Searching for signals of inhomogeneity using multiple probes of the cosmic expansion rate H(z)

TL;DR

This paper discusses how different methods of measuring the cosmic expansion rate can reveal inhomogeneities in the universe, with potential signals indicating deviations from the standard FLRW model due to cosmic backreaction.

Contribution

It clarifies under what conditions measurements of H(z) from cosmic chronometers and redshift drift can detect inhomogeneity and backreaction effects in the universe.

Findings

Cosmic chronometers estimate the average expansion rate even in inhomogeneous universes.

Redshift drift measurements align with the average expansion rate only in FLRW-like universes.

Discrepancies between the two methods can signal significant cosmic inhomogeneity.

Abstract

It is argued that cosmic chronometers yield estimates of the spatially averaged expansion rate even in a universe that is not well described by a global FLRW model - as long as the Universe is statistically homogeneous and isotropic with a sufficiently small homogeneity scale. On the other hand, measurements of the expansion rate based on observations of redshift drift will not in general yield estimates of the spatially averaged expansion rate - but it will in the case where the universe is described well by a single FLRW model on large scales. Therefore, a disagreement between measurements of the expansion rate based on cosmic chronometers versus redshift drift is an expected signal of non-negligible cosmic backreaction.