# Théorie

# The tensor track to quantum gravity

Random tensors generalize random matrices hence could be useful to quantize gravity. Recent progress led to the discovery of a 1/N expansion for large random tensors of size N and to the definition of a new class of quantum field theories, called tensor group field theories (TGFTs). Natural models in this class have been proved renormalizable and asymptotically free. They could therefore lead to ultraviolet consistent scenarios for the emergence of effective space-time and gravity in more than two dimensions.

# Dark Matter in the Universe and Novel Detection Mechanism using DNA

Weakly Interacting Massive Particles (WIMPs) may constitute most of the matter in the Universe. While there are intriguing results from DAMA/LIBRA, CoGeNT and CRESST-II, there is not yet a compelling detection of dark matter. The ability to detect the directionality of recoil nuclei will considerably facilitate detection of WIMPs by means of "annual modulation effect" and "diurnal modulation effect". Directional sensitivity requires either extremely large gas (TPC) detectors or detectors with a few nanometer spatial resolution.

# Effective action for the Abelian Higgs model in FLRW

We compute the divergent contributions to the one-loop action of the U(1) Abelian Higgs model. The calculation allows for a Friedmann-Lemaitre-Robertson-Walker space-time and a time-dependent expectation value for the scalar field. Treating the time-dependent masses as two-point interactions, we use the in-in formalism to compute the first, second and third order graphs that contribute quadratic and logarithmic divergences to the effective scalar action. Working in $R_xi$ gauge we show that the result is gauge invariant upon using the equations of motion.

# Effective Field Theory of Multi-Field Inflation a la Weinberg

We generalize Weinberg's effective field theory approach to multiple-field inflation. In addition to standard terms in the Lagrangian we consider terms containing up to the fourth derivative of the scalar fields and the metric.

# Symmetries of Inflation

Symmetries play an important role in the physics of inflation. Global symmetries and/or supersymmetry are often employed to explain the radiative stability of the inflationary background. Spontaneously broken de Sitter symmetries constrain the effective theory of inflationary fluctuations. In this talk, I will discuss both of these aspects of symmetry in inflation and their observational consequences.

# The Blackfold Approach

The Blackfold Approach is an effective theory for higher dimensional black holes. In this approach black holes are described as materials: they are characterised by speeds of sound, viscosities and elastic moduli. I will explain how to derive this effective theory from first principles in General Relativity by generalising techniques developed in AdS/CFT contexts.

# Forecast constraints on cosmic string parameters from future gravitational wave experiments

Gravitational waves (GWs) are one of the key signatures of cosmic strings. If GWs from cosmic strings are detected in future experiments, not only their existence can be confirmed but also their properties might be probed. In this talk, I will present the determination of cosmic string parameters through future GW experiments. We consider two types of direct detection GW signals, bursts and the stochastic GW background, which provide us with different information about cosmic string properties.

# Black holes in higher-derivative scalar-tensor theories

We consider classes of higher-derivative scalar-tensor theories, relevant for applications to modified theories of gravity (Galileons). They can be obtained from dimensional reduction and admit exact analytical black hole solutions. We describe their properties, in particular the presence of geometric horizons which dress otherwise naked singularities. If time permits, we will discuss some applications to holography.

# 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.