<p> SEMINARIS HEP</p><p>"Effective actions for relativistic fluids from holography"</p><p>Resum: Motivated by recent progress in developing action formulations of relativistic hydrodynamics, we use holography to derive the low energy dissipationless effective action for strongly coupled conformal fluids. Our analysis is based on the study of novel double Dirichlet problems for the gravitational field, in which the boundary conditions are set on two codimension one timelike hypersurfaces (branes). We provide a geometric interpretation of the Goldstone bosons appearing in such constructions in terms of a family of spatial geodesics extending between the ultraviolet and the infrared brane. Furthermore, we discuss supplementing double Dirichlet problems with information about the near-horizon geometry. We show that upon coupling to a membrane paradigm boundary condition, our approach reproduces correctly the complex dispersion relation for both sound and shear waves. We also demonstrate that upon a Wick rotation, our formulation reproduces the equilibrium partition function formalism, provided the near-horizon geometry is properly accounted for. Finally, we define the conserved hydrodynamic entropy current as the Noether current associated with a particular transformation of the Goldstone bosons.</p><p>Autor: Michal Heller (Perimeter Institute)</p><p>************************************************************ Dia: Dijous, 9 de juliol Hora: 14:00 Lloc: Aula 507 ************************************************************</p><p>"Resonant Primordial Gravitational Waves amplification" </p><p>Resum We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.</p><p>Autor: Chunshan Lin (YITP, Kyoto University)</p><p>************************************************************ Dia: Divendres, 10 de juliol Hora: 14:00* Lloc: Aula 507 </p><p>* Note unusual time ************************************************************</p>