The influence of structure formation on the cosmic expansion

TL;DR

This paper examines how the average effects of structure formation influence cosmic expansion, revealing potential biases in dark energy measurements and intrinsic errors in cosmological parameter estimation.

Contribution

It provides a second-order perturbative analysis of backreaction effects in the concordance model, highlighting an intrinsic homogeneity scale and its impact on expansion rate measurements.

Findings

Backreaction can increase the deceleration parameter by over 10%.

An intrinsic homogeneity scale affects the residual offset in expansion measurements.

Intrinsic variance introduces errors that challenge high-precision cosmological parameter estimation.

Abstract

We investigate the effect that the average backreaction of structure formation has on the dynamics of the cosmological expansion, within the concordance model. Our approach in the Poisson gauge is fully consistent up to second-order in a perturbative expansion about a flat Friedmann background, including a cosmological constant. We discuss the key length scales which are inherent in any averaging procedure of this kind. We identify an intrinsic homogeneity scale that arises from the averaging procedure, beyond which a residual offset remains in the expansion rate and deceleration parameter. In the case of the deceleration parameter, this can lead to a quite large increase in the value - more than 10% - and may therefore have important ramifications for dark energy measurements, even if the underlying nature of dark energy is a cosmological constant. We give the intrinsic variance that affects the value of the effective Hubble rate and deceleration parameter. These considerations serve to add extra intrinsic errors to our determination of the cosmological parameters, and, in particular, may render attempts to measure the Hubble constant to percent precision overly optimistic.