How well is our universe described by an FLRW model?

TL;DR

The paper reviews the accuracy of the FLRW model in describing our universe, showing it is highly effective on large scales despite local inhomogeneities, and discusses the validity of the $ ext{Lambda}$CDM model within general relativity.

Contribution

It provides a rigorous analysis demonstrating that small-scale inhomogeneities do not significantly affect large-scale cosmological dynamics, supporting the validity of the FLRW and $ ext{Lambda}$CDM models.

Findings

FLRW metric approximates the universe within 1 part in 10^4.

Large inhomogeneities do not produce significant large-scale backreaction.

Newtonian cosmologies are excellent approximations to Einstein's solutions.

Abstract

Extremely well! In the $\Lambda$CDM model, the spacetime metric, $g_{ab}$, of our universe is approximated by an FLRW metric, $g_{ab}^{(0)}$, to about 1 part in $10^4$ or better on both large and small scales, except in the immediate vicinity of very strong field objects, such as black holes. However, derivatives of $g_{ab}$ are not close to derivatives of $g_{ab}^{(0)}$, so there can be significant differences in the behavior of geodesics and huge differences in curvature. Consequently, observable quantities in the actual universe may differ significantly from the corresponding observables in the FLRW model. Nevertheless, as we shall review here, we have proven general results showing that---within the framework of our approach to treating backreaction---the large matter inhomogeneities that occur on small scales cannot produce significant effects on large scales, so $g_{ab}^{(0)}$ satisfies Einstein's equation with the averaged stress-energy tensor of matter as its source. We discuss the flaws in some other approaches that have suggested that large backreaction effects may occur. As we also will review here, with a suitable "dictionary," Newtonian cosmologies provide excellent approximations to cosmological solutions to Einstein's equation (with dust and a cosmological constant) on all scales. Our results thereby provide strong justification for the mathematical consistency and validity of the $\Lambda$CDM model within the context of general relativistic cosmology.