Observable Deviations from Homogeneity in an Inhomogeneous Universe

TL;DR

This paper uses numerical relativity to study how inhomogeneities in the universe cause observable deviations from FLRW models, affecting cosmological measurements and interpretations.

Contribution

It demonstrates that inhomogeneities lead to measurable deviations in cosmological observables, highlighting the importance of path-dependent effects in inhomogeneous universes.

Findings

Inhomogeneous structures cause deviations in Hubble diagrams.

Observable effects grow with source redshift.

Inhomogeneities impact the relationship between angular diameter distance and source extent.

Abstract

How does inhomogeneity affect our interpretation of cosmological observations? It has long been wondered to what extent the observable properties of an inhomogeneous universe differ from those of a corresponding Friedman-Lemaitre-Robertson-Walker (FLRW) model, and how the inhomogeneities affect that correspondence. Here, we use numerical relativity to study the behavior of light beams traversing an inhomogeneous universe and construct the resulting Hubble diagrams. The universe that emerges exhibits an average FLRW behavior, but inhomogeneous structures contribute to deviations in observables across the observer's sky. We also investigate the relationship between angular diameter distance and the angular extent of a source, finding deviations that grow with source redshift. These departures from FLRW are important path-dependent effects with implications for using real observables in an inhomogeneous universe such as our own.