Mercredi 17 Juin à 14h, salle 114
William G. Unruh (University of British Columbia) : Deaf and Dumb holes— Analogs of white and black holes
The discovery by Hawking that black holes were not black but rather suffered some sort of
quantum instability which caused them to emit thermal radiation, was one of
the biggest surprises of gravitational physics in the last century. That this
instability is still poorly understood over 30 years later is an embarassement.
Condensed matter systems can form an analog to black holes, including having
the same instability to the emission of radiation as black holes do, and give
us hope of understanding what is happening in black holes better. I will
introduce these analogs and point out the possibilities of using them to
understand black holes, both theoretically and experimentally.
Mardi 16 Juin à 11h, salle 110
Maulik Parikh (IUCAA, India) : Black Holes and the Structure of Spacetime
The intensive study of black holes over the last few decades has taught us a variety of sometimes surprising lessons about the structure of gravity and spacetime. I will describe several of these of relevance to cosmology, including the mysterious manner in which classical gravitation seems to appear as the thermodynamics of spacetime itself.
Mardi 9 Juin à 11h, salle 110
Elias Kiritsis (U. of Crete & Ecole Polytechnique) : Hořava-Lifshitz gravity : potential interest and potential problems
Mardi 2 Juin à 11h, salle 110
Jérôme Novak (LUTH, Meudon) : The Einstein equations on a computer : formulations and numerical solution
In recent years, the field of numerical relativity has obtained several
important results, among which the simulation of the coalescence and merger of
two black holes. These achievements rely not only on complex numerical
algorithms, but primarily on the analytic study of the Einstein equations to
obtain stable and numerically well-suited formulations (and gauge choices) for
this set of partial differential equations. In this seminar I shall introduce
some of these formulations, based on the 3+1 approach, before turning to the
description of the high-precision numerical methods, so-called spectral
methods, particularly adapted for the description of fields in spherical
coordinates. Finally, a brief review about the numerical representation of
black holes (puncture of excision methods) shall be sketched.
Mardi 19 Mai à 11h, salle 110
Riad Ziour (APC) : Magnification corrections to Galaxy-Lensing correlations
Magnification bias of background galaxies due to foreground mass can induce corrections to galaxy-shear and galaxy-magnification surveys. I will present in detail this effect and give the conditions under which it becomes non-negligible and should be taken into account.
If there is some time left, I will also evoke some current work on spherical collapse in f(R) theories.
Mercredi 13 Mai à 11h, salle 110
Christiano Germani (LUTH, Meudon) : p-nflation
I will show that an inflationary background might be realised by using any p-form non-minimally coupled to gravity. Standard scalar field inflation corresponds to the 0-form case and vector inflation to the 1-form. Moreover, I will prove that the 2- and 3-form fields are dual to a new vector and scalar inflationary theories where the kinetic terms are non-minimally coupled to gravity. I will then conclude by discussing primordial scalar and tensor perturbations of the 3-form inflation.
Mardi 5 Mai à 11h, salle 110
Filippo Vernizzi (CEA, Saclay) : The w<-1 side of dark energy
I will discuss generic single-field dark energy models by parametrization of the most general theory of their perturbations around a given background, including higher-derivative terms. In particular, I will cross the "phantom divide" and study the stability of models with equation of state w <-1, deriving theoretical constraints on their clustering properties.
Mardi 7 Avril à 11h, salle 110
Charalampos Bogdanos (LPT, Orsay) : Genetic algorithms and non-parametric Dark Energy reconstructions
Genetic algorithms are being used in applied sciences as a stochastic method of problem-solving. Borrowing ideas from evolution of biological systems, they can act as efficient search algorithms in large problem spaces for which there is poor theoretical knowledge. We discuss how this scheme can be used in a cosmological context as an alternative non-parametric reconstruction approach, to determine Dark Energy parameters . The method is used to analyse supernova Ia data and provide fits for the temporal evolution of the equation of state.
Mercredi 25 Mars à 11h, salle 110
Cyril Pitrou (University of Oslo) : The non-linear evolution of the cosmic microwave background
Non-Gaussian effects in the cosmic microwave background (CMB) can arise
either from the primordial phase of the universe or from the subsequent
non-linear evolution. I will focus on the latter point and review the
perturbation theory beyond linear order. I will detail how the kinetic
theory can be used in cosmology to derive the evolution of perturbations
for polarized radiation. Finally I will present why the collapse of dark
matter is the main source of non-Gaussianity in the CMB on small scales.
Mardi 17 Mars à 11h, salle 110
Michele Liguori (DAMTP, Cambridge) : Can cosmological structure form without dark matter ?
One of the prime pieces of evidence for dark matter is the observation of large overdense regions in the universe. Since we know from the cosmic microwave background that the regions that contained the most baryons when the universe was 400,000 years old were overdense by only one part in ten thousand, perturbations had to have grown since then by a factor greater than 1+z_rec 1180 where z_rec is the epoch of recombination. This enhanced growth does not happen in general relativity, so dark matter is needed in the standard theory. We show here that enhanced growth can occur in alternatives to general relativity, in particular in Bekenstein’s relativistic version of MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory for a completely different reason plays a key role in generating the instability that produces large cosmic structures today.
Mercredi 11 Mars à 11h, salle 110
Alberto Iglesias (UC Davis) : Dynamical Stability and Plasmid Strings
I’ll introduce a particular class of fundamental string configurations in
the form of closed loops stabilized by their internal dynamics. Then, I’ll
describe their classical treatment and embedding in models of string
cosmology, present their quantum version in flat space and the semiclassical
limit that provides the microscopic description of certain type of black
rings. I’ll show the parametric matching between the degeneracy of
microstates and the entropy of the supergravity solution and conclude by
commenting on the progress towards a similar construction on an AdS
Mercredi 4 Mars à 11h, salle 110
Julien Serreau (APC, Paris) : Non equilibrium QFT and cosmology
Mardi 17 Février à 11h, salle 110
Nathalie Deruelle (APC, Paris) : A rotating, Kerr-Schild, solution in Einstein-Gauss-Bonnet gravity
As is well-known, Kerr-Schild metrics linearize the Einstein tensor.
We shall see that they also simplify the Gauss-Bonnet tensor. This has
allowed us to find an exact, rotating, solution of the
Einstein-Gauss-Bonnet field equations. I shall present and discuss this
Mardi 10 Février à 11h, salle 110
Julien Serreau (APC, Paris) : Nonequilibrium field theory and cosmology
Lundi 21 Janvier à 14h
Olindo Corradini (INFN, Bologna) : Induced gravity on intersecting brane worlds
Cosmology of higher-codimensional brane world models is a
challenging problem. We discuss a possible way to tackle such problem by
constructing higher-codimensional models as intersections of
codimension-one brane worlds and by allowing induced gravity terms on all
the branes worldvolumes.
Lundi 8 Décembre au Mercredi 10 Décembre
Informal Workshop in Orsay on Higher Dimensional Black Holes : Exact Solutions and their stability
Invited external speakers include, Barak Kol, Sandro Fabbri, Ruth Gregory,
Simon Ross as well as Oscar Dias, Tony Padilla and Harvey Reall, Nemanja
Kaloper (the participation of the last two is to be confirmed).
Jeudi 27 Novembre à 16h30, salle 114, Bâtiment 210
SEMINAIRE CONJOINT DE COSMOLOGIE ET DE PHYSIQUE DES PARTICULES
Fernando Quevedo (University of Cambridge) : The LARGE volume string scenario and closed moduli inflation
An overview will be presented of the LARGE volume scenario of
moduli stabilisation in IIB string compactifications. Potential implications
to achieve cosmological inflation from the geometric (closed string) moduli
in this scenario will be reviewed.
Mercredi 26 Novembre à 10h30, salle 110
Steven Gratton (Institute of Astronomy, University of Cambridge) : Neutrinos Masses from Cosmology : What We Will Learn from Planck and Other Cosmological Datasets
Mercredi 19 Novembre à 10h30, salle 110
Christian Byrnes (University of Heidelberg) : Large non-Gaussianity from slow-roll inflation
Non-Gaussianity of the primordial curvature perturbation is a powerful and
topical way of distinguishing between the many possible models of
inflation. While the simplest models of inflation predict a tiny level of
non-Gaussianity, there are many models which predict an observable level
of non-Gaussianity such as the curvaton scenario, modulated reheating etc.
In this talk I show that it is possible to generate a large level of
non-Gaussianity $(f_NL\sim 100)$ during multiple-field inflation, without
breaking the slow-roll conditions. The non-Gaussianity is generated on
super-Hubble scales and is of the local form. I present the general
conditions required to generate a large non-Gausssianity and quantify the
fine tuning of the initial conditions required to achieve this. Finally I
apply these conditions to the specific model of hybrid inflation.
Mardi 18 Novembre à 10h30, salle 110
Eugeny Babichev (APC Paris) : Spherically symmetric solutions of massive gravity and the Goldstone picture
Mercredi 12 Novembre à 11h, salle 110
Guillermo Ballesteros (Madrid) : Scale dependence of the spectral index and high energy physics
The WMAP data currently allows an important scale dependence of the
spectrum of primordial perturbations. In the context of single field
slow-roll inflation, there is a tension between solving the horizon
problem and obtaining a large running of the spectral index. Therefore, if
the Planck probe confirms such a remarkable scale dependence, many
inflationary models may have to be discarded. I will discuss a class of
potentials (based on flat tree-level directions) that can overcome this
difficulty. I will also comment on the risk of using the standard
parameterization of the spectrum to assess the performance of models that
permit a non-trivial scale dependence.