f(R) Gravity as a Dark Energy Fluid

by Boris Bolliet (ENS Lyon)

Friday 11 Mar 2016, 11:30 → 12:30 Europe/Zurich

Room 234 (Geneva University)

Using the so-called equation of state approach to linear perturbation applied to f(R) gravity, we reproduce most of the results presented in the article "The Large Scale Structure of f(R) Gravity" (2007) by Y.-S. Song, W. Hu and I. Sawicki. More precisely, we study the equations for the evolution of cosmological perturbations in f(R) gravity and conclude that this modified gravity model can be interpreted as a dark energy fluid at background and linearised perturbation order. By eliminating the extra scalar degree of freedom known to be present in such theories, we are able to characterise the evolution of the perturbations in the scalar sector in terms of equations of state for the entropy perturbation and anisotropic stress which are written in terms of the density and velocity perturbations of the dark energy fluid and those in the matter, or the metric perturbations. We also do the same in the much simpler vector and tensor sectors. In order to illustrate the simplicity of this formulation, we numerically evolve cosmological perturbations in a small number of cases.