Dear all,

I am pleased to invite you to my PhD defense which will take place on November 15th at 14:00 in room 454A Luc Valentin, 4th floor.

I will present my work on strong coupling physics, holography and cosmology which has been done at APC under the supervision of Francesco Nitti.

The defense will be followed by a pot in the salle de convivialité.

Best regards,

Valentin Nourry

zoom link:

https://u-paris.zoom.us/j/82940704158?pwd=U0xhcTgxMEdheGZzSnBqb1VWc3ZrUT09

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Title:

Strong coupling physics, holography and cosmology

Abstract:

This thesis studies the interplay between gravity and quantum effects of a conformal field theory (CFT).

In the first part of this thesis, the main goal is to investigate the stability of maximally symmetric spacetimes under small metric perturbations. We use AdS/CFT as a non-perturbative tool to compute the 2-point function of the CFT stress-energy tensor in the presence of a dynamical metric. Interpreting the framework of semi-classical gravity as an effective field theory (EFT), we compute the spectral functions of metric perturbations around maximally symmetric backgrounds. These spectral functions contain all the necessary information to settle the problem of stability in the validity regime of the EFT. By studying the poles of these functions, we track the presence (or absence) of instabilities whether they are ghost-like or tachyonic. These instabilities are located on the whole parameter space of the problem (the background curvature, the central charge of the CFT and the quadratic curvature coefficients of the gravity action). This project aims at distinguishing the contributions of the CFT and the gravity action. We also compare the instabilities coming from different sectors of the scalar and tensor decomposition of the metric perturbations.

The second part of this thesis is dedicated to the study of holographic 2-point functions of a CFT defined in AdSd. The most generic system is made of two different CFTs which are connected through a bulk AdSd+1 space whose boundary is made of two distinct AdS_d patches. The coupling between these two CFTs is responsible for breaking conformal invariance which is recovered by turning off one the two CFTs. Alternatively, we can define boundary conditions on a tensionless end-of-the-world brane to impose conformal invari- ance at the level of holographic correlators. We find an expression for the non-local action induced on such brane. This brane, as well as the AdS_d spaces, extend to the conformal boundary of AdS_{d+1}. We then investigate the possibility of defining sources there, but leave the solution of this problem for future work.

Keywords: conformal field theory, stability, gravity, holography, AdS/CFT, effective field theory

## Dates:

## Localisation / Location:

## Salle / Local:

- Soutenance de thèse

## Nom/Prénom // Last name/First name:

## Affiliation:

## Equipe(s) organisatrice(s) / Organizing team(s):

- Théorie