A rule of thumb for cosmological backreaction

Abstract

In the context of second order perturbation theory, cosmological backreaction is seen torescale both time and the scale factor. The issue of the homogeneous limit oflong-wavelength perturbations is addressed and backreaction is quantified in terms of agauge-invariant metric function that is the true physical degree of freedom in thehomogeneous limit. The time integral of this metric function controls whether backreactionhastens or delays the expansion of the universe. As an example, late-time acceleration ofthe universe is shown to be inconsistent with a perturbative approach. Any tendency toaccelerate the expansion requires negative non-adiabatic pressure fluctuations.