# Théorie

# Two-point functions for the Maxwell field in Roberston-Walker spacetimes and conformal gauges

In this talk, we will show how obtain an explicit two-point function for the Maxwell field in flat Robertson-Walker (cosmological) spacetimes. This result, which as far as we know is new, is obtained thanks to a simple gauge condition which takes the conformal flatness of the space into account. We will discuss in particular the applicability of the Gupta-Bleuler quantization scheme in the context of the flat Robertson-Walker spacetimes.

# Dynamical Casimir effect in Bose Einstein condensates

Bose Einstein condensate allow to experimentally test for analogue gravity, mainly because of their low temperature. The quasiparticles are however produced both by classical amplification of waves and spontaneous production from vacuum. Our aim is to characterize when the former dominates over the latter.

# Healthy theories beyond Horndeski

In search for a candidate that could explain the current acceleration of the Universe, a lot of attention has been given recently to Galileon theories, or in their generalized form, Horndeski theories. They are interesting as they represent the most general scalar tensor theories that do not lead to equations of motion containing more than two derivatives. This restriction is generally thought to be of great importance, as generically, higher order derivatives lead to ghost instabilities.

# Dark matter with t-channel mediators

From the experimental point of view, two of the most profound reasons to search for physics beyond the Standard Model come from cosmological observations: the baryon asymmetry and dark matter. In this talk we discuss the interplay of indirect, direct and collider searches for WIMP dark matter that interacts with the Standard Model fermions via a t-channel mediator.

# Geodesic “curve”-of-sight formulae for the cosmic microwave background: an extension of the line-of-sight integration method to second order

# Loop Quantum Gravity: The role of the Barbero-Immirzi parameter

Loop Quantum Gravity (LQG) is an attempt to quantize general relativity in the Hamiltonian formalism. The resolution of the cosmological singularity and the microscopic explanation of black hole thermodynamics are certainly its most important achievements. These results relies fundamentaly on the discreteness of space at the Planck scale predicted in LQG. The discreteness itself is strongly related to the presence of a free parameter in the theory called the Barbero-Immirzi parameter.

# Collective motion of natural objects: about the use of minimal models

I will discuss about two unusual systems for physicists: flocks and deserts. Minimal statistical models have been used for those systems in the last two decades. They gave some results, but they also induced some mistakes. I will try to underline what the advances are in this field, called "complexity".

# Primordial Inflation and the Higgs Boson

Primordial Inflation is thought to be the stage of quasi exponential expansion of the Universe which produces the initial conditions for the radiation dominated era from which the Universe has evolved. I will show that the Higgs field may have played a crucial role for Inflation. In fact, for a narrow band of values of the top quark and Higgs masses, the Higgs potential can develop a second minimum at energies of about 10^15-10^16 GeV and so we propose that it can be a source of energy for primordial Inflation, provided a transition from Inflation to a radiation era can be achieved.

# Gauge fixing in Yang-Mills theories and Gribov ambiguities: an insight from statistical mechanics

The analytic approaches to gauge theories rely on a gauge-fixing procedure, the standard textbook implementation of which involves the Faddeev-Popov method. It is well established that this procedure is in general ambiguous because it only *partially *fixes the gauge, a phenomenon called the Gribov ambiguity. The aim of this talk is to shed light on this Gribov ambiguity by describing a model of statistical physics that shares many similarities with the issue of gauge fixing in nonabelian theories.

# Fake hair for black holes

We will review modified gravity theories and in particular scalar

tensor theories, the mildest of modifications, where

we have an additional interacting scalar field coupling to the metric