black hole collapse, and the late inspiral and merger of binary neutron stars.

02/12/2021, 16:00, Lecture Hall
Ville Vaskonen (IFAE, Barcelona, Spain)
Probing dark matter through gravitational waves
The LIGO-Virgo observations have already demonstrated the power of gravitational wave astronomy. In near future various experiments will probe gravitational wave signals across a broad range of frequencies providing invaluable insight into astrophysics, cosmology and fundamental physics. In this talk I will discuss how we can use these observations to test dark matter properties. I will focus on two signatures of compact dark matter objects: gravitational waves produced by their mergers, and lensing of gravitational waves. I will show that the LIGO-Virgo observations already provide constraints on compact dark matter and I will discuss the future prospects of gravitational wave probes of dark matter.

25/11/2021, 16:00, Lecture Hall
Anamaria Hell (LMU, Munich, Germany)
Exploring the dualities of massive gauge theories: Aμ vs. Bμν
We compare the massive Kalb-Ramond and Proca fields with a quartic self-interaction and show that the same strong coupling scale is present in both theories. In the Proca theory, the longitudinal mode enters the strongly coupled regime beyond this scale, while the two transverse modes propagate further and survive in the massless limit. In contrast, in case of the massive Kalb-Ramond field, the two transverse modes become strongly coupled beyond the Vainshtein scale, while the pseudo-scalar mode remains in the weak coupling regime and survives in the massless limit. This contradicts the numerous claims in the literature that these theories are dual to each other, which is shown to be false. We show that the difference between the theories can be traced already to the free theories without a self-interaction by studying the behavior of quantum fluctuations for different modes.

18/11/2021, 16:00, Zoom
Graham White (University of Tokyo, Japan)
Archaeology on the origin of matter
One of the most convincing reasons to expect physics beyond the Standard model is the inbalance between matter and anti-matter. Some fantastic paradigms exist that can be probed at a low scale including electroweak baryogenesis, mesogenesis and resonant leptogenesis. While these paradigms or worthy of dedicated attention, the elephant in the room is that there are two paradigms that are very minimal and involve physics at scales we cannot possibly reach with Earth based colliders in our life time. I will first discuss the nightmare scenario of thermal leptogenesis implemented with no BSM particle content beyond sterile neutrinos and an inflaton. In this case, measurements of the top and Higgs mass along with inflationary observables can shed some light on the plausibility, or lack thereof, of vanilla leptogenesis. I will then discuss the GUT leptogenesis and Affleck Dine baryogenesis. I argue in both these cases there are generic predictions of a primordial gravitational wave background that can be measured today. The presence of such a signal would lend plausibility to one of these scenarios. Finally I discuss the discriminating power of GWs in discerning the symmetry breaking path through the variable signals of hybrid defects.

11/11/2021, 16:00, Lecture Hall
Tomislav Prokopec (Utrecht University, Netherlands)
Quantum origin of dark energy and the Hubble tension
Local measurements of the Hubble parameter obtained from the distance ladder at low redshift are in tension with global values inferred from cosmological standard rulers. A key role in the tension is played by the assumptions on the cosmological history, in particular on the origin of dark energy. Here we consider a scenario where dark energy originates from the amplification of quantum fluctuations of a light field in inflation. We show that spatial correlations inherited from inflationary quantum fluctuations can reduce the Hubble tension down to one standard deviation, thus relieving the problem with respect to the standard cosmological model. Upcoming missions, like Euclid, will be able to test the predictions of models in this class.

04/11/2021, 16:30, Lecture Hall
Camilo Garcia Cely (DESY, Germany)
The CMB as a detector of gravitational waves
In complete analogy with axion dark matter, gravitational waves are converted into electromagnetic radiation when they propagate in magnetic fields. I will explain how this effect can be used to detect gravitational waves. With this in mind, I will examine gamma-ray observations by Fermi-LAT and HESS strongly suggesting the existence of a non-vanishing cosmic magnetic field. Then, I will show how the consequent conversion of gravitational waves into radio waves might distort the CMB, leading to bounds that exceed those from current terrestrial experiments. I will discuss prospects from the future gamma-ray observatory CTA and argue that future advances in 21 cm astronomy might push these bounds below the Neff constraint on the radiation density present during the CMB formation.

27/10/2021, 16:00, Lecture Hall
Stian Hartman (University of Oslo, Norway)
Self-interacting Bose-Einstein condensed dark matter; cosmological constraints and simulations
In this seminar I will talk about a particular kind of light scalar field dark matter, namely those with self-interactions, which I will call self-interacting Bose-Einstein condensed (SIBEC) dark matter. These have been found to possibly solve some of the small-scale issues of LCDM, such as the core-cusp problem, by providing an interaction pressure that supports hydrostatic halo cores on the order of kpc. Unlike their non-interacting ultra-light counterparts (fuzzy dark matter), there is not yet a large body of work dedicated to providing constraints on the strength of the SIBEC-DM self-interaction using large-scale observables. I will present such constraints, which weakly rules out the self-interactions generally thought to be needed to solve the cusp-core problem in the simplest scenario of SIBEC-DM. I will also talk about ongoing efforts to study structure formation in a SIBEC-DM universe using cosmological simulations.

21/10/2021, 16:00, Lecture Hall
Farbod Hassani (University of Oslo, Norway)
Characterizing the non-linear evolution of dark energy and modified gravity models
Understanding the reason behind the observed accelerating expansion of the Universe is one of the most notable puzzles in modern cosmology, and conceivably in fundamental physics. In the upcoming years, near future surveys will probe structure formation with unprecedented precision and will put firm constraints on the cosmological parameters, including those that describe properties of dark energy. In light of this, in the first part of my talk, I'm going to show a systematic extension of the Effective Field Theory of Dark Energy framework to non-linear clustering. As a first step, we have studied the k-essence model and have developed a relativistic N-body code, k-evolution. I'm going to talk about the k-evolution results, including the effect of k-essence perturbations on the matter and gravitational potential power spectra and the k-essence structures formed around the dark matter halos. In the second part of my talk, I'm going to show that for some choice of parameters the k-essence non-linearities suffer from a new instability and blow up in finite time. This talk will be based on the following publications and an ongoing work: arXiv:2107.14215, arXiv:2007.04968, arXiv:1910.01105, arXiv:1910.01104, arXiv:1906.04748.

14/10/2021, 16:00, Lecture Hall
Roman Konoplya (Peoples' Friendship U., Moscow, Russia)
Traversable wormholes in General Relativity without exotic matter
I will start by briefly reviewing wormholes - exotic compact objects the interest to which has been recently largely revived. In [J. Blazquez-Salcedo, C. Knoll, E. Radu, Phys. Rev. Lett. 126 (2021) no.10, 101102] asymptotically flat traversable wormhole solutions were obtained in the Einstein-Dirac-Maxwell theory without using exotic matter. The normalizable numerical solutions found in this work above require a peculiar behavior at the throat: the mirror symmetry relatively the throat leads to the nonsmoothness of gravitational and matter fields. In particular, one must postulate the changing of the sign of the fermionic charge density at the throat requiring coexistence of particle and antiparticles without annihilation and posing a membrane of matter at the throat with specific properties. Apparently this kind of configurations could not exist in nature. We show that there are wormhole solutions, which are asymmetric relative to the throat and endowed by smooth gravitational and matter fields, being, thereby, free from all the above problems. This indicates that such wormhole configurations could also be supported in a realistic scenario.

30/09/2021, 16:00, Zoom
Anton Baleato Lizancos (UC Berkeley, USA)
Fundamental Physics with CMB Lensing and Delensing
Gravitational lensing of CMB photons by the matter distribution of the Universe can be both a blessing and nuisance. It's a blessing because of the way it can be harnessed to map the structures responsible for the deflections, and from this, constrain any physics affecting the growth of cosmic structure, such as the sum of the neutrino masses or dark matter. But lensing is also a nuisance because it generates B-mode polarization which obscures the highly-sought-after primordial signal associated with gravitational waves generated during cosmic inflation, our most accessible portal to physics near the GUT scale. In this talk, I will focus on key systematic effects that need to be controlled in order to harness the full potential of the Simons Observatory (SO), CMB-S4, and other upcoming experiments to make progress in these exciting areas. In the first part of my talk, I will briefly review the ways in which emission from galaxies and clusters can bias power spectra and cross-correlations of CMB lensing reconstructions, and describe our ongoing efforts to understand these biases analytically. Then, in the second part, I will explain how the lensing contamination to CMB B-modes can be removed — what is known as delensing — and discuss our recent findings regarding the performance of different delensing methods. I will also summarize preparatory work to delens SO data, and highlight biases to watch out for (and how to mitigate them) when the matter proxy used for delensing is either the cosmic infrared background or a lensing reconstruction derived from the CMB itself.

23/09/2021, 16:00, Lecture Hall
Jan Burger (University of Iceland, Reykjavik, Iceland)
Conservation of radial actions in time-dependent spherical potentials
The Hamiltonian of a particle orbiting in a static, spherically symmetric potential can be written as a function of only two actions, the angular momentum and the radial action, which is a conserved quantity in adiabatically evolving potentials (a so-called adiabatic invariant). For that reason, canonical action-angle coordinates are frequently used as a basis for perturbative calculations, for instance in Hamiltonian perturbation theory, when considering the long-term evolution of near-equilibrium systems. In impulsively (fast) evolving potentials, conservation of radial actions breaks down and actions change discontinuously by a non-deterministic amount. Here, I focus on the transition between adiabatically and impulsively evolution. I show that the evolution of radial actions in mildly time-dependent potentials is given by an oscillation around a constant value, the amplitude of which is set by the rate at which the potential changes. As a consequence, the evolution of a distribution of radial actions is governed by a diffusion equation. Based on the derived drift and diffusion coefficients, I qualitatively discuss the non-linear regime and demonstrate that the non-linear evolution of radial action distributions is given by an asymmetric drift towards lower actions. I illustrate the relevance of these results on two astrophysical examples, accretion onto a cold dark matter (CDM) halo and the cusp-core transformation in a dwarf-sized self-interacting dark matter (SIDM) halo.

 

Academic year 2000/2001

 

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

Time:  16:00

Place: Zoom seminar


Analytical and regular solutions in four-dimensional General Relativity representing multiblack hole systems immersed in external gravitational fields are discussed. The external fields background is composed by an infinite multipolar expansion, which allows to regularise the conical singularities of an array of collinear static black holes. Charged, Rotating, NUT and accelerating generalisations are presented. Limits to the binary Majumdar–Papapetrou, Bonnor–Swaminarayan and the Bičák–Hoenselaers–Schmidt metrics are recovered.

 

 

10/06/2021 -- Francis-Yan Cyr-Racine (University of New Mexico)

Time:  16:00

Place: Zoom seminar


Observations of dark matter structure at the smallest scales can tell us about physical processes taking place in the dark sector at very early times. Here, we point out that the presence of light degrees of freedom coupling to dark matter in the early Universe introduces a localized feature in the halo mass function. This leads to a mass function that is distinct in shape than either warm dark matter or cold dark matter, hence distinguishing these models from other leading classes of dark matter theories. We present analytical calculations of these mass functions and show that they closely match N-body simulations results. We also discuss the impact of these mass functions at high-redshift on the 21-cm signal from cosmic dawn. We briefly discuss how current constraints on the abundance of small-scale dark matter structure do not directly apply to these models due to the multi-scale nature of their mass function.

 

27/05/2021 -- Daniel Litim (University of Sussex, UK)

Time:  16:00

Place: Zoom seminar


Fixed points under the renormalisation group are key for a fundamental definition of quantum field theory. They can be free such as in asymptotic freedom of QCD, or interacting, such as in asymptotic safety. In this talk, I provide rigorous results for interacting UV and IR fixed points in general 4d QFTs with or without supersymmetry. I will also discuss the state of the art for fixed points in 4d quantum gravity with or without matter, including an overview of results and open challenges.

 

 

20/05/2021 -- Macarena Lagos (Columbia University, NY, USA)

Time:  16:00

Place: Zoom seminar


Gravitational waves (GWs) allow us to probe the content of the Universe and the behaviour of gravity on cosmological scales, through information contained in their propagation. For instance, the presence of dynamical fields interacting non-minimally with gravity may induce a non-trivial propagation of GWs, changing their propagation speed, dispersion relation, or detected amplitude, among others. In this talk, I will discuss particular cosmological scenarios where GWs interact with another tensor field, such as in the theory of massive bigravity. I will illustrate explicitly how the GW signal from a coalescence of black holes gets distorted during propagation, generating specific features such as echoes of the GW signal emitted. These strong features suggest that stringent constraints on interacting GWs can be placed with current and future GW detectors.

 

 

13/05/2021 -- Swetha Bhagwat (Sapienza, University of Rome, Italy)

Time:  16:00

Place: Zoom seminar


In this talk I will talk about a recent work where we propose a new test of GR. The gravitational waves emitted during the coalescence of binary black holes offers an excellent probe to test the behaviour of strong gravity at different length scales. In this work, we propose a test called the merger-ringdown consistency test that focuses on probing horizon-scale dynamics of strong-gravity using the binary black hole ringdowns. This test is a modification of the more traditional inspiral-merger-ringdown consistency test. I will present a proof-of-concept study of this test using simulated binary black hole ringdowns embedded in the Einstein Telescope-like noise. Furthermore, we use a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.

 

 

06/05/2021 -- Viviana Niro (APC Paris, France)

Time:  16:00

Place: Zoom seminar


The HAWC telescopes has recently revealed new spectra for gamma-ray sources in the Galactic plane. In this talk I will review the possibility of detecting these sources at KM3 detectors. I will consider, with particular emphasis, the 2HWC J1825-134 source. Amongst the HAWC sources, it is indeed the most luminous in the multi-TeV domain and therefore is one of the first that should be searched for with a neutrino telescope in the northern hemisphere. I will show the prospects to detect this source at the KM3NeT detector and comment on the possibilities for others neutrino telescopes. I will consider, moreover, the gamma-ray sources eHWC J1907+063, eHWC J2019+368 and 2HWC J1857+027. For these sources, I will show the prediction for neutrinos at the IceCube detector, presenting the calculation of the statistical significance, considering 10 and 20 years of running time, and I will comment on the current results reported by the collaboration.

 

 

29/04/2021 -- Emel Altas (Karamanoglu Mehmetbey University, Turkey)

Time:  16:00

Place: Zoom seminar


Using the time evolution equations of (cosmological) general relativity in the first order Fischer-Marsden form, we construct an integral that measures the amount of nonstationary energy on a given spacelike hypersurface in D dimensions. We also construct analytical initial data for a slowly moving and rotating black hole for generic orientations of the linear momentum and the spin. We solve the Hamiltonian constraint approximately and work out the properties of the apparent horizon and show the dependence of its shape on the angle between the spin and the linear momentum. In particular, a dimple, whose location depends on the mentioned angle, arises on the two-sphere geometry of the apparent horizon. We exclusively work in the case of conformally flat initial metrics.

 

 

22/04/2021 -- Teodor Borislavov Vasilev (Universidad Complutense de Madrid, Spain)

Time:  16:00

Place: Zoom seminar


The big rip, the little rip and the little sibling of the big rip are cosmological doomsdays predicted by some phantom dark energy models that could describe the future evolution of our own Universe. When the Universe evolves towards either of these future cosmic events, all bounded structures and, ultimately, space-time itself are ripped apart. Nevertheless, it is commonly belief that quantum gravity effects may smooth or avoid these (classical) singularities. In this talk I will review the occurrence of these rip-like events in the scheme of alternative metric $f(R)$ theories of gravity from both classical and quantum points of view. The quantum analysis will be performed in the framework of $f(R)$ quantum geometrodynamics. This is a canonical quantization procedure based on the Wheeler-DeWitt equation for the case of $f(R)$ theories of gravity. In this context, I will discuss the avoidance of these (classical) singularities by means of the DeWitt criterion.

 

 

15/04/2021 -- Famaey Benoit (Observatoire astronomique de Strasbourg, France)

Time:  16:00

Place: Zoom seminar


In this talk I will summarize the intriguing phenomenology associated to galaxy scaling relations, a phenomenology which is still challenging to understand in the standard context. I will show how the MOND paradigm of Milgrom naturally solves most of these puzzles. I will also show that it however comes with new puzzles. I will focus the end of the talk on some of the main constraints that should be taken into account for MONDian model-building, especially on galaxy cluster scales.

 

 

25/03/2021 -- Johannes Noller (Cambridge University, DAMTP, UK)

Time:  16:00

Place: Zoom seminar


Recent years have seen great progress in probing gravitational physics on a vast range of scales, from the very largest cosmological scales to the microscopic ones associated with high energy particle physics. In this talk I will give a whistle stop tour of some of the different physical systems we can use to learn more about gravity in this way, with a focus on how we can use them synoptically to learn more about dark energy. Stops will include gravitational waves emitted by binary systems, the cosmic microwave background, large scale structure formation, and (theoretical) bounds on the behaviour of gravity on scales inaccessible to current experiments.

 

 

18/03/2021 -- Kazuya Koyama (Portsmouth U., ICG, UK)

Time:  16:00

Place: Zoom seminar


Future galaxy surveys such as Euclid, LSST and SKA will cover larger and larger scales where general relativistic effects become important. On the other hand, our study of large scale structure still relies on Newtonian N-body simulations. I show how standard Newtonian N-body simulations can be interpreted in terms of the weak-field limit of general relativity. Our framework allows the inclusion of radiation perturbations and the non-linear evolution of matter. I show how to construct the weak-field metric by combining Newtonian simulations with results from Einstein-Boltzmann codes. I discuss observational effects on weak lensing and ray tracing, identifying important relativistic corrections. Finally, I show that this framework can be extended to gravitational theories beyond general relativity.

 

 

11/03/2021 -- Raissa Mendes (Universidade Federal Fluminenense, Brazil)

Time:  16:00

Place: Zoom seminar


In this talk, I will discuss how extreme properties that may be present in the interior of some neutron stars can turn them into unique laboratories for tests of modified theories of gravity. In particular, I will focus on the case of scalar-tensor theories with screening mechanisms. These theories offer an interesting framework for cosmology, since the scalar degree of freedom could help to drive the accelerated expansion of the universe, while screening off its effects in solar system scales, where general relativity is very well tested. Although it is typically understood that screening becomes more effective in high density environments, we will show in a few interesting models how it can actually fail in the densest places in nature - the core of some neutron stars.

 

 

04/03/2021 -- Ali Seraj (Universite Libre de Bruxelles)

Time:  16:00

Place: Zoom seminar


In this talk, I will review the gravitational wave memory effect in Einstein general relativity and discuss its relation to BMS symmetries. Then I will consider Brans-Dicke theory containing an additional gravitational degree of freedom. This mode is associated with novel memory effects. However, being a scalar, it is not obvious which symmetry this memory corresponds to. I will show that the memories associated with the breathing mode correspond to the asymptotic symmetries of a dual 2 form representation of the scalar field.

 

 

18/02/2021 -- François Larrouturou (IAP Paris, France)

Time:  16:00

Place: Zoom seminar


In 2011, F. Hassan and R. Rosen achieved the construction of the first gosth-free theory of two interacting spin-2 fields. But despite its great elegance and interesting phenomenological implications, this theory of "bigravity" suffers from a gradient-type instability. Moreover, it postulates the existence of vectorial and scalar gravitational modes, the latter being severely constrained by observations of binary pulsars. This stimulated us to construct a "minimal" theory of bigravity, ie. a theory of two interacting spin-2 fields that propagates only four tensorial degrees of freedom. This talk will first introduce the motivations that led to this "minimal" theory of bigravity, and review its construction by a Hamiltonian procedure. I will then present its cosmological phenomenology, and show that it provides a stable nonlinear completion of the cosmology of the Hassan-Rosen bigravity. We will end by discussing interesting phenomenological features and possible ways to test the theory.

 

 

11/02/2021 -- Ondrej Hulik (Charles University, Prague)

Time:  16:00

Place: Zoom seminar


I will review the formulation of generalized and exceptional geometry as the underlying geometry of supergravity and M-theory. With suitable generalized framework one can treat these distict types of geometries as a special case of one object "G-algebroid". I will sketch main underlying idea and how is this formulation useful in treating T/U duality.

 

 

04/02/2021 -- Giulia Cusin (Universite de Geneve, Switzerland)

Time:  16:00

Place: Zoom seminar


There are two possible approaches to describe a population of astrophysical gravitational wave (GW) sources: one can focus on high signal-to-noise sources that can be detected individually, and build a catalogue. Alternatively, one can take a background-approach and study the incoherent superposition of GW signals emitted by the entire population (both resolved and unresolved sources) from the onset of stellar activity until today. A detailed description of signals from resolvable sources, and of the properties of a stochastic background, including propagation effects, is crucial to extract accurate information on the underlying source population. Moreover, these two observables contain complementary astrophysical information and, once combined, they can provide insight on the properties of a faint and distant sub-population that cannot be accessed with any other means of observation. In my talk I will outline the differences and the complementarity of these two approaches, from the point of view of observations and of theoretical modeling, and stress a few caveats to be kept in mind when deriving predictions to be compared with (present and future) datasets.

 

 

28/01/2021 -- Paolo Benincasa (IFT Madrid, Spain)

Time:  16:00

Place: Zoom seminar


QFT in nearly dS space-times and, more generally, in FRW backgrounds allows us to describe correlations at the end of inflation. However, how to extract fundamental physics out of them is still a challange: we do not even know how fundamental pillars such as causality and unitarity of the time evolution constrain them. In this talk I will present a recent program which aims to construct the wavefunction of the universe, which generates these correlations, directly from first principles without making any reference to time evolution: these observables naturally live at the boundary of the nearly dS/FRW space-times and the time evolution is integrated out. I will discuss two approaches: one approach aims to construct the wavefunction from the knowledge of its general analytic properties, in a similar fashion as scattering amplitudes in flat space-time (which can be formulated directly from on-shell data with no reference to fields whatsoever); the second approach aims to find new mathematical objects, with their own first principle definition, which has the very same properties we ascribe to the wavefunction of the universe, with all the basic physical principles such as causality and unitarity, emerging from their intrinsic definition.

 

 

21/01/2021 -- Timothy Anson (Universite Paris-Saclay, France)

Time:  16:00

Place: Zoom seminar


Starting from a recently constructed stealth Kerr solution of higher order scalar tensor theory, I will discuss disformal versions of the Kerr spacetime with a constant disformal factor and a regular scalar field. While the disformed metric has only a ring singularity and asymptotically is quite similar to Kerr, it is neither Ricci flat nor circular. Non-circularity has far reaching consequences on the structure of the solution. In particular, I will discuss the properties of important hypersurfaces in the disformed spacetime: ergosphere, stationary limit and event horizon, and highlight the differences with the Kerr metric.

 

 

14/01/2021 -- Siddharth Prabhu (Tata Institute of Fundamental Research, India)

Time:  16:00

Place: Zoom seminar


In the last couple of decades, we have learnt a great deal about quantum gravity and its holographic nature in asymptotically AdS spacetimes. Here, we explore this idea in asymptotically flat spacetimes with the following question: Can an observer on the boundary of the spacetime distinguish between two states that are deemed distinguishable by an observer in the bulk? We argue that semiclassical gravity is an effective tool to answer this question, and extrapolate its results to make a few reasonable assumptions regarding the low energy structure of any complete theory of quantum gravity. Using these, we argue that an asymptotic observer indeed has access to all the information in a theory with only massless bulk excitations, provided they are at the past boundary of future null infinity. We also show that information available in any cut of future null infinity is also available in any later cut, but the converse doesn't hold. Similar results hold for past null infinity. We will comment on several interesting questions that this line of investigation sheds light on.

 

17/12/2020 -- Oksana Iarygina (Leiden University, NL)

Time:  16:00

Place: Zoom seminar


The reheating era in the early universe, that connects inflation and big-bang nucleosynthesis, is still very weakly constrained. However, inefficient preheating can lead to a prolonged matter-dominated phase after inflation, changing the time during inflation when the Cosmic Microwave Background (CMB) modes exit the horizon. This shifts CMB predictions and thus can break the degeneracy of otherwise indistinguishable inflation models. Typically, models that allow a UV completion, include many distinct fields, often with curved field-space manifolds. Based on arXiv:2005.00528, arXiv:1810.02804, the present talk focuses on the physical mass scales that control the dynamics and observable predictions of all multi-field models with a non-zero field-space curvature: the Hessian of the potential, the turning rates of the trajectory and the field-space curvatures. We analyse how their interplay affects reheating and shifts inflationary predictions. We also demonstrate the existence of a region in parameter space, where the symmetric and asymmetric multi-field alpha-attractors, that are known by the universality of their single-field inflationary predictions, are explicitly not the same: one preheats and one does not. This leads to a different cosmic history for the two models, with one possibly exhibiting a long matter-dominated phase, and a shift in the observational predictions for ns and r.

 

 

10/12/2020 -- Horng Sheng Chia (Institute for Advanced Study, Princeton, USA)

Time:  16:00

Place: Zoom seminar


Black holes are never isolated in realistic astrophysical environments; instead, they are often perturbed by complicated external tidal fields. How does a black hole respond to these tidal perturbations? In this talk, I will discuss both the conservative and dissipative responses of the Kerr black hole to a weak and adiabatic gravitational field. The former describes how the black hole would change its shape due to these tidal interactions, and is quantified by the so-called “Love numbers”. On the other hand, the latter describes how energy and angular momentum are exchanged between the black hole and its tidal environment due to the absorptive nature of the event horizon. I will show that the Love numbers of the Kerr black hole vanish identically — in other words, you cannot stretch a black hole. I will also describe how the Kerr black hole's dissipative response implies that energy and angular momentum can either be lost to or extracted from the black hole, with the latter process commonly known as the black hole superradiance. I will end by discussing how these tidal responses leave distinct imprints on the gravitational waves emitted by binary black holes.

 

 

03/12/2020 -- Antony Lewis (University of Sussex, UK)

Time:  16:00

Place: Zoom seminar


Cosmological measurements from the cosmic microwave background, large-scale structure, lensing, supernovae and other data are now able to constrain multiple cosmological parameters to percent-level precision within in the context of the standard Lambda-CDM cosmology. Disagreements between these measurements assuming Lambda-CDM could provide strong evidence for beyond-Lambda-CDM physics. I review the status of current measurements and their agreement (or otherwise) within the standard cosmological model. I’ll mention some possible types of model extensions that could help to resolve H0 tensions and how new physics might be pinned down by forthcoming data. Galaxy and CMB lensing provide interesting complementary constraints; I’ll show the state-of-the-art results from CMB lensing, compare with galaxy lensing and discuss possible implications.

 

 

26/11/2020 -- Eiichiro Komatsu (Max-Planck-Institute for Astrophysics, Germany)

Time:  16:00

Place: Zoom seminar


Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics. In this presentation we report on a new measurement of parity violation from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.

 

19/11/2020 -- Mikhail Shaposhnikov (EPFL)

Time:  16:00

Place: Zoom seminar


It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it incorporates Higgs inflation at energies below the onset of the strong-coupling of the theory. Second, it contains a four-fermion interaction that originates from torsion associated with spin degrees of freedom. This interaction leads to a novel universal mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. Finally, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect.

 

12/11/2020 -- William Barker (Kavli Institute for Cosmology, Cambridge, UK)

Time:  16:00

Place: Zoom seminar


Several novel Poincare gauge theories of gravity (curvature and torsion) were recently found to be unitary/power-counting renormalizable in the weak regime, and pass solar system tests [1,2]. We show these theories contain LCDM as an attractor state, despite neither the Einstein-Hilbert or cosmological constant terms appearing the action: the only extra parameter (xLCDM) adds effective dark radiation to relieve the Hubble tension [3]. We show that the phenomenology of the general ten-parameter theory, including novel theories, can be easily understood through a non-canonical bi-scalar-tensor analogue [4]. We discuss ongoing Dirac-Bergmann analysis of the Hamiltonian in the strong regime.([1] arXiv:1812.02675, [2] arXiv:1910.14197, [3] arXiv:2003.02690, [4] arXiv:2006.03581)

 

 

05/11/2020 -- Fedor Bezrikov (University of Manchester, UK)

Time:  16:00

Place: Zoom seminar


I'll make a review of the attempts to understand a seemingly simple process of reheating in Higgs inflation. Although reheating can be readily expected to happen at rather high temperatures, its details leave imprint on the number of inflationary e-foldings and, thus, on predictions for CMB parameters. The quest for the understanding reheating took some time, starting form simple, but incomplete approach, evolving into realisation that careful study in the strong coupled regime is inevitable. The approach of perturbative UV completion of the model by $R^2$ inflation was hoped to provide immediate answer to preheating dynamics in a weakly coupled theory, but turned into an ongoing study of evolution in non-linear potentials. I will also mention that in some cases even pure non-regularised Higgs inflation can allow for calculable predictions for preheating.

 

 

 

22/10/2020 -- Eugene Lim (King's College London, UK)

Time:  16:00

Place: Zoom seminar


Inflation is now the paradigmatic theory of the Big Bang. But is it deserved? I will describe the conceptual and theoretical challenges that Inflation is still facing, argue that we should keep an open mind. In particular, I will argue that while it is a theory that claims to be a theory of initial conditions of the Universe, successful inflation actually depends on an intimate interplay between its own initial conditions and the inflationary model. I will show how one might go about probing this interplay by testing whether inflation can begin if its own initial conditions are not homogenous.

 

 

 

15/10/2020 -- Filippo Camilloni (University of Perugia, IT and Niels Bohr Institute, DK)

Time:  16:00

Place: Zoom seminar


Force-free electrodynamics is a non-linear regime of Maxwell’s equations often employed to provide a minimal non-trivial level of description for pulsar and black hole magnetospheres. For a solution of this system to be physically meaningful the field has to be magnetically dominated, Fˆ2=Bˆ2-Eˆ2>0, however no analytic solution is known to respect this requirement in the background of a highly-spinning black hole. In this talk I will show how the Near-Horizon Extreme Kerr (NHEK) region might play a crucial role for the construction of sensible models of extreme Kerr magnetospheres . Any stationary and axisymmetric force-free solution in the extreme Kerr background is observed to converge to an attractor in the NHEK region. We used this attractor as an universal starting point to develop a new perturbative approach, showing that at the second order in perturbation theory it is possible to find magnetically-dominated force-free fields. A similar attractor mechanism occurs in the Near-Horizon Near-Extreme Kerr (near-NHEK) region of a nearly-extreme Kerr black hole, thus providing a way to extend this formalism outside extremality.

 

 

08/10/2020 -- Anne Green (University of Nottingham, UK)

Time:  16:00

Place: Lecture Hall


Primordial Black Holes (PBHs) are black holes formed in the early Universe, for instance from the collapse of large density perturbations generated by inflation. The discovery of gravitational waves from mergers of ~10 Solar mass black hole binaries has led to increased interest in PBHs as a dark matter candidate. I will review the formation of PBHs and the limits on their abundance, with particular emphasis on microlensing constraints in the Solar mass region. I will also emphasise key open questions in the field (e.g. clustering, and methods for constraining asteroid mass PBHs).

 

 

12/03/2020 -- Keigo Shimada (Tokyo Institute of Techonology)

Time:  16:00

Place: 226


Scalar-tensor theories in metric-affine geometry are formulated. General Relativity is currently the most successful gravitational theory which has surpassed countless observations. However, in recent years, it has been noticed that GR cannot explain some cosmological phenomena such as inflation, dark energy and dark matter. To solve this, countless alternative gravitational theories beyond General Relativity has been proposed. However, most require the geometry to be Riemannian, just as in GR. In this talk, it will be shown how one could extend theories of gravity by 'deforming' Riemann Geometry into what is called metric-affine geometry, in which not only the metric but also that connection is an independent variable that is decided from the gravitational action. By applying metric-affine formalism to scalar-tensor theories, one notices that there are different and fruitful characteristics that appear when compared to the Riemann counterpart. Especially, through the novel symmetry of the connection called 'projective symmetry', one may find natural ways to eliminate ghosts that are caused by higher derivatives. Finally, some possible applications would be discussed. References: Phys.Rev. D98 (2018) no.4, 044038 Phys. Rev. D 100, 044037 (2019).

 

 

27/02/2020 -- Elias Kiritsis (APC, Paris & Crete University)

Time:  16:00

Place: 226


I will discuss ideas on how gravity can be an emergent interaction in QFT, what guarantees the emergent diffeomorphism invariance, what are its general features and properties and what could be the possible implications for realistic gravitational physics.

 

02/03/2020 -- Tarek Anous (Amsterdam University)

Time:  14:00

Place: 226


The BFSS matrix model provides an example of gauge-theory / gravity duality where the gauge theory is a model of ordinary quantum mechanics with no spatial subsystems. If there exists a connection between areas and entropies in this model similar to the Ryu-Takayanagi formula, the entropies must be more general than the usual subsystem entanglement entropies. I will give a brief overview of the BFSS/D0 brane geometry duality and describe general features of the extremal surfaces in the bulk. I will then discuss the possible entropic quantities in the matrix model that could be dual to the ‘regulated areas’ (which I will define) of these extremal surfaces.

 

20/02/2020 -- Georgios Loukes-Gerakopoulos (Astronomical Institute, Prague)

Time:  16:00

Place: 226


In this talk three different studies we have undertaken to address cosmological issues, like dark energy, will be presented and their results will be discussed. In these studies we have tried to remain model agnostic as much as possible. In particular, our first study (arxiv:1902.11051) concerns a cosmic fluid obeying rest-mass conservation of unspecified equation of state (EoS) in an unspecified background assuming just that the fluid's speed of sound is positive and less than the speed of light. Our second study (arXiv:2001.00825) performs a dynamical analysis of a barotropic fluid of unspecified EoS with positive energy density in spatially curved Friedmann-Robertson-Walker (FRW) spacetimes. While, the third study (arXiv:1905.08512) performs a dynamical analysis of a broad class of non-minimally coupled real scalar fields in spatially curved FRW spacetime with unspecified positive potential.

 

13/02/2020 -- Paolo Creminelli (ICTP, Trieste)

Time:  16:00

Place: 226


Cosmic inflation makes the universe flat and homogeneous, but under which conditions inflation will start? I will discuss some analytical results that show, with very weak assumptions, that inflation starts somewhere and some (partial) results about a de Sitter ho-hair theorem.

 

30/01/2020 -- Tomas Ledvinka (Charles University, Prague)

Time:  16:00

Place: 226


Despite the tremendous success of mathematical general relativity which revealed among others surprising features of the geometry of rotating (Kerr) black holes and developed approximation techniques to study early stages of their inspiral, the necessity to describe completely the merger of two black holes lead to a substantial progress of numerical relativity. This field necessarily uses techniques of modern computer science to amass and command number crunching capabilities of current computers as well as numerical methods for partial differential equations, but the successful computer simulations also required new type of answers to questions "what is the black hole" and "what kind of equations are the Einstein ones". From this perspective I will also mention some results on hyperbolicity analysis of 3+1 reductions of Einstein equations and coordinate choice and horizon formation for collapse of gravitational waves into a black hole.

 

29/01/2020 -- Harold Erbin (University of Turin)

Time:  14:00

Place: 226


Machine learning has revolutionized most fields it has penetrated, and the range of its applications is growing rapidly. The last years has seen efforts towards bringing the tools of machine learning to lattice QFT. After giving a general idea of what is machine learning, I will present two recent results on lattice QFT: 1) computing the Casimir energy for a 3d QFT with arbitrary Dirichlet boundary conditions, 2) predicting the critical temperature of the confinement phase transition in 2+1 QED at different lattice sizes.

 

23/01/2020 -- Chris Clarkson (Queen Mary University of London)

Time:  16:00

Place: 226


Over the coming decade new surveys will map the cosmos over huge volumes. This will allow us to probe general relativity on unprecedented scales. I shall discuss some of the new relativistic effects that may be significant on these scales. Though corrections to the Newtonian picture of observations of structure formation are small, they should be detectable, and offer new insights into gravity on scales approaching the horizon.

 

16/12/2019 -- Pavel Motoloch (CITA, Toronto)

Time:  16:00

Place: 226


Measurements of galaxy angular momenta can, at least in principle, be used to probe fundamental physics such as primordial gravitational waves and non-Gaussianity. In my talk I explain how galaxy spins arise from the initial density perturbations, describe how they are sensitive to various physical parameters of interest and finally detail our related observational effort.

 

12/12/2019 -- Filippo Vernizzi (Universite Paris Saclay)

Time:  16:00

Place: 226


The observed accelerated expansion of the Universe opens up the possibility that general relativity is modified on cosmological scales. While this has motivated the theoretical study of many alternative theories that will be tested by the next generation of cosmic large-scale structure surveys, I will show that the recent observations of gravitational waves by LIGO/Virgo has dramatic consequences on these theories.

 

28/11/2019 -- Guillermo Ballesteros (Autonoma University, Madrid)

Time:  16:00

Place: 226


I will discuss the idea that black holes may constitute a large fraction of the Universe’s dark matter, focusing mostly on their formation from large primordial fluctuations generated during inflation. I will summarize the ups and downs of this mechanism and explain some ideas that help to alleviate its main shortcomings.

 

21/11/2019 -- Giovanni Acquaviva (Charles University, Prague, CZ)

Time:  16:00

Place: 226


It is known that the entropy of a system contained in a certain volume is bounded from above by the entropy of a black hole with corresponding surface area.  We relate such universal bound to the existence of fundamental degrees of freedom and provide model-independent considerations about their features.  In particular, both geometry and fields propagating on it are seen as phenomena emergent from the more fundamental dynamics, in analogy with many examples in condensed matter physics.  An immediate consequence is that, even though the fundamental evolution is considered unitary, the fields develop an entanglement with the spacetime geometry, hence leading to an effective non-unitary evolution on the emergent level.  We exemplify some consequences of this scenario by providing a toy-model of black hole evaporation: the entanglement between geometry and fields is interpreted at our low-energy scales as an effective loss of information in Hawking radiation.  A question currently under scrutiny is how can unitary and continuum quantum field theory emerge from such fundamental picture.


04/11/2019 -- Dionysios Anninos (King's College London, UK)

Time:  16:00

Place: 226

We discuss quantum fields on a Euclidean sphere and their relation to de Sitter space. Various cases are considered, including particles of different spins, and masses. Some emphasis will be placed on the three-dimensional case. Time permitting, we will consider higher spin theories.






17/10/2019 -- Sk Jahanur Hoque (Chennai Mathematical Institute, India)

Time:  16:00

Place: 226

We discuss different notion of charges for asymptotically de Sitter space-time. We present a covariant phase space construction of hamiltonian generators of asymptotic symmetries with `Dirichlet' boundary conditions in de Sitter spacetime, extending a previous study of J\"ager. We show that the de Sitter charges so defined are identical to those of Ashtekar, Bonga, and Kesavan (ABK). We then present a comparison of ABK charges with other notions of de Sitter charges. We compare ABK charges with counterterm charges, showing that they differ only by a constant offset, which is determined in terms of the boundary metric alone. We also compare ABK charges with charges defined by Kelly and Marolf at spatial infinity of de Sitter spacetime. When the formalisms can be compared, we show that the two definitions agree.






10/10/2019 -- Cosimo Bambi (Fudan University)

Time:  16:00

Place: 226

Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. However, the strong field regime is still largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations only when gravity becomes strong. X-ray reflection spectroscopy is potentially a powerful tool for testing the strong gravity region around astrophysical black holes with electromagnetic radiation. In this talk, I will present the reflection model RELXILL_NK designed for testing the metric around black holes and the current constraints on possible new physics from the analysis of a few sources.






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

Time:  14:00

Place: 226

Ultraviolet consistency of quantum gravitational theories requires the presence of new states at or below the Planck scale. In the setting of AdS3/CFT2, this statement follows from the modular bootstrap. It has been a long-standing problem to improve the best upper bound on the mass of the lightest non-graviton state in this context. I will explain how this can be done using the "analytic extremal functionals", which were originally developed for the four-point bootstrap in 1D. The new analytic upper bound on the dimension of the lightest nontrivial primary is c/8.503... at large c (central charge) -- an improvement over the previous best bound c/6 due to Hellerman. I will also explain that the sphere packing problem of Euclidean geometry can be studied using a version of the modular bootstrap. The analytic functionals apply also in this context. They lead directly to the recent solution of the sphere-packing problem in 8 and 24 dimensions due to Viazovska and Cohn+Kumar+Miller+Radchenko+Viazovska.

The talk will be based on https://arxiv.org/pdf/1905.01319.pdf

 



02/10/2019 -- Thales Azevedo (Institute of Physics - UFRJ)

Time:  11:00

Place: 226

Recently, a gauge theory built out of dimension-six operators such as (DF)^2 appeared in the double-copy construction of conformal supergravity amplitudes. In this talk, I will show how theories of that kind are related to conventional, sectorized and ambitwistor string theories.

 

 


01/10/2019 -- Poulami Nandi (Indian Institute of Technology Kanpur, India)

Time:  14:00

Place: 117

Conformal Carrollian groups are known to be isomorphic to Bondi-Metzner-Sachs (BMS) groups that arise as the asymptotic symmetries at the null boundary of Minkowski spacetime. The Carrollian algebra is obtained from the Poincare algebra by taking the speed of light to zero, and the conformal version similarly follows. In this paper, we construct explicit examples of Conformal Carrollian field theories as limits of relativistic conformal theories, which include Carrollian versions of scalars, fermions, electromagnetism, Yang-Mills theory and general gauge theories coupled to matter fields. Due to the isomorphism with BMS symmetries, these field theories form prototypical examples of holographic duals to gravitational theories in asymptotically flat spacetimes. The intricacies of the limiting procedure lead to a plethora of different Carrollian sectors in the gauge theories we consider. Concentrating on the equations of motion of these theories, we show that even in dimensions d = 4, there is an infinite enhancement of the underlying symmetry structure. Our analysis is general enough to suggest that this infinite enhancement is a generic feature of the ultra-relativistic limit that we consider.





30/09/2019 -- Antoine Bourget (Imperial College London, UK)

Time:  14:00

Place: 226

I will explore the geometrical structure of Higgs branches of quantum field theories with 8 supercharges in 3, 4, 5 and 6 dimensions. They are symplectic singularities, and as such admit a decomposition (or foliation) into so-called symplectic leaves, which are related to each other by transverse slices. We identify this foliation with the pattern of partial Higgs mechanism of the theory and, using brane systems and recently introduced notions of magnetic quivers and quiver subtraction, we formalise the rules to obtain the Hasse diagram which encodes the structure of the foliation.

 


 


19/09/2019 -- Gizem Sengor (Czech Academy of Sciences, CZ)

Time:  16:00

Place: 226


The symmetry group of de Sitter, can accommodate fields of various mass and spin among its unitary irreducible representations. These unitary representations are labeled by the spin and scaling dimension. The scaling dimension depends on the mass and spin of the field and can have purely imaginary values. This talk focuses on scalar fields on de Sitter and aims to show that even the purely imaginary weights correspond to unitary operators on de Sitter, which seems contrary to the case on Anti de Sitter.

By studying the late time limit of scalar field solutions with different masses (conformally coupled, heavy and light fields); we identify the unitary representations they correspond to with respect to their scaling dimension and recognize them as late time boundary operators. The definition for a positive definite inner product on de Sitter is subtle. For operators with real scaling dimension it involves a so called intertwining operator. By carefully accounting for the presence or the absence of the intertwining operator we show that all of the identified boundary operators have positive definite norm and are thus unitary representations.

 



  

12/08/2019 -- Camilo Garcia-Cely (DESY, Hamburg)

Time:  14:00

Place: 226

In this talk, I will discuss MeV spin-2 dark matter. In particular, I will show that such a particle typically self-interacts and undergoes self-annihilations via 3-to-2 processes. I will discuss its production mechanisms and also identify the regions of the parameter space where self-interactions can alleviate the discrepancies at small scales between the predictions of the collisionless dark matter paradigm and cosmological N-body simulations.





  

01/08/2019 -- Andrei Frolov (Simon Fraser University)

Time:  16:00

Place: 226

The simple story of primordial gravitational waves produced by inflation sourcing B-modes of Cosmic Microwave Background polarization in reality is complicated by the fact that we are looking through the coloured glass of astrophysical foregrounds originating much closer to home. I will talk about amplitude of primordial B-modes we expect from inflation, characterization of polarized dust foregrounds from Planck Legacy data, and where do we go from here. In particular, I will show new reconstruction of the large-scale galactic magnetic field responsible for the patterns we see in the dust polarization, and explain how it allows accurate modelling of the polarized dust emission for design and analysis of future CMB experiments.





  

22/07/2019 -- Agnes Ferte (Jet Propulsion Laboratory, Pasadena, USA)

Time:  14:00

Place: 226

The universe has been going through a phase of accelerated expansion for the last 6 billion years. Understanding the origin of this cosmic acceleration is one of the main goals of observational cosmology: is it caused by a cosmological constant or a dynamical dark energy? Or is it a sign that we don’t understand the laws of gravity on cosmological scales? In this talk I will first describe weak lensing, a powerful observable that helps addressing these fundamental questions. I will then give an overview of the current experimental context for weak lensing. In the main part of the talk, I will present my results on tests of gravity on large scales through weak lensing and end by presenting the Precision Projector Laboratory, which goal is to characterize the new generation of detectors that will be used in future galaxy surveys.





  

19/07/2019 -- Xingang Chen (Harvard University)

Time:  14:00

Place: 226

How to model-independently distinguish the inflation scenario from alternatives to inflation, as the origin of the Big Bang Cosmology, is an important challenge in modern cosmology. In this talk, we show that massive fields in the primordial universe function as standard clocks and imprint clock signals in the density perturbations, which directly record the scale factor of the universe as a function of time, a(t). This function is the defining property of any primordial universe scenario, so can be used to identify the inflation scenario, or one of its alternatives, in a model-independent fashion. The signals also encode the mass and spin spectra of the particle physics at the energy scale of the primordial universe.





  

16/07/2019 -- Leonardo Modesto (SUSTech, Shenzhen, China )

Time:  14:00

Place: 117

In order to have a unitary and finite quantum gravity, we propose a weakly nonlocal completion of the Einstein-Hilbert's action compatible with causality (a Shapiro's time advance never occurs in Nonlocal Gravity). As a consequence of finiteness, there is no Weyl anomaly and the theory turns out to be conformal invariant at classical as well at quantum level. Therefore, finite nonlocal quantum gravity is a conformal invariant theory in the spontaneously broken phase of the Weyl symmetry. The coupling to matter enjoy the same properties with and without supersymmetry. As an application, Weyl conformal symmetry solves the black hole's singularity issue and the cosmological singularity problem, otherwise unavoidable in a generally covariant local or non-local gravitational theory. Following and extending the seminal paper by Narlikar and Kembhavi, we are able to provide explicit examples of singularity free black hole exact solutions. The absence of divergences is based on the finiteness of the curvature invariants and on the geodesic completion. Indeed, no massive or massless particles can reach the former singularity in a finite amount of proper time or of affine parameter.





  

15/07/2019 -- Mirek Rapcak (Perimeter Institute, Canada)

Time:  11:00

Place: 226

I will discuss generalizations of the $\mathcal{W}_{1+\infty}$ algebra denoted as $\mathcal{W}_{m|n\times \infty}$ generated by a super-matrix of fields for each integral spin $i=1,2,3,\dots$. Truncations of the algebra are in correspondence with holomorphic functions on singular Calabi-Yau three-folds given by the zero locus of $xy=z^mw^n$. I propose a free-field realization of such truncations generalizing the Miura transformation for $\mathcal{W}_N$ algebras. Relations in the ring of holomorphic functions lead to bosonisation-like relations between different free-field realizations. The algebras are expected to be AGT dual to gauge theories supported on divisors corresponding to the zero locii of such holomorphic functions. The discussion uncovers many non-trivial relations between vertex operator algebras, algebraic geometry and gauge theory.





  

11/07/2019 -- David Svoboda (Perimeter Institute, Canada)

Time:  14:00

Place: 226

We present a para-complex analogy of the Generalized Kähler (GK) geometry, generalized para-Kähler (GpK) geometry. We show that similarly to GK geometry describing targets of 2D (2,2) supersymmetric sigma models, the GpK geometry describes the targets of (2,2) twisted supersymmetric sigma models. We then discuss topological twists of such sigma models. Because the involved geometries are para-complex, they provide new examples -- in particular of topological theories -- on manifolds that are not complex, contrary to the usual (2,2) case.





  

08/07/2019 -- Adolfo Cisterna (Chile University)

Time:  14:00

Place: 226

In this talk a new method for the construction of homogenous black strings is shown. The method, which is based on a particular scalar-dressing of the extra dimensions of the spacetime under consideration, allow us to construct the black string generalization of the AdS Schwarzschild black hole in any dimension in General Relativity. Furthermore the method can be generalized to provide the black string extension of the Boulware-Deser black hole, or the black string extension of any black hole contained in the Lovelock theory. It will be also discussed how to construct black string with non-trivial matter fields.





  

04/07/2019 -- Cesar Arias (Riemann Center for Geometry and Physics (Leibniz University of Hannover))

Time: 14:00

Place: 226

It has been proposed that Vasiliev’s nonlinear equations can be extracted from a cubic action principle of the Chern–Simons type, built up from a set of differential forms, a trace operation and a star product inherited from the associative algebra, and a nilpotent differential containing the (gauged) de Rham differential. In this talk, we argue that all of these algebraic structures can be naturally modeled by a class of two-dimensional topological models, referred to as differential Poisson sigma models, which we analyse in some detail.





  

24/06/2019 -- Andrea Fontanella (Instituto de Fisica Teorica UAM/CSIC, Madrid)

Time:  14:00

Place: 226

I will present how we found the hidden relativistic symmetry in the context of AdS2 and AdS3 integrable superstring theories (arXiv:1903.10759). Then I shall discuss how such symmetry can be used in AdS3 to write down the Thermodynamic Bethe Ansatz for massless non-relativistic modes from the one available in literature for massless relativistic modes.





  

10/06/2019 -- Dmitry Gorbunov (Institute for Nuclear Research, Moscow)

Time:  14:00

Place: 226

Inflation can explain why the Universe is flat and homogeneous at large scales. However, it is not falsifiable unless also responsible for the matter perturbations sourcing the cosmic structure formation and anisotropy of cosmic microwave background. Moreover, even in that case different models often give (almost) the same predictions for the cosmological spectra, and it would be nice to test these inflationary models in other ways. The Higgs inflation is one of the examples naturally providing with such independent tests. A recently suggested modification with $R^2$-term solves the strong coupling problem in the original Higgs inflation allowing for perturbative matching of high-energy and low-energy model coupling constants, which is required to perform such direct tests. A remarkable feature of the model is instant preheating due to tachyonic instabilities in Higgs and vector boson sectors, which ask for a special study.





  

07/06/2019 -- Mairi Sakellariadou (King's College London)

Time:  14:00

Place: 226

The direct detections of Gravitational Waves (GWs) by the Advanced LIGO and Advanced Virgo interferometers have opened a new era of astronomy. Aside the current detections associated with individual loud events, one expects a superposition of coincident unresolved events leading to a stochastic GW background (SGWB). After reviewing briefly the SGWB, I will discuss how the anisotropic distribution of sources and the inhomogeneous geometry of the intervening spacetime can induce anisotropies. I will consider a SGWB produced by (1) cosmic strings and (2) by compact binary coalescences. I will show that while the SGWB monopole is sensitive to the particular model one uses, the anisotropic angular power spectrum is basically insensitive to the cosmic string model or the nature of binary black holes population. I will then discuss the noise in the anisotropies of the astrophysical GW background sourced by the finite sampling of both the galaxy distribution and the compact binary coalescence event rate.





  

04/06/2019 -- Tanmay Vachaspati (Arizona State University)

Time:  13:30

Place: FZU Lecture Hall

We show that certain quantum systems in non-trivial classical backgrounds can be mapped into entirely classical systems in higher dimensions. The evolution of the classical system can be used to obtain the particle production rate as well as the quantum backreaction on the classical background. The technique has many potential applications, including breather/oscillon dynamics, Hawking radiation and black hole evaporation, and particle production during inflation.





  

21/05/2019 -- Julian Adamek (Queen Mary University of London)

Time:  16:00

Place: 117

I will give a brief overview of the latest code release v1.2 of gevolution, with particular attention to the new features that facilitate the analysis on observers' past light cones. This provides a framework to include all interesting relativistic contributions in the prediction of large-scale structure observables.





  

13/05/2019 -- Tomislav Prokopec (Utrecht University)

Time:  14:00

Place: 226

In this talk I will give an introduction on how to compute quantum corrections in inflation. I will review quantum effects in interacting scalar theories, scalar quantum electrodynamics and summarize on the quantum one loop corrections to dynmical gravitons and scalar gravitational potentials. Understanding these type of corrections is of crucial importance for our understanding of how large can be the quantum corrections to cosmological perturbations during inflation.





  

10/04/2019 -- Ogan Ozsoy (Swansea University)

Time:  14:00

Place: 117

Observations of Cosmic Microwave Background ( CMB ) radiation appear to be consistent with the simplest realizations of the inflationary paradigm: single field slow-roll inflation. However, in practice, CMB probes can provide us information about the inflationary dynamics only for a limited range of scales that correspond to a small portion of the dynamics compared to required time span of inflation in solving the standard problems of Hot Big Bang cosmology. This leaves us with a large portion of the dynamics together with a vast range of scales that are pretty much uncharted and yet to be explored. In this talk, I will focus on two possible observational windows together with a simple primordial mechanism that can provide us the opportunity to probe the inflationary dynamics on small scales compared to the CMB. In this context, I will show two exemplary scenarios that has potential to accomplish this goal through enhanced scalar and tensor fluctuations during inflation.





  

25/03/2019 -- James Bonifacio (Case Western Reserve University)

Time:  14:00

Place: 226

A free massless scalar in flat space has an infinite number of shift symmetries. In (A)dS, each of these symmetries is preserved only for particles with particular discrete masses. I will show how these shift symmetries generalize to massive higher-spin particles and explain how these are related to partially massless symmetries. For the case of scalar fields, I discuss deformations of the underlying symmetry algebras and whether there exist invariant interactions. This leads to a ghost-free theory in (A)dS that is invariant under a deformed quadratic shift symmetry and which reduces in flat space to the special Galileon. This theory has a rich structure of interactions that are completely fixed by the nonlinear symmetry, including a nontrivial potential. Lastly, I will speculate on possible generalizations to interacting massive higher-spin particles.





  

18/03/2019 -- Masahide Yamaguchi (Tokyo Institute of Technology)

Time:  14:00

Place: 226

We propose a new class of higher derivative scalar-tensor theories without the Ostrogradsky's ghost instabilities. The construction of our theory is originally motivated by a scalar field with spacelike gradient, which enables us to fix a gauge in which the scalar field appears to be non-dynamical. We dub such a gauge as the spatial gauge. Though the scalar field loses its dynamics, the spatial gauge fixing breaks the time diffeomorphism invariance and thus excites a scalar mode in the gravity sector. We generalize this idea and construct a general class of scalar-tensor theories through a non-dynamical scalar field, which preserves only spatial covariance. We perform a Hamiltonian analysis and confirm that there are at most three (two tensors and one scalar) dynamical degrees of freedom, which ensures the absence of a degree of freedom due to higher derivatives. Our construction opens a new branch of scalar-tensor theories with higher derivatives.





  

14/03/2019 -- Stefano Camera (University of Turin)

Time:  16:00

Place: 226

'Synergy' means 'the interaction of two or more agents to obtain a combined effect greater than the sum of their separate effects'. With this in mind, in this talk I shall present my current lines of research, all focussed on developing novel combinations of astrophysical and cosmological observables to the aim of testing the foundations of the concordance cosmological model. Specifically, I shall discuss how innovative cross-correlations can mitigate the impact of systematic effects, noise and cosmic variance, to the end of studying dark energy and modified gravity models, detecting particle dark matter signatures, and testing gravity and inflation on the largest cosmic scales. All, with a view on the current and oncoming generation of cosmological experiments and large-scale surveys.





  

28/02/2019 -- Gizem Sengor (FZU)

Time:  16:00

Place: 226

Cosmological backgrounds in general posses time dependence. On these backgrounds scalar degrees of freedom that transform nonlinearly under time diffeomorphisms arise to guarantee the time diffeomorphism invariance of the action. In the early universe these time dependent backgrounds can be attributed to the presence of time dependent scalar fields that dominate the energy momentum density of the universe. Then the species of the scalar degree of freedom that transforms nonlinearly under time diffeomorphisms correspond to perturbations of the scalar field that gives rise to the time dependence of the cosmological background at a given era. Effective field theories (EFT) of cosmological perturbations generalize the interactions between cosmological perturbations of different species based on their transformation properties under diffeomorphisms. Preheating refers to the stage at the end of inflation where the inflaton field continues to dominate the energy momentum density but transfers its energy to other fields through resonance, as opposed to perturbative decays. The aim of this talk is to consider general interactions between the perturbations of the inflaton and a second scalar field during Preheating, to understand the scales these interactions introduce and explore which species propagate as effective degrees of freedom at different scales.





  

27/02/2019 -- Antonio Racioppi (NICPB, Tallinn, Estonia)

Time:  14:00

Place: 117

We study models of chaotic inflation where the inflaton field $\phi$ is coupled non-minimally to gravity via $\xi \phi^n R$, a.k.a. $\xi$-attractors. We focus on the Palatini theory of gravity and we show that in this case Starobinsky inflation is not any more a universal attractor. On the other hand we prove that, once quantum corrections are taken into account, the strong coupling limit of (a certain class of) $\xi$-attractor models will move into linear inflation regardless of the adopted gravity formulation metric or Palatini).





  

25/02/2019 -- Jakub Vicha (NICPB, Tallinn, Estonia)

Time:  14:00

Place: 226

The Pierre Auger Observatory is the currently largest cosmic-ray detector covering ultra-high energies from 10^18 eV to 10^20 eV. The size of exposure accumulated since 2004 granted measurements of unprecedented precisions on energy spectrum, mass composition and anisotropy searches. These measurements guide us slowly to the sources of ultra-high energy cosmic rays, which is a tantalizing mystery of physics. A brief introduction to the field of ultra-high energy cosmic rays will be given together with a description of the Pierre Auger Observatory and its detection techniques. Then, an overview of the most interesting results will follow.





  

18/02/2019 -- Vojtech Witzany (Astronomical Institute of the Czech Academy of Sciences)

Time:  14:00

Place: 226

A compact stellar mass object inspiralling onto a massive black hole deviates from geodesic motion due to radiation-reaction forces as well as finite-size effects. Such deviations need to be included with sufficient precision into wave-form models for the upcoming space-based gravitational-wave detector LISA. I will present the formulation and solution of the Hamilton-Jacobi equation of a generic geodesic in Kerr space-time perturbed by the spin-curvature coupling, the leading order finite-size effect. In return, this solution allows to compute a number of observables such as the turning points of the orbits as well as the fundamental frequencies of motion. These results essentially solve the question of conservative finite-size effects in extreme mass ratio inspirals.





  

10/12/2018 -- Shinji Mukohyama (Kyoto University and Tokyo University)

Time:  14:00

Place: 226

It is generally believed that modification of general relativity inevitably introduce extra physical degree(s) of freedom. In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology.



  

26/11/2018 -- Ondrej Pejcha (Charles University, Prague)

Time:  14:00

Place: 226

Interest in the transient astronomical sky has increased tremendously thanks to modern time-domain surveys, which have discovered unexpected diversity in previously known phenomena and identified many new classes of transients. I will focus on two types of transients that are important for the nucleosynthesis in the Universe and the evolution of gravitational wave sources. I will argue that the deaths of massive stars marked by core-collapse supernovae are highly sensitive to initial conditions, which leads to a complex pattern of neutron star and black hole formation. Many stars are members of binary systems and their evolution can be significantly affected by a catastrophic interaction, which results in the rapid loss of mass, energy and angular momentum, and sometimes even merger of the binary star. This phase was recently connected to a newly identified group of red transients. I will present surprising findings from the theoretical interpretation of observations of these red transients.



  

23/11/2018 -- Rachel Houtz (IFT Madrid)

Time:  14:00

Place: 117

In this talk I present a model with an enlarged color sector which solves the strong CP problem via new massless fermions. QCD color is unified with another non-Abelian group with a large confinement scale. The spontaneous breaking of the unified color group provides a source of naturally large axion mass due to small size instantons, and as a result no very light axions are present in the low-energy spectrum. The axion scale may be around a few TeV which translates to observable signals at colliders. This model naturally enlarges the parameter space for axions which solve the strong CP problem well beyond that of invisible axion models.



  

22/11/2018 -- Antonino Marciano (Fudan University, Shanghai)

Time:  16:00

Place: 226

We discuss how inflation and bounce cosmology can emerge from a four-fermion interaction induced by torsion. Inflation can arise from coupling torsion to Standard Model fermions, without any need of introducing new scalar particles beyond the Standard Model. Within this picture, the inflaton field can be a composite field of the SM-particles and arises from a Nambu-Jona-Lasinio mechanism in curved space-time, non-minimally coupled with the Ricci scalar. The model we specify predicts small value of the r-parameter, namely r ~ 10-3 - 10-2, which nonetheless would be detectable by the next generation of experiments, including BICEP 3 and the ALI-CMB projects. On the other hand, bouncing cosmology can be also accounted for in terms of fermion condensates, with the remarking appearance of an ekpyrotic phenomenon, which is solely due to the quantum corrections to the fermion potential. We finally argue about the richness of the phenomenological perspective encoded in both the schemes.



  

20/11/2018 -- Andrea Addazi (Fudan University, Shanghai)

Time:  14:00

Place: 117 or 226

We will discuss dark matter models which are related to dark first order phase transitions (D.F.O.P.T.) in the early Universe. D.F.O.P.T. sources an efficient materialization of dark bubbles. Dark bubbles can scatter each others producing a gravitational radiation background, detectable in next generation of experiments. We will discuss the specific framework of Majoron dark matter, as a neutrino mass and a Warm Dark Matter genesis model. Majorons may also be detected in next laboratory experiments: i) electron-positron high luminosity colliders; ii) neutrino-less-double-beta decays processes; iii) experiments searching for baryon violating Neutron-Antineutron transitions.



  

05/11/2018 -- Subodh Patil (Niels Bohr Institute, Copenhagen)

Time:  14:00

Place: 226

In this talk, we present an amusing observation that primordial gravitational waves, if ever observed, can be used to bound the hidden field content of the universe. This is because a large number of hidden fields can resum to potentially observable logarithmic runnings for the graviton two-point function in the context of single field inflation, courtesy of a `large N' expansion. This allows one to translate ever more precise bounds on the tensor to scalar consistency relation into bounds on the hidden field content of the universe, with potential implications for phenomenological constructions that address naturalness with a large number of species. Along the way, we'll review how the cutoff for an EFT that includes gravity changes as we incorporate matter, identifying two distinct scales for gravity. We'll also need to address certain subtleties regarding loop corrections on cosmological backgrounds, especially with regards to the correct implementation of dimensional regularization.



  

25/10/2018 -- Alexey Golovnev (St.-Petersburg State University)

Time:  16:00

Place: 117 or 226

I will review the basic construction of teleparallel gravity and its modifications, with a special emphasis on local Lorentz transformations in the tetrad space. One of these modified models, the f(T), is widely used for cosmological model building. I will explain how (linear) cosmological perturbations should be treated in f(T) and in similar models. Finally I will discuss the problem of dynamical structure and Hamiltonian analysis for modified teleparallel gravity.



  

16/10/2018 -- Lasha Berezhiani (Max Planck Institute of Physics, Munich)

Time:  14:00

Place: 117

After a brief review of some of the empirical correlations between dark and baryonic sectors within galaxies, I will discuss a novel theory of dark matter superfluidity as a potential explanation of this observations. I will argue that, depending on the mass and self-interaction cross section of dark matter particles, the superfluid may in principle be formed in the central regions of galactic halos. After this, I will discuss the criteria that need to be met by superfluid properties in order to account for the above-mentioned empirical correlations.



  

8/10/2018 -- Katherine Freese (Nordita, Sweden)

Time:  14:00

Place: 226

Inflation, a period of accelerated expansion at the beginning of the Universe, seeks to explain the (otherwise mysterious) large scale smoothness, isotropy, and “oldness” of the Universe. An important product of this inflationary epoch is the origin of density perturbations that are the seeds of galaxies and other large structures today. The density perturbations and gravitational waves produced by inflation provide sensitive tests of both the inflationary paradigm and of individual inflationary models. In the past decade predictions of inflation have been tested by Cosmic Microwave Background data, most recently with the Planck satellite observations. The basic idea of inflation matches the data and sensitive tests have been made of individual models. Planck data have ruled out most inflation models. I will discuss the status of Natural Inflation, a model that my collaborators and I originally proposed in 1990, as well as modern variants. Natural inflation uses “axions” as the inflaton, where the term “axion” is used generically for a field with a flat potential as a result of a shift symmetry. The successes of inflation as well as the potential discoveries in upcoming data will be emphasized.





  

17/09/2018 -- Elena De Paoli (Marseille, CPT)

Time:  14:00

Place: 117

We identify a symplectic potential for general relativity in tetrad and connection variables that is fully gauge-invariant, using the freedom to add surface terms. When torsion vanishes, it does not lead to surface charges associated with the internal Lorentz transformations, and reduces exactly to the symplectic potential given by the Einstein-Hilbert action. In particular, it reproduces the Komar form when the variation is a Lie derivative, and the geometric expression in terms of extrinsic curvature and 2d corner data for a general variation. As a direct application of this analysis we prove that the first law of black hole mechanics follows from the Noether identity associated with the covariant Lie derivative, and that it is independent of the ambiguities in the symplectic potential provided one takes into account the presence of non-trivial Lorentz charges that these ambiguities can introduce.



  

 

30/07/2018 ­­ Shun-Pei Miao (National Cheng Kung University, Taiwan)

Time:  1:30pm

Place: 226

We consider an additional fine-tuning problem which afflicts scalar-driven models of inflation. The problem is that successful reheating requires the inflaton be coupled to ordinary matter, and quantum fluctuations of this matter induces Coleman-Weinberg potentials which are not Planck-suppressed. Unlike the flat space case, these potentials depend upon a still-unknown nonlocal functional of the metric which agrees with the Hubble parameter for de Sitter. Such a potential cannot be completely subtracted off by any local action. We numerically consider the effect of subtracting it off at the beginning of inflation in a simple model. For fermions the effect is to prevent inflation from ending unless the Yukawa coupling to the inflaton is so small as to endanger reheating. For gauge bosons the effect is to make inflation end almost instantly, again unless the gauge charge is unacceptably small.



  

 

27/07/2018 ­­ Richard Woodard (University of Florida, USA)

Time:  2pm

Place: 226

MOND is a phenomenological model which modifies the extreme weak field regime of Newtonian gravity so as to explain galactic rotation curves without dark matter. If correct, it must be the non-relativistic, static limit of some relativistic modified gravity theory. I show how the only possible metric-based modification of gravity is nonlocal, and I construct the action using the Tully-Fisher relation and weak lensing. Then I explore the consequences of this model for cosmology. This talk is based on four arXiv papers: 1106.4984, 1405.0393,1608.07858 and 1804.01669.



  

 

26/07/2018 ­­ Dam Thanh Son (Kadanoff Center for Theoretical Physics, University of Chicago, USA)

Time:  2pm

Place: FZU lecture hall





  

 

25/07/2018 ­­ Oleg Teryaev (Joint Institute for Nuclear Research, Dubna, Russia)

Time:  2pm

Place: FZU lecture hall

The energy-momentum tensor matrix elements describe the particle coupling to gravitational field. They are responsible for gravity action on particle spin which may result, in particular, in neutrino spin-flip in anisotropic Universe. One of the proton's formfactors, related to the pressure of quarks, was recently experimentally extracted from the data obtained in Jefferson Lab. (Nature, V. 557, p. 396, May 17, 2018). The pressure is extremely large with the distribution analogous to that in macroscopic stable object, like star.



  

 

24/07/2018 ­­ Renato Costa (University of Cape Town, South Africa)

Time:  2pm

Place: 117

The singularity problem is one of the hints that the \LambdaCDM models has to be extended at very high energies. We use the guiding principle of symmetries to extend the FLRW background to an explicitly T-dual one which is well described by double field theory (DFT). We show that, at the level of the background, one can have a singularity-free cosmology once the dual time coordinate introduced by DFT is inversely related to the standard time coordinate of general relativity. We also show that introducing matter in DFT cosmology naturally leads to the correct equation of state for the winding modes and to a more clear interpretation of the connection between the two time coordinates.



  

 

  

 

18/07/2018 ­­ Massimiliano Rinaldi (Trento University, Italy)

Time:  2pm

Place: 117

In this talk I will present a scalar-tensor model of modified gravity that is globally scale-invariant. Such a symmetry spontaneously breaks to give rise to a mass scale, and an inflationary scenario naturally emerges. The same model will be presented both in the Jordan and in the Einstein frame and the compatibility with current observations will be discussed.



17/07/2018 ­­ Alessandro Drago (Ferrara University, Italy)

Time:  2pm

Place: room 117

I will discuss what we have learnt from the first merger of two neutron stars observed in gravitational waves and in E.M. waves. My discussion will include information coming from new theoretical analyses and also from x-ray data collected by satellites.



  

 

12/07/2018 ­­ Jarah Evslin (Institute of Modern Physics, Lanzhou, China)

Time:  14:00

Place: room 226

There are at least two 3 sigma anomalies in the cosmic expansion rate. One is the discrepancy between the local Universe measurement of the Hubble constant by Riess et al and also using strong lensing time delays vs the best fit Planck result assuming LCDM. The other is the Lyman alpha forest Baryon Acoustic Oscillation (BAO) measurement, which disagrees with LCDM when combined with other BAO measurements or Planck. We note that unanchored BAO provides a robust geometric probe, free of all but the most basic cosmological assumptions. Using it, we find that if these anomalies are confirmed, the first necessarily implies a change in pre-recombination cosmology while the second implies dynamical dark energy between the redshifts z=2 and z=0.6.



  

 

 

  

 

03/07/2018 ­­ Yi-Zen Chu (National Central University, Taiwan)

Time:  14:00

Place: Room 226

Despite being associated with massless particles, electromagnetic and gravitational waves do not propagate strictly on the null cone in curved spacetimes. They also develop tails, traveling inside the light cone. This tail effect, in particular, provides a contribution to the self-force of compact bodies orbiting super-massive black holes, which in turn are believed to be important sources of gravitational waves for future space based detectors like LISA, TianQin and Taiji. For the first portion of my talk I will describe my efforts to explore novel methods to understand the tail effect in curved geometries -- primarily in cosmological spacetimes. Some of the spin-offs include the (small) discovery of new type of gravitational wave memory effect induced by tails. If time permits, for the second part of my talk, I will address a seemingly basic aspect of gravitational wave theory that -- as far as I am aware -- has not received proper clarification in the literature to date. Specifically, the "transverse-traceless" gravitational wave (GW) is usually touted as the gauge-invariant observable; while practical computations actually do not strictly yield this "TT" GW. Furthermore, the gauge-invariant TT GW is actually acausally related to its matter source, as can be seen by simply computing its associated Green's function. I will clarify the situation for the spin-1 photon, as an analogy to the gravitational case.



  

 

28/06/2018 ­­ Andreas Albrecht (University of California at Davis, USA)

Time:  14:00

Place: FZU Lecture Hall

I review the current status of cosmic inflation, including successes and open questions. I also scrutinize the question of the famous cosmological "tuning puzzles" and analyze the extent to which inflation does and does not resolve these. I explain why I think the open questions about inflation are deeply scientifically exciting. They should not be regarded as "failures" of inflation, nor should they be swept under the rug.



  

 

27/06/2018 ­­ Andreas Albrecht (University of California at Davis, USA)

Time:  14:00

Place: room 117

Decoherence and "einselection" have important roles in quantum physics, and are understood to be important in the emergence of classical behavior. Traditional discussions of einselection all assume an arrow of time. The extent to which einselection (and thus the emergence of classicality) is tied to an arrow of time has possibly deep implications for cosmology. In this talk I present some early results on this topic based on calculations in a toy model related to the classic Caldeira Leggett model, which I solve unitarily in all regimes. This talk will include introductory material, and will not assume prior familiarity with decoherence, einselection or cosmology.



  

 

26/06/2018 ­­ Eugeny Babichev (LPT, Orsay, France)

Time:  14:00

Place: room 117

A Hamiltonian density bounded from below implies that the lowest-energy state is stable. I will discuss that, contrary to common lore, an unbounded Hamiltonian density does not necessarily imply an instability: this is a coordinate-dependent statement. I will give the correct stability criterion, using the relative orientation of the causal cones for all propagating degrees of freedom. I will then apply this criterion to an exact Schwarzschild-de Sitter solution of a beyond-Horndeski theory, while taking into account the recent experimental constraint regarding the speed of gravitational waves coming from GW170817.



  

 

25/06/2018 ­­ Wojciech Hellwing (Warsaw, Poland)

Time:  14:00

Place: room 226

While the Earth-base laboratories keep trying very hard to elucidate on the nature of the elusive dark matter particles the other very promising avenue to test and/or falsify potential dark matter candidates resides in astrophysical observations. In this context our own Galaxy - the Milky Way - with its unique set of satellites shows potential to serve as a extraterrestrial laboratory for dark matter. The very physical nature of dark matter particles and especially the differences between the main candidate, the neutralino of Cold Dark Matter (CDM), and its currently strongest competitor, the sterile neutrino of Warm Dark Matter candidate, may lead to significant differences in the properties of dwarf galaxies. Such objects are dominated (by mass) by their host DM haloes and therefore provide an unique view on the physical properties of DM. I shall discuss our recent efforts to use the state-of-the-art galaxy formation hydrodynamical simulation scheme of the EAGLE project as well as high-resolution Copernicus Complexio N-body simulations to study the galaxy formation of Milky Way like systems in CDM and WDM scenarios. Our results render new insights on potential ways to use astronomical observations for falsifying the CDM paradigm and testing its competitors.



  

 

15/06/2018 -- Emre Kahya (Istanbul)

Time:  11:00

Place: 226

I will discuss quantum gravitational loop effects to observable quantities such as curvature power spectrum and primordial non-gaussianity of Cosmic Microwave Background (CMB) radiation. We first review the previously shown case where one gets a time dependence for zeta-zeta correlator due to loop corrections. Then we investigate the effect of these loop corrections to primordial non-gaussianity of CMB.



  

 

 

14/06/2018 -- Emre Kahya (Istanbul)

Time:  16:00

Place: 226

The gravitational wave (GW) signal (GW170817) from the coalescence of binary neutron stars was simultaneously seen throughout the electromagnetic (EM) spectrum from radio waves to gamma rays. We point out that this simultaneous detection rules out a class of modified gravity theories, and provides another indirect evidence for the existence of dark matter.



 

06/06/2018 -- Sébastien Clesse (University of Namur)

Time:  14:00

Place: 226

I will present the current status of primordial black holes as a Dark Matter candidate, a scenario that has recently seen a strong revival of interest. Formation models, astrophysical and cosmological constraints, as well as observations pointing towards the possible existence of primordial black holes, with abundances comparable to the one of dark matter, will be reviewed and discussed, including the gravitational waves from massive black hole mergers detected by LIGO/VIRGO. Finding evidence of even a single primordial black hole could have groundbreaking consequences for our understanding of the early Universe and of High Energy physics.



  

 

  

 

21/05/2018 -- Maksym Ovchynnikov (Leiden University)

Time:  14:00

Place: 226

It is well-known that the Standard Model of particle physics does not explain dark matter, neutrino masses and matter-antimatter asymmetry of the Universe and therefore has to be extended. This means that there should exist some new particles that are either too heavy to be found before (the “energy frontier”) or interact too feebly (the “intensity frontier”). In the absence of a good guiding principle predicting where we should look for new physics, we consider the so-called “portals” — renormalizable interactions between new particles and the Standard Model. We review these portals and their phenomenology at the “intensity frontier” (in particular at SHiP). We pay special attention to the searches of dark matter particle through these portals and discuss the cosmological status of “light dark matter”.



  

 

17/05/2018 -- Lorenzo Pizzuti (Trieste)

Time:  16:00

Place: 226

I will provide a brief overview on my work concerning constraints on modified gravity models obtained using galaxy cluster mass profile determinations. In particular, I will present the results of a paper in which we combined the information given by the kinematic of galaxies in cluster with the information provided by lensing analyses for 2 galaxy clusters of the CLASH-CLASH\VLT collaboration to get constraints on f(R) models. In order to discuss the applicability of the proposed method in view of future imaging and spectroscopic surveys, I will further introduce my current study of cosmological simulations, aiming at estimating and calibrating the impact of systematics.



  

 

15/05/2018 -- Santiago Casas (CEA Paris-Saclay)

Time:  15:00

Place: 117

The large freedom in the free functions affecting linear perturbations in theories of modified gravity and dark energy leads to the burden of parametrization, which means that the observational constraints depend strongly on the way these free functions are parametrized. Using a model-independent test of gravity, alleviates this problem and it even frees us from assumptions about initial conditions, galaxy bias or the nature of dark matter. In this talk I will present the first model-independent reconstruction of the gravitational slip as a function of redshift, using present data on large scale structure and the Hubble function. For future data I will show how we can use these tests to rule out entire classes of modified gravity models and how we have to handle, in a Bayesian way, the constraints from models which are very close to LCDM and might not even be clearly distinguishable, even with next generation surveys.



  

 

07/05/2018 -- Diego Blas (King's College London)

Time:  14:00

Place: 226

The high quality of the data from pulsar timing makes of it a fantastic resource to understand gravitational phenomena. Traditionally this has been used to test general relativity. In this talk I will describe a less explored possibility: using pulsar timing to understand dark matter properties. I will focus on (possibly) detectable modifications of binary orbits due to the interaction with dark matter in different scenarios.



04/05/2018 -- Peter Tinyakov (University Of Brussels)

Time:  14:00

Place: 226

Compact stars - neutron stars and white dwarfs - can capture and accumulate dark matter. Even though only a tiny fraction of the star mass can be accumulated in realistic conditions, this may lead to dramatic consequences such as the star collapse into a black hole. Thus, mere existence of neutron stars and white dwarfs sets constraints on DM models where this phenomenon occurs. Alternatively, if only a fraction of NS is converted into black holes, these may be identified with the gravitational wave detectors: the masses of such BH are around one solar mass, while stellar evolution does not lead to BH lighter than ~2 solar masses. We will discuss in detail two examples: the DM composed of primordial black holes, and asymmetric DM with self-interactions.



  

 

03/05/2018 -- Jan Novák (Technical University of Liberec)

Time:  14:00

Place: 226

We investigate the Universe at the late stage of its evolution and inside the cell of uniformity 150 - 370 MPc. We consider the Universe to be filled at these scales with dust like matter, a minimally coupled Galileon field and radiation as matter sources. We will use the mechanical approach and therefore the peculiar velocities of the inhomogeneities as well as fluctuations of other perfect fluids are nonrelativistic. Such fluids are said to be coupled, because they are concentrated around inhomogeneities. We investigate the conditions under which the Galileon field can become coupled. We know from previous work that at background level coupled scalar field behave as a two-component perfect fluid: a network of frustrated cosmic string and cosmological constant. We found a correction for the Galileon field, which behaves like matter. We investigate a similar task for K-essence models and we try to find the conditions under which the K-essence scalar field with the most general form for its action can become coupled. We investigate at the background level three particular examples of the K-essence models: (1) the pure kinetic K-essence field, (2) a K-essence with a constant speed of sound and (3) the K-essence model with the Lagrangian bX+cX2−V (φ). We demonstrate that if the K-essence is coupled, all these K-essence models take the form of multicomponent perfect fluids where one of the component is the cosmological constant. Therefore, they can provide the late-time cosmic acceleration and be simultaneously compatible with the mechanical approach.



  

 

10/04/2018 -- Luca Marzola (National Institute of Chemical Physics and Biophysics, Tallinn)

Time:  14:00

Place: 117

In this talk I review the origin of the 21-cm line and explain why the particle physics community is making such a big deal out of it. I will show two possible ways to use the new results of the EDGES experiment and try to convince you that, maybe, you should have a look into the matter too.



12/04/2018 -- Tomi Koivisto (NORDITA, Stockholm)

Time:  14:00

Place: 226

Teleparallel gravity is formulated in terms of a flat spacetime affine connection. In the symmetric teleparallelism, the affine connection is further torsion-free. These simplifications may improve the theory of gravity both technically (only first derivatives and no boundary term in the action) and conceptually (resolution of the gravitational energy, separation of the inertial effects). In the talk we will review these formulations and discuss some recent developments in the symmetric teleparallel geometry that were reported in the pre-print arXiv:1803.10185.



27/03/2018 -- Julian Adamek (Queen Mary University London)

Time:  14:00

Place: 117

I present a general (relativistic) framework for numerical simulations of cosmic large-scale structure in the context of generic metric theories of gravity. The full spacetime metric is evolved within a weak-field description, while cold dark matter is represented as an N-body ensemble that follows timelike geodesics. The framework allows one to study phenomena that lead to generic modifications of the metric perturbations, either by introducing new relativistic sources or by modifying the theory of gravity.


  

20/03/2018 -- Pat Stengel (University of Stockholm)

Time:  14:00

Place: 117

The Standard Model Higgs boson, which has previously been shown to develop an effective vacuum expectation value during inflation, can give rise to large particle masses during inflation and reheating, leading to temporary blocking of the reheating process and a lower reheat temperature after inflation. We study the effects on the multiple stages of reheating: resonant particle production (preheating) as well as perturbative decays from coherent oscillations of the inflaton field. Specifically, we study both the cases of the inflaton coupling to Standard Model fermions through Yukawa interactions as well as to Abelian gauge fields through a Chern-Simons term. We find that, in the case of perturbative inflaton decay to SM fermions, reheating can be delayed due to Higgs blocking and the reheat temperature can decrease by up to an order of magnitude. In the case of gauge-reheating, Higgs-generated masses of the gauge fields can suppress preheating even for large inflaton-gauge couplings. In extreme cases, preheating can be shut down completely and must be substituted by perturbative decay as the dominant reheating channel. Finally, we discuss the distribution of reheating temperatures in different Hubble patches, arising from the stochastic nature of the Higgs VEV during inflation and its implications for the generation of both adiabatic and isocurvature fluctuations.


 

15/03/2018 -- Ilidio Lopes (University of Lisbon)

Time:  14:00

Place: 226

For the past decade asteroseismology has opened a new window into studying the physics inside stars. Today, it is well known that more than ten thousand stars have been found to exhibit solar-like oscillations. This large amount of high-quality data for stars of different masses and sizes is having a profound impact in our understanding of the structure of stars in the main and the post-main sequence, on the formation and evolution of stellar clusters in our Galaxy. Moreover, it can be used to test new fundamental laws of nature including the existence of dark matter. While many particle candidates have been proposed as the main constituents of dark matter, the impact of such candidates in the evolution of stars has been sparsely addressed. In this talk, I will focus on the impact that dark matter has in the evolution of stars, and how stellar oscillations have been used to constrain the properties of dark matter. I will discuss the potential of the next generation of asteroseismic missions helping us to address this problem.




20/02/2018 -- Eric A. Bergshoeff (University of Groningen)

Time:  14:00

Place: 226

A Schroedinger equation proposed for the GMP gapped spin-2 mode of fractional Quantum Hall states is found from a novel non-relativistic limit, applicable only in 2+1 dimensions, of the massive spin-2 Fierz-Pauli field equations. It is also found from a novel null reduction of the linearized Einstein field equations in 3+1 dimensions, and in this context a uniform distribution of spin-2 particles implies, via a Brinkmann-wave solution of the non-linear Einstein equations, a confining harmonic oscillator potential for the individual particles.

 





11/12/2017 -- Martin Roček (SUNY, Stony Brook)
 
 

Time:  16:00

Place: auditorium

WZW models and generalized geometry

I'll review (2,2) superspace and explore how to describe the generalized kahler structure of (2,2) supersymmetric WZW models, presenting surprising new results for SU(3).
 

  



8/11/2017 -- Pierre Fleury (University of Geneva)
 
 

Time:  14:00

Place: 117

Weak lensing with finite beams

The standard theory of weak gravitational lensing relies on the infinitesimal light beam approximation. In this context, images are distorted by convergence and shear, the respective sources of which unphysically depend on the resolution of the distribution of matter—the so-called Ricci-Weyl problem. In this talk, I will discuss a strong-lensing-inspired formalism designed to deal with finite light beams. I will show that it solves the Ricci-Weyl problem. Furthermore, finite-size effects systematically enhance the beam’s distortions, which could affect the interpretation of cosmic shear data.
 

  



6/11/2017 -- Andrei Gruzinov (New York University)
 
 

Time:  14:00

Place: 226

Particle production by real (astrophysical) black holes

The rate of production of light bosons (if they exist) by astrophysical black holes is calculated. Observability of this effect is discussed.
 

  



23/10/2017 -- George Pappas (Lisbon Centre for Astrophysics)
 
 

Time:  14:00

Place: 226

Neutron stars as matter and gravity laboratories

Compact objects in general and neutron stars (NSs) in particular open a window to some of the most extreme physics we can find in nature. On the one hand in the interior of NSs we can find matter in very extreme densities, exceeding nuclear densities and anything we can probe in the laboratory, while on the other hand NSs are related to the strongest gravitational fields next only to those found in black holes. Therefore studying NSs gives us access to both supranuclear densities as well as strong gravity and can be used to get information and test our theories of matter (equation of state) and gravity. The relevant properties of the structure of NSs are encoded on the spacetime around them and by studying the astrophysical processes that take place around NSs we can map that spacetime and extract these properties (i.e., the multipole moments, the equation of state, etc). In this talk we will discuss these properties of NSs and how they are related to the properties of the spacetime around them both in GR and in one of the proposed alternative theories of gravity. We will also talk about the relation of these properties to astrophysical observables and how one could tell these theories apart.
 

  



16/10/2017 -- Tessa Baker (Oxford University)
 
 

Time:  14:00

Place: 226

Tests of Beyond-Einstein Gravity

Corrections to General Relativity on large distance scales are under consideration as an explanation of cosmic acceleration. However, studying extended gravity models on an individual basis is a labour-intensive way of testing these ideas. I will explain how instead EFT-inspired parameterised methods can be used as a powerful and efficient way of testing for deviations from GR. I will outline the theoretical foundations of these techniques, and describe the current status of their observational constraints.
 

  



8/9/2017 -- Dani Figueroa (CERN)
 
 

Time:  14:00

Place: 117

Higgs Cosmology: implications of the Higgs for the early Universe.

I will discuss some of the consequences arising when we take into account the existence, and hence the presence, of the Standard Model Higgs during Inflation. In particular, I will derive stringent constraints on the couplings of the Higgs to the inflationary and gravitational sectors. I will also discuss the circumstances under which the Higgs can be resposible for the origin of the Standard Model species required by the 'hot Big Bang' paradigm. If there is enough time, I will also discuss the implications of all this for primordial gravitational waves.
 

 



6/9/2017 -- Sergey Ketov (Tokyo Metropolitan University)
 
 

Time:  14:00

Place: 117

Starobinsky inflation in supergravity

I begin with an introduction to Starobinsky inflation based on (R+R^2) gravity, in light of Planck data about CMB. Next, I introduce the supergravity extensions of Starobinsky inflation, review their problems and possible solutions. I conclude with a discussion of reheating after Starobinsky inflation in the context of supergravity.



29/6/2017 -- Bruce Bassett (University of Cape Town)
 
 

Time:  14:00

Place: Room 117

Rise of the Machine: AI and Fundamental Science

With the recent spectacular advances in machine learning we are naturally confronted with the question of the limits of Artificial Intelligence (AI). Here we will review how AI is being used in astronomy, discuss the future role of AI in fundamental science and finally discuss whether AI will ever be able to undertake its own original research.




28/6/2017 -- Dmitry Semikoz (APC, Paris)
 
 

Time:  14:00

Place: Lecture hall

Signatures of a two million year old nearby supernova in antimatter data.

In this talk I will  show how one can explain multiple anomalies in the cosmic ray data by adding the effects of a 2 million year old nearby supernova to static model of galactic cosmic rays. In particular, this supernova can explain the excess of positrons and antiprotons above 20 GeV found by PAMELA and AMS-02, the discrepancy in the slopes of the spectra of cosmic ray protons and heavier nuclei in the TeV-PeV energy range and the plateau in cosmic ray dipole anisotropy in the 2-50 TeV energy range. Same supernova was responsible for Fe60 measured in ocean crust.



2/6/2017 -- David Alonso (University of Oxford)
 
 

Time:  14:00

Place: Room 117

Science with future ground-based CMB experiments.

After the findings of Planck, the immediate future of CMB observations lies with the next generation of ground-based experiments. In this talk I will first introduce the most compelling science objectives for these experiments in combination with future large-scale-structure surveys. Then I will describe a number of novel observational methods to tackle these objectives enabled by the enhanced angular resolution and reduced noise levels of Stage-3 and Stage-4 observatories, as well as the main challenges they will face.
 
 

22/5/2017 -- Mathieu Langer (Université Paris-Sud)
 
 

Time:  14:00

Place: Room 117

Magnetizing the intergalactic medium during reionization.

An increasing amount of evidence indicates that cosmological sheets, filaments and voids may be substantially magnetised. The origin of magnetic fields in the the Intergalactic Medium is currently uncertain. It seems now well known that non-standard extensions to the physics of the Standard Model are capable of providing mechanisms susceptible of magnetising the Universe at large. Much less well known is the fact that standard, classical physics of matter-radiation interactions possesses actually the same potential. After reviewing briefly our current knowledge about magnetic fields on the largest scales, I will discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous Intergalactic Medium. Operating in the neighbourhood of ionising sources during the Epoch of Reionization, this mechanism is capable of generating magnetic seeds of relevant strengths on scales comparable to the distance between ionising sources. In addition, summing up the contributions of all ionising sources and taking into account the distribution of gas inhomogeneities, I will show that this mechanism leaves the IGM, at the end of Reionization, with a level of magnetization that might account for the current magnetic fields strengths in the cosmic web.

--based on Durrive & Langer, MNRAS, 2015, and Durrive et al. MNRAS 2017 (submitted)--



16/5/2017 -- Sergey Sibiryakov (CERN, EPFL, INR RAS)
 
 

Time:  16:00

Place: Room 117

Counts-in-cells statistics of cosmic structure and non-perturbative methods of  quantum field theory

I will show how the probability distribution for matter over/under- densities in spherical patches of the universe can be derived from first principles using the instanton technique borrowed from quantum field theory. The spherical collapse solution plays the role of an instanton, whereas deviations from sphericity are consistently accounted for by a Gaussian integral over small perturbations around the instanton. The method is valid even for large — by a factor ten — deviations from the mean density and provides a way to probe the dynamics of dark matter and statistics of initial fluctuations in the regime where perturbative treatment does not apply.
 
 

25/4/2017 -- Ippocratis Saltas (University of Lisbon)
 

Time:  11:00

Place: Room 117

What can unimodular gravity teach us about the cosmological constant?

Unimodular gravity became very popular over the last years as a theory that could shed light on the cosmological—constant problem. In this talk, I will explain the idea behind unimodular gravity, and discuss its (in)ability to bring a new perspective to the problem of the cosmological vacuum.
 
 

12/4/2017 -- Andrei Nomerotski (Brookhaven National Lab, USA)

 
 

Time:  14:00

Place: Lecture Theatre

Status and Plans for Large Synoptic Survey Telescope

Investigation of Dark Energy remains one of the most compelling tasks for modern cosmology. It can be studied with several probes which are accessible through precise and deep surveys of the Universe. In the talk I will review the status and plans for Large Synoptic Survey Telescope, which will precisely measure the positions and shapes of billions of galaxies along with estimates of their distances, providing an order-of-magnitude improvement relative to current experiments. LSST Camera employs thick, fully depleted CCDs with extended infrared sensitivity. The talk will provide more detail on the camera design and will discuss limitations on the achievable precision coming from the instrumentation.



6/4/2017 -- Alex Vikman (CEICO, Institute of Physics)
 
 

Time:  14:00

Place: Lecture Theatre

The Phantom of the Cosmological Time-Crystals

I will discuss a recently proposed new cosmological phase where a scalar field moves exactly periodically in an expanding spatially-flat Friedmann universe. On average this phase has a vacuum or de Sitter equation of state and can be interesting to model Inflation and Dark Energy in a novel way. This phase corresponds to a limiting cycle of the equations of motion and can be considered as a cosmological realization of a general idea of a "time-crystal" introduced by Wilczek et. all in 2012. Recently we showed that this cosmological phase is only possible, provided the Null Energy Condition is violated and the so-called Phantom divide is crossed. Using methods from the dynamical systems, we proved that in a rather general class of single scalar field models called k-essence: i) this crossing causes infinite growth of quantum perturbations on short length-scales, and ii) exactly periodic solutions are only possible, provided the limiting cycle encircles a singularity in the phase plane. The configurations neighboring this singular curve in the phase space are linearly unstable on one side of the curve and superluminal on the other side. Moreover, the increment of the instability is infinitely growing for each mode by approaching the singularity, while for the configurations on the other side, the sound speed is growing without limit. We illustrated our general results by analytical and numerical studies of particular models proposed by Wilczek and collaborators. Finally I will briefly discuss systems where this idea of time-crystals may be realized.

 


 3/4/2017 -- Jnan Maharana (Institute of Physics, Sachivalaya Marg, India)

 

Time:  14:00

Place: Room 226.

Scattering of Stringy States and T-duality

First a brief overview of target space duality will be presented. Compactification of a closed bosonic string in its massless backgrounds wil be considered when it is compactified on a d-dimensional torus. The vertex operators associated with the moduli of the compactified closed string will be constructed. The Kawai-Llewellyn-Tye factorization technique will be utilized to show the T-duality transformation properties of S-matrix for the moduli.

 



27/3/2017 -- Michal Bilek (Astronomical Institute of the Czech Academy of Sciences)
 

Time: 14:00

Place: Room 117.

Galaxy interactions in MOdified Newtonian Dynamics (MOND)

MOdified Newtonian Dynamics (MOND) is a promising attempt to solve the missing mass problem by changing the standard laws of physics rather than by postulating the dark matter. MOND has been inspired by observations of isolated disk galaxies. It is thus important to test it also in other objects. First, I will give a short introduction to MOND and review the published simulations of interacting galaxies. I will then present my work on testing MOND in elliptical galaxies using remnants of accreted satellites and my simulation of the past close encounter of Milky Way and the Andromeda galaxy that MOND predicts. I will discuss observational evidence for this encounter.
 
 
 


27/2/2017 -- Misao Sasaki (Yukawa Institute for Theoretical Physics, Kyoto -- director)
 

Time: 16:00

Place: Room 117.

Signatures from inflationary massive gravity.

Inflation is a natural platform for modified gravity. In this talk, we consider
a theory that spontaneously violates the local SO(3) symmetry, which gives
rise to a preferred spatial frame during inflation.
As a result, the tensor modes become massive. We argue that this theory
leads to several interesting observational signatures.

 


 
23/2/2017 -- Misao Sasaki

Time: 14:00

Place: Lecture hall.


Colloquium: Inflation and Beyond.

There is strong observational evidence now that the Universe has
experienced an almost exponential expansion at its very early stage, called
inflation. In this talk I first review the inflationary universe and its observational
predictions. Then I discuss possible future directions beyond and behind theory
of inflation, and their observational signatures.

 

 
14/12/2016 -- Giovanni Acquaviva (Charles University, Prague)

Time: 14:00

Place: Room 226

Dark matter perturbations with causal bulk viscosity

We analyse the evolution of perturbations of cold dark matter endowed with bulk viscosity.  Focusing on structure formation well within the Hubble radius, the perturbative analysis is carried out in the Newtonian approximation while the bulk viscosity is described by Israel-Stewart's causal theory of dissipation.  Differently from previous analysis based on non-causal theories, we obtain a density contrast evolution governed by a third order equation.  This framework can be employed to address some of the current inconsistencies in the observed clustering of galaxies.
 



 9/12/2016 -- David Pirtskhalava (EPFL, Lausanne, Switzerland)
 
Time: 14:00
 
Place:Room 117.

Relaxing the Cosmological Constant
 
 

 14/11/2016 --Glenn Barnich (Université Libre de Bruxelles & International Solvay Institutes)
 
Time: 14:00
 
Place: Room 117

Finite BMS transformations
 
 

 18/10/2016 -- Eugeny Babichev (Laboratoire de Physique Théorique d'Orsay, Orsay, France)
 
Time: 14:00
 
Place: Room 117

Gravitational origin of dark matter
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