**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 Models**

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