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Theorique d'Orsay

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Agenda > Séminaires / Seminars > Cosmologie Dernier ajout : jeudi 2 octobre 2014.

Séminaires Cosmologie 2008-2009

Les séminaires se tiennent en salle 110 au 1er étage du bâtiment 210, généralement le mardi à 10h30, sauf indication contraire.

Contact pour les séminaires : Christos Charmousis.


-  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 background.


-  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 solution.


-  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.