On the onset of cosmological backreaction

TL;DR

This paper investigates the initial phase of cosmological backreaction using perturbation theory and Buchert's framework, revealing that boundary terms significantly influence averaged observables and that backreaction effects are scale-dependent.

Contribution

It combines perturbation theory with Buchert's non-perturbative approach to analyze the onset of backreaction, highlighting the importance of boundary terms and gauge dependence.

Findings

Boundary terms dominate averaged observables at the onset of backreaction.

Backreaction effects are scale-dependent and influence the effective equation of state.

Surface terms are generally nonzero, confirming the physical relevance of backreaction.

Abstract

Cosmological backreaction has been suggested as an explanation of dark energy and is heavily disputed since. We combine cosmological perturbation theory with Buchert's non-perturbative framework, calculate the relevant averaged observables up to second order in the comoving synchronous gauge, and discuss their gauge dependence. With the help of an integrability condition the leading second order contributions follow from the first order calculation. We focus on the onset of cosmological backreaction, as a perturbative treatment is necessarily restricted to the era when the effect is still small. We demonstrate that the leading contributions to all averaged physical observables are completely specified on the boundary of the averaged domain. For any finite domain these surface terms are nonzero in general and thus backreaction is for real. We map the backreaction effect on an effectively homogeneous and isotropic fluid. The generic effective equation of state is not only time dependent, but also depends on scale.