Backreaction effects on the matter side of Einstein's field equations

TL;DR

This paper discusses how matter perturbations and viscosity effects in the dark sector can alter the universe's expansion, potentially explaining acceleration without dark energy, and explores ways to constrain these effects observationally.

Contribution

It introduces a new expression for matter perturbation backreaction effects on cosmic expansion and discusses their potential observational implications.

Findings

Viscous damping can modify the universe's expansion history.

Backreaction effects could account for cosmic acceleration.

Observable consequences are possible in precision cosmology.

Abstract

Recently, we have derived a novel and compact expression for how perturbations in the matter fields of the cosmological fluid can lead to deviations from the standard Friedmann equations. Remarkably, the dissipative damping of velocity perturbations by bulk and shear viscosity in the dark sector can modify the expansion history of the universe on arbitrarily large scales. In universes in which this effect is sufficiently sizeable, it could account for the acceleration of the cosmological expansion. But even if dark matter should be less viscous and if the effect would be correspondingly smaller, it may have observable consequences in the era of precision cosmology. Here, we review the origin of this backreaction effect and possibilities to constrain it further.