# Théorie

# Gauss-Bonnet braneworld redux: A novel scenario for the bouncing universe

We propose a new scenario for the bouncing universe in a simple five-dimensional braneworld model in the framework of Einstein-Gauss-Bonnet gravity, which works even with ordinary matter on the brane. In this scenario, the so-called branch singularity located at a finite physical radius in the bulk spacetime plays an essential role. We show that a three-brane moving in the bulk may reach and safely pass through it in spite that it is a curvature singularity. From the bulk point of view, this process is the collision of the three-brane with the shell of branch singularity.

# Brane interactions: space dimensionality, inflation, cosmic superstrings

I will discuss various aspects of brane cosmology.

I will first highlight how brane interactions can lead to our three-dimensional universe.

I will then investigate brane inflationary models and their consistency with supergravity constraints.

I will finally discuss some aspects of cosmic superstrings, formed at the end of brane inflation, whose rich phenomenology

may offer a window to test string theory.

# Supersymmetric hybrid inflation along waterfall trajectories

I will identify a new inflationary regime for which more than 60 e-folds are generated classically during the waterfall phase occuring after the usual hybrid inflation. By performing a bayesian Monte-Carlo-Markov-Chain analysis, this scenario can be shown to take place in a large part of the parameter space of the model. When this occurs, the observable perturbation modes leave the Hubble radius during waterfall inflation.

# A statistical Approach to Multifield Inflation: Many-field perturbations beyond Slow Roll

We develop a numerical statistical method to study linear cosmological fluctuations in inflationary scenarios with multiple fields, and apply it to an ensemble of six-field inflection-point models in string theory corresponding to a D3-brane moving in a conifold region. The latter are concrete microphysical realizations of quasi-single field inflation, in which scalar masses are of order the Hubble parameter and an adiabatic limit is reached before the end of inflation. We find that slow-roll violations, bending trajectories and ``many-field'' effec

# Higher order Post-Newtonian corrections via Effective Field Theory

Over the last decade considerable worldwide efforts have been invested in order to detect gravitational radiation. Inspiralling binary systems of black holes, which can be described analytically by the post-Newtonian (PN) approximation of GR, are promising candidate sources of gravitational waves signals. For a successful detection of such signals, PN corrections are required to high orders.

# Black holes, fluids and the Gregory-Laflamme instability

In dimensions higher than four, black holes exhibit a much richer dynamics, and qualitatively new phenomena emerge. I will review their properties and show how deviations from the familiar four-dimensional behavior can be traced back to the emergence of widely separated scales in the system. We will see that those are the regimes in which instabilities can arise and the black hole behaves effectively as a fluid that lives on a dynamical world volume.

# The first law of binary black hole mechanics

First laws of black hole mechanics, or thermodynamics, come in a variety of different forms. We establish a first law of mechanics for binary systems of point masses moving along circular orbits. This relation is derived from first principles in General Relativity, and is explicitly shown to hold up to very high orders in the post-Newtonian approximation. Analogies are drawn with the single and binary black hole cases, revealing intriguing formal relations between point masses and black holes.

# How to Solve a CFT in D > 2: The Resurgent Bootstrap and the 3D Ising Model

`In recent years the conformal bootstrap has emerged as a surprisingly powerful tool to study CFTs in dimensions greater than two. In this talk I will explain how crossing symmetry of the four-point function of scalar operators can be used to extract very non-trivial constraints on the spectrum of a putative CFT in arbitrary spacetime dimension. Applying these techniques in D=3 we will find that the 3D Ising model lies at a special point in the space of CFTs.`

# A new window on primordial non-Gaussianity

We know very little about primordial curvature perturbations on scales smaller than about a Mpc. I review how mu-type distortion of the Cosmic Microwave Background spectrum provides the unique opportunity to probe these scales over the unexplored range from 50 to $10^4 Mpc^-1$.

# Superluminality of Matter in the Multicritical Universe

First I will review the ideas and concepts of quantum gravity with

anisotropic scaling, which has attracted a lot of attention in

recent years both because of its improved ultraviolet behavior

and for its applications ranging from holographic renormalization,

to the causal dynamical triangulations approach to quantum gravity,

to the mathematics of the Ricci flow on Riemannian manifolds.

Then I will present the "multicritical universe" scenario,

in which gravity with anisotropic scaling is minimally coupled to