Early recombination as a solution to the $H_0$ tension

TL;DR

This paper proposes that earlier recombination, achieved through a time-varying electron mass, can resolve the H_0 tension while maintaining a good fit to CMB, BAO, and SNeIa data, and also addresses the CMB lensing anomaly.

Contribution

It introduces a model with a time-varying electron mass that successfully resolves the H_0 tension and fits multiple cosmological datasets without needing local H_0 measurements.

Findings

Early recombination can reconcile H_0 measurements with CMB data.

A time-varying electron mass model fits CMB, BAO, and SNeIa data well.

The model yields H_0 around 72 km/sec/Mpc, consistent with local measurements.

Abstract

We show that the $H_0$ tension can be resolved by making recombination earlier, keeping the fit to cosmic microwave background (CMB) data almost intact. We provide a suite of general necessary conditions to give a good fit to CMB data while realizing a high value of $H_0$ suggested by local measurements. As a concrete example for a successful scenario with early recombination, we demonstrate that a model with time-varying $m_e$ can indeed satisfy all the conditions. We further show that such a model can also be well fitted to low-$z$ distance measurements of baryon acoustic oscillation (BAO) and type-Ia supernovae (SNeIa) with a simple extension of the model. Time-varying $m_e$ in the framework of $\Omega_k\Lambda$CDM is found to be a sufficient and excellent example as a solution to the $H_0$ tension, yielding $H_0=72.3_{-2.8} ^{+2.7}\,$km/sec/Mpc from the combination of CMB, BAO and SNeIa data even without incorporating any direct local $H_0$ measurements. Apart from the $H_0$ tension, this model is also favored from the viewpoint of the CMB lensing anomaly.