Averaged universe confronted with cosmological observations: A fully covariant approach

TL;DR

This paper uses a covariant averaging framework to analyze the impact of cosmic inhomogeneities on the universe's expansion, constraining backreaction effects with multiple cosmological data sets including the first full Planck CMB analysis.

Contribution

It introduces a fully covariant approach to averaging in cosmology and constrains the backreaction parameter using comprehensive observational data, including the first full Planck CMB analysis.

Findings

Backreaction density parameter $oxed{ ext{−0.0155} ext{ to } 0}$ at 68% CL.

Backreaction is strongly correlated with $oxed{ ext{Ω}_ ext{ ext{Λ}}}$, $oxed{ ext{σ}_8}$, and $oxed{ ext{H}_0}$.

A small backreaction effect of a few percent could influence precision cosmology.

Abstract

One of the outstanding problems in general relativistic cosmology is that of the averaging. That is, how the lumpy universe that we observe at small scales averages out to a smooth Friedmann-Lemaitre-Robertson-Walker (FLRW) model. The root of the problem is that averaging does not commute with the Einstein equations that govern the dynamics of the model. This leads to the well-know question of backreaction in cosmology. In this work, we approach the problem using the covariant framework of Macroscopic Gravity (MG). We use its cosmological solution with a flat FLRW macroscopic background where the result of averaging cosmic inhomogeneities has been encapsulated into a backreaction density parameter denoted $\Omega_\mathcal{A}$. We constrain this averaged universe using available cosmological data sets of expansion and growth including, for the first time, a full CMB analysis from Planck temperature anisotropy and polarization data, the supernovae data from Union 2.1, the galaxy power spectrum from WiggleZ, the weak lensing tomography shear-shear cross correlations from the CFHTLenS survey and the baryonic acoustic oscillation data from 6Df, SDSS DR7 and BOSS DR9. We find that $-0.0155 \le \Omega_\mathcal{A} \le 0$ (at the 68\% CL) thus providing a tight upper-bound on the backreaction term. We also find that the term is strongly correlated with cosmological parameters such $\Omega_\Lambda$, $\sigma_8$ and $H_0$. While small, a backreaction density parameter of a few percent should be kept in consideration along with other systematics for precision cosmology.