On the physical effects of consistent cosmological averaging

TL;DR

This paper investigates how averaging in cosmology affects the universe's expansion, finding small shifts in the Hubble parameter but large effects on the deceleration parameter, using perturbation theory.

Contribution

It provides a self-consistent calculation of backreaction effects on cosmological dynamics using second-order perturbation theory in a specific gauge.

Findings

Fractional shift in Hubble parameter ~ 10^{-5}

Effective energy density ~ a few times 10^{-5}

Large fractional shift in deceleration parameter Q

Abstract

We use cosmological perturbation theory to study the backreaction effects of a self-consistent and well-defined cosmological averaging on the dynamics and the evolution of the Universe. Working with a perturbed Friedman-Lemaitre-Robertson-Walker Einstein-de Sitter cosmological solution in a comoving volume-preserving gauge, we compute the expressions for the expansion scalar and deceleration parameter to second order, which we use to characterize the backreaction. We find that the fractional shift in the Hubble parameter with respect to the input background cosmological model is Delta~10^{-5}, which leads to an effective energy density of the order of a few times 10^{-5}. In addition, we find that an appropriate measure of the fractional shift in the deceleration parameter Q is very large.