Is there an early Universe solution to Hubble tension?

TL;DR

The paper investigates whether a descending $H_0$ with redshift can be detected in a low-redshift data set ($z\leq0.7$) by binning megamasers, cosmic chronometers, Type Ia SNe, and BAO. Using flat $\Lambda$CDM with $H_0$ and $\Omega_m$, and treating $M$ and $r_d$ as free parameters, the authors perform an $MCMC$ fit and find an overall $H_0$ near 70, with a statistically significant (2.1$\sigma$) trend of $H_0$ decreasing with $z$ across bins. The result persists across data types but is sensitive to the inclusion of cosmic chronometers and priors on $r_d$; with Planck priors on $r_d$, the trend weakens. If the trend is real, it challenges early-Universe solutions to the Hubble tension and points toward late-time physics, offering a new diagnostic for cosmological model building beyond $\Lambda$CDM.

Abstract

We consider a low redshift $(z<0.7)$ cosmological dataset comprising megamasers, cosmic chronometers, type Ia SNe and BAO, which we bin according to their redshift. For each bin, we read the value of $H_0$ by fitting directly to the flat $Λ$CDM model. Doing so, we find that $H_0$ descends with redshift, allowing one to fit a line with a \textit{non-zero} slope of statistical significance $2.1 \, σ$. Our analysis rests on the use of cosmic chronometers to break a degeneracy in BAO data and it will be imperative to revisit this feature as data improves. Nevertheless, our results provide the first independent indication of the descending trend reported by the H0LiCOW collaboration. If substantiated going forward, early Universe solutions to the Hubble tension will struggle explaining this trend.

Figures (2)

• Figure 1: Overlapping probability distribution functions for each bin plotted alongside the overall constraint.
• Figure 2: The best-fit line against the binned data. The line is $2.1 \, \sigma$ removed from the flat line null hypothesis.