Evolution of non-linear cosmological perturbations

TL;DR

Full nonperturbative generalizations of the uniform density and comoving curvature perturbations are defined geometrically, independently of any coordinate system, and the equations governing their evolution on all scales are given.

Abstract

We define fully non-perturbative generalizations of the uniform density and comoving curvature perturbations, which are known, in the linear theory, to be conserved on sufficiently large scales for adiabatic perturbations. Our non-linear generalizations are defined geometrically, independently of any coordinate system. We give the equations governing their evolution on all scales. Also, in order to make contact with previous works on first and second order perturbations, we introduce a coordinate system and show that previous results can be recovered, on large scales, in a remarkably simple way, after restricting our definitions to first and second orders in a perturbative expansion.