Does Hubble Tension Signal a Breakdown in FLRW Cosmology?

TL;DR

This paper investigates whether the Hubble tension indicates a failure of the FLRW cosmological model by analyzing low redshift data and early Universe physics, suggesting a possible breakdown of the standard framework.

Contribution

It demonstrates that low redshift data restrict modifications to FLRW cosmology to be close to a cosmological constant, and explores how early Universe physics could reconcile the Hubble tension.

Findings

Low redshift data favor a dark energy sector near the cosmological constant

Late Universe modifications within FLRW are ruled out by current data

Early Universe physics can set an upper limit on H0 around 71 km/s/Mpc

Abstract

The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the Hubble parameter $H(z)$, and a generic low redshift modification to flat $\Lambda$CDM, we find that low redshift data ($z \lesssim 2.5$) and well-motivated priors only permit a dark energy sector close to the cosmological constant $\Lambda$. This restriction rules out late Universe modifications within FLRW. We show that early Universe physics that alters the sound horizon can yield an upper limit of $H_0 \sim 71 \pm 1$ km/s/Mpc. Since various local determinations may be converging to $H_0 \sim 73$ km/s/Mpc, a breakdown of the FLRW framework is a plausible resolution. We outline how future data, in particular strongly lensed quasar data, could also provide further confirmations of such a resolution.