Title: Gauges in cosmological perturbation theory and in the separate-universe approach

Abstract: The so-called “separate-universe” (SU) approach aims to provide an effective description of cosmological perturbations at large scales. The gauge prescriptions of general relativity are often transposed in SU in order to derive some results valid at large scales, such as the stochastic noise arising from quantum fluctuations.

In this talk, we will show that such a procedure for fixing gauges in SU may lead to some results that differ from the full general-relativistic description. We will introduce a geometrical framework in the Hamiltonian framework that may be relevant for obtaining a correct gauge-fixing procedure and a gauge-invariant formulation of the SU approach.