# Théorie

# 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

# Holographic RG flows and Quantum Effective Actions

# Spectral distortions in the cosmic microwave background polarization

Deviations of the cosmic microwave background (CMB) spectrum from the one of a blackbody, so-called spectral distortions, have recently emerged as a powerful probe of many physical phenomena, ranging from inflation to dark matter and reionization. However, most studies so far have concentrated on the distortions of the monopole of the CMB intensity, while future missions will characterize the spectrum of its anisotropies, both in intensity and in polarization.

# The confining trailing string

We extend the holographic trailing string picture of a heavy quark diffusing through the plasma to the case of a bulk geometry dual to a confining gauge theory. We compute the classical trailing confining string solution for a static as well as a uniformly moving quark. The trailing string is infinitely extended and approaches a confining horizon, situated at a critical value of the radial coordinate, along one of the space-time directions, breaking boundary rotational invariance.

# Inflation, BICEP2, and an interesting discrepancy with Planck

# An introduction to the non perturbative renormalization group: method and some applications in statistical mechanics

An introduction to the modern version of Wilson's renormalization group will be given together with the two nonperturbative approximation schemes that have effectively been implemented (the derivative expansion and the Blaizot-Mendez-Wschebor scheme). The O(N) scalar models will be our favourite playground. Then, some results obtained in out of equilibrium statistical mechanics will be presented showing that fully nonperturbative physics can be captured by the nonperturbative renormalization group.