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Theorique d'Orsay

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

Séminaires Cosmologie 2009-2010

Les séminaires se tiennent en salle 110 au 1er étage du bâtiment 210.

Contact pour les séminaires : Christos Charmousis.


-  Mardi 29 juin à 11h, salle 110

Theodore Jacobson (Maryland) : Black Hole thermodynamics in Einstein-Aether theory


-  Mardi 22 juin à 11h, salle 110

Marina Seikel (Bielefeld) : Probing Backreaction Effects with Supernova Data

As the Einstein equations are non-linear, spatial averaging and temporal evolution do not commute. Therefore, the evolution of the averaged universe is affected by local inhomogeneities. It is, however, highly controversial how large these cosmological backreaction effects are. Using Buchert’s averaging formalism and perturbation theory up to second order, one can calculate the effects of backreaction on the measurements of the Hubble rate in the nearby universe. The size of the effect depends on the size and shape of the volume that is averaged over. The theory is then compared to actual measurements of the Hubble rate using the supernova data of the Constitution set up to a redshift of 0.1.


-  Mardi 7 juin à 11h, salle 110

Lorenzo Sorbo (Amherst) : A natural framework for chaotic inflation


-  Mardi 18 mai à 11h, salle 110

Nicolas Regnault (Tours) : Can we measure reliable distances from type Ia supernovae ?

Comparing nearby and distant type Ia supernova luminosities allows one to study the relation between luminosity distance and redshift and to constrain the expansion history of the Universe. Twelve years ago, relying on the first distant supernova samples obtained in the 1990s, two independant teams reported that the expansion of the Universe seems to be accelerating. This acceleration implies either the existence of a fluid with negative pressure usually called "Dark Energy", or modifications of gravity on cosmological scales.

Several new cosmological measurement techniques, as well as new Type Ia supernova (SN Ia) datasets allowed to narrow in on the properties of dark energy. SN Ia measurements remain a key ingredient in all current determinations of cosmological parameters. However, given the size of the current SN Ia samples, systematic uncertainties now dominate the error budgets. Currently, the dominant sources of systematics are (1) the photometric calibration of the surveys, especially that of the nearby supernova samples, (2) the empirical modeling of SN Ia used to obtain luminosity distances (3) the possible evolution of SN Ia properties with redshift.

Recent studies seem to suggest that the uncertainty budget is much larger than previously assessed and that the uncertainties related to the empirical modeling of SNe Ia have been largely underestimated. In this talk, we will examine these claims, and review the main source of uncertainties affecting the current results.


-  Mardi 11 mai à 11h, salle 110

Chiara Caprini (CEA) : CMB temperature anisotropy at large scales induced by a causal primordial magnetic field

A possible explanation for the origin of the large scale magnetic fields observed in galaxies and clusters is that they have been generated in the primordial universe. A stochastic background of primordial magnetic fields generated in the early universe with a mean amplitude well below micro-Gauss level can leave imprints on the temperature anisotropy of the cosmic microwave background. In this talk I present an analytical derivation of the CMB temperature anisotropy induced at large scales from a primordial magnetic field. The magnetic field is assumed to be generated by a causal process, as for example a phase transition, which consistently sets the initial conditions. Einstein’s and conservation equations for the perturbations at large scales are then solved analytically including neutrinos, in order to derive the magnetic Sachs Wolfe effect. Indeed, the presence of a non-zero anisotropic stress in the primordial fluid after neutrino decoupling strongly affects the scale-dependence of the magnetic Sachs Wolfe spectrum : this analysis confirms that it increases quadratically in the angular scale, as found in the latest numerical analyses.


-  Mercredi 24 mars à 11h, salle 110

Thomas Sotiriou (Cambridge) : Astrophysical Black Holes as particle Colliders

It has recently been shown that particles falling freely from rest at infinity outside an extremal Kerr black hole can in principle collide with arbitrarily high center of mass energy. The mechanism behind this exciting result will be analyzed and particular attention will be paid in understanding the role of the maximal black hole spin assumption. A number of practical limitations will be discussed, which seem to prohibit ultra-energetic collisions near black holes in nature.


-  Mercredi 10 mars à 11h, salle 110

Olindo Corradini (Bologna) : Smooth higher codimension braneworlds with form fields


-  Mardi 9 mars à 11h, salle 110

Cyril Pitrou (Portsmouth) : The y-sky ; diffuse spectral distortions of the Cosmic Microwave Background


-  Mercredi 24 février à 11h, salle 110

Alberto Iglesias (MPI, Munich) : Across the forbidden range of massive spin-2 on de Sitter

The theory of a spin-2 field with Pauli-Fierz mass term on a de Sitter background is non-unitary for a range of values of the mass parameter squared, namely, 0


-  Jeudi 18 février à 11h, salle 110

Christina Volpe (Orsay, IPN) : Challenges in Neutrino (Astro)physics

Important progress in our knowledge of neutrino properties has been made, in the last decade, after the discovery of neutrino oscillations, with an impact in various fields of physics. In particular, this phenomenon is essential to understand how neutrino propagate in astrophysical environments, e.g. in our Sun, in core-collapse supernovae, in the accretion-disk around black-holes, as well as in the Early Universe just before Big-Bang nucleosynthesis. Neutrino physics is now entering a crucial phase since, in the upcoming years, several experiments will address crucial open issues, among which the third neutrino mixing angle value and the possible existence of leptonic CP violation.

In this talk we will review the status in the field as well as the recent advances in core-collapse supernova neutrinos. In particular, new phenomena have been shown to emerge after the inclusion of the neutrino coupling to matter and to neutrinos on one hand and of density profiles with shock waves effects on the other. We will discuss the importance to observe neutrinos from an (extra)galactic explosion as well as the diffuse supernova neutrino background.


-  Mardi 9 février à 11h, salle 110

Camille Bonvin (CEA) : Full-sky gravitational lensing : new relativistic effects in convergence and second-order shear

Future lensing surveys aim at observing almost the whole sky. They will measure lensing correlations at very large scales. At those scales, new relativistic effects will come into play and open the way to new type of studies. In this talk, I will present a fully relativistic calculation of weak gravitational lensing up to second order in the gravitational potential. First, I will show that the convergence part of weak lensing contains new effects that are neglected at small scales. I will show that one of these contributions, generated by peculiar velocities of galaxies, becomes important at large scales. At small redshifts, it has an observable impact on the convergence power spectrum. Then I will present a calculation of the shear part of weak lensing up to second order. I will show that at large scales various non-linear couplings appear that can potentially provide new tests of gravity.


-  Mardi 26 janvier à 11h, salle 110

Eugeny Babichev (Max Planck Inst., Munich) : Recovering General Relativity from Massive Gravity

I will discuss static spherically symmetric solutions of massive gravity. In the first part of my talk, I will concentrate on the decoupling limit and consider the Vainshtein mechanism for the system of equations left over in the decoupling limit. This system has regular solutions featuring a Vainshtein-like recovery of solutions of General Relativity. In the second part of the talk, I consider the full system of equations of massive gravity and obtain static, spherically symmetric, and asymptotically flat numerical solutions of full massive gravity with a source. Those solutions show a recovery of the Schwarzschild solution of General Relativity via the so-called Vainshtein mechanism. Possible realizations of Vainshtein mechanism in scalar-tensor models are also discussed.


ATTENTION : changement d’horaire

-  Mardi 19 janvier à 10h30, salle 110

Savvas Nesseris (Niels Bohr Institute) : Cosmological constraints on Modified Gravity


Mardi 1er décembre 14h - Meeting commun IPhT-Saclay / LPT-Orsay au LPT, salle 110

-  14:30-15:10 - Julien Serreau (APC) : Decoherence in quantum field theory

-  15:15-16:25 - Jorge Ivan Norena (SISSA) : Spherical collapse in quintessence models with zero speed of sound

Coffee break

-  17:00-17:40 - Jean-Francois Dufaux (APC) : Gravitational Waves from Primordial Sources : Preheating after Inflation and Super-Symmetric Flat Directions

Discussion Session


-  Mercredi 18 novembre à 11 h

David Campo (U. of Gottingen) ; Quantum corrections during inflation : conservation of adiabatic perturbations and perspective on the backreaction problem

The possibility to learn about the very early universe rests on the ability to relate cosmological perturbations between eras seperated by a vast energy range about which we know very little. This ability is expressed classically as the conservation of certain combinations of the matter and metric perturbations, e.g. the one commonly noted $\zeta$. This conservation law is related to a symmetry, to wit the invariance by dilatation of the field equations in the infinite wavelength limit. Its violation at the quantum level would therefore constitute an important clue that the backreaction cannot be neglected.

I will present the calculation of the quantum corrections to the two-point function of $\zeta$ at one loop order. I found that the conservation law is preserved. I will insist in particular on the physical transparency and relative simplicity of the Hamiltonian formalism used to do this calculation, and on its application to further investigations on the backreaction problem.


-  Mercredi 4 novembre à 11 h

Eugeny Babichev (APC) : Relativistic stars in f(R) and chameleon theories

I will discuss relativistic stars in the context of scalar tensor theories of gravity that try to account for the observed cosmic acceleration and pass the local gravity experiments. I will concentrate on two types of models : chamelon-like and f(R) theories. The existence of static star solutions is explored numerically. The choice of the equation of state for the star is crucial for the existence of solutions. Indeed, if the pressure exceeds three times the energy density in a large part of the star, static configurations do not exist. In our analysis, I will use a polytropic equation of state, which is not plagued with this problem and, moreover, provides a better approximation for a realistic neutron star.


-  Mercredi 28 octobre à 11 h

Bruno Moraes (LPTA, Universite Montpellier 2) : The Growth of Matter Perturbations in f(R) Models

In this work, we study different aspects of the growth of matter density perturbations for some viable f(R) models that satisfy both cosmological and local gravity constraints, where the Lagrangian density f is a function of the Ricci scalar R. More specifically, we’ll look at some properties that may be useful in the quest for f(R) modified gravity models using future high-precision observations and that confirm the possibility to distinguish these models from the \LambdaCDM model for a certain range of parameters.


-  Mercredi 21 octobre à 11 h

Francois Bourliot (Ecole Polytechnique) : Quantum and Thermal superstring cosmology

In this talk I will discuss a recent attempt to describe early Universe cosmology thanks to superstring theory, taking into account thermal and quantum effects. I will first emphasize the different physical ansatz and aspects of the models considered before presenting very concrete results and showing that in a wide range of models considered, the early Universe is attracted to a Radiation Dominated Universe. This RDU appears to be stable for small and large fluctuations around it. I will also mention different extensions of the models considered and explain why our results are quite robust and apply for lots of different string theoretic constructions. If I have time, I will present another very recent project aiming at building a map between gravitational instantons (with or without cosmological constant) and geometric flows.


-  Mercredi 14 octobre à 11 h

Eric Gourgoulhon (LUTh, Meudon) : Les trous noirs : de l’horizon des événements aux horizons de piégeage

Après des rappels sur le concept de trou noir et son histoire, je présenterai une nouvelle approche géométrique, développée par plusieurs auteurs depuis une dizaine d’année. Cette approche ne repose pas sur le concept classique d’horizon des événements (qui est une structure globale de l’espace-temps), mais sur celui d’une structure quasi-locale appelée horizon de piégeage. Nous nous focaliserons ensuite sur une analogie hydrodynamique, semblable à celle développée pour la dynamique des horizons des événements dans les années 1970 ("paradigme de la membrane"). Nous verrons en particulier que le signe de la viscosité de volume des horizons piégeage est positif, alors qu’il était négatif pour les horizons des événements et discuterons des implications physique de cette propriété.


Documents joints
 Eric Gourgoulhon (1.1 Mo)