# PhD Defense announcement at CPHT

**Charles Marteau (String theory)**

**Boundary structures and holographic fluids in gravity**

__Abstract__

This thesis is devoted to the study of several aspects of dynamical spacetimes with boundaries. An emphasis is put on asymptotic boundaries such as the conformal boundary of AdS or the null infinity in flat space. In AdS the geometry of the conformal boundary is pseudo-Riemannian since the boundary is time-like. In flat space, we will show how the geometry of the null infinity can be described in terms of Carroll structure. The latter emerges as the ultra-relativistic limit, or c → 0 limit, of a pseudo-Riemannian geometry. In particular, the flat limit in the bulk maps to this ultra-relativistic limit on the boundary. We will also see how the symmetries of asymptotically flat gravity translate into global symmetries of this exotic boundary geometry. This analysis is of central importance in fluid/gravity correspondence since the fluid is expected to live on the boundary. In this context we find integrability conditions on the boundary fluid that allow for a resummation of the so-called Derivative Expansion in AdS. The flat limit gives rise to the notion of Carrollian fluid on the boundary whose hydrodynamical expansion maps to a flat version of the Derivative expansion in the bulk, thus providing a notion of fluid/gravity correspondence in flat space. A second type of boundary that we study is the one formed by the horizon of a black hole. There, another type of fluid/gravity correspondence exists: the membrane paradigm. We revisit this concept and propose a novel interpretation of the Damour–Navier–Stokes equation in terms of ultra-relativistic conservation laws.

__Jury__

Dionysios ANNINOS, Kings College, Londres

Glenn BARNICH, Université Libre de Bruxelles

Daniel GRUMILLER, TU Wien, Vienne

Niels OBERS, Niels Bohr Institute, Copenhague et Nordita, Stockholm

Marios PETROPOULOS, École Polytechnique, IP Paris (directeur de thèse)

Andrea PUHM, École Polytechnique, IP Paris

__Organisation__

The defense will take place by zoom conference on Friday, June 19, 2020 at 10:30 am. Reception will open from 10 am. Latecomers will not be allowed in the virtual hall before intermission.

Connection details can be obtained on request from Malika Lang, Charles Marteau or Marios Petropoulos.