The effect of cosmic inhomogeneities on the average cosmological dynamics

TL;DR

This paper reviews recent theoretical work suggesting that cosmic inhomogeneities have a negligible effect on large-scale universe dynamics if homogeneity occurs at scales much smaller than the Hubble length, though observational evidence is inconclusive.

Contribution

It synthesizes multiple approaches to averaging in cosmology, arguing that back-reaction effects are likely small if homogeneity is established at sufficiently small scales.

Findings

Back-reaction from inhomogeneities is negligible if homogeneity scale is much smaller than Hubble length.

Multiple approaches agree on the small impact of averaging in the real universe.

No definitive observational evidence for the homogeneity scale being much smaller than the Hubble scale.

Abstract

It is generally assumed that on sufficiently large scales the Universe is well-described as a homogeneous, isotropic FRW cosmology with a dark energy. Does the formation of nonlinear cosmic inhomogeneities produce a significant effect on the average large-scale FLRW dynamics? As an answer, we suggest that if the length scale at which homogeneity sets in is much smaller than the Hubble length scale, the back-reaction due to averaging over inhomogeneities is negligible. This result is supported by more than one approach to study of averaging in cosmology. Even if no single approach is sufficiently rigorous and compelling, they are all in agreement that the effect of averaging in the real Universe is small. On the other hand, it is perhaps fair to say that there is no definitive observational evidence yet that there indeed is a homogeneity scale which is much smaller than the Hubble scale, or for that matter, if today's Universe is indeed homogeneous on large scales. If the Copernican principle can be observationally established to hold, or is theoretically assumed to be valid, this provides strong evidence for homogeneity on large scales. However, even this by itself does not say what the scale of homogeneity is. If that scale is today comparable to the Hubble radius, only a fully non-perturbative analysis can establish or rule out the importance of cosmological back-reaction. This brief elementary report summarizes some recent theoretical developments on which the above inferences are based.