The trouble with Hubble: Local versus global expansion rates in inhomogeneous cosmological simulations with numerical relativity

TL;DR

This study uses numerical relativity to simulate inhomogeneous cosmologies, finding local expansion rates closely match global ones, and concludes local variance cannot explain discrepancies in Hubble constant measurements.

Contribution

It demonstrates that local inhomogeneities have minimal impact on the measured Hubble parameter, challenging the idea that local variance resolves the Hubble tension.

Findings

Local Hubble measurements differ by less than 1% from global values.

Local variance cannot account for the Hubble constant discrepancy.

Results align with Newtonian gravity predictions.

Abstract

In a fully inhomogeneous, anisotropic cosmological simulation performed by solving Einstein's equations with numerical relativity, we find a local measurement of the effective Hubble parameter differs by less than 1\% compared to the global value. This variance is consistent with predictions from Newtonian gravity. We analyse the averaged local expansion rate on scales comparable to Type 1a supernova surveys, and find that local variance cannot resolve the tension between the \citet{riess2018b} and \citet{planck2018a} measurements.