The mass effect -- Variations of the electron mass and their impact on cosmology

TL;DR

Variations in the electron mass could significantly reduce the Hubble tension in cosmology, with current data mildly favoring such models, though constraints remain weak and future observations are needed.

Contribution

This paper reviews and analyzes how electron mass variations can alleviate the Hubble tension, providing updated significance levels and discussing observational constraints.

Findings

Electron mass variations can reduce Hubble tension from ~5σ to below 2σ.

CMB, BAO, and supernova data mildly favor electron mass variation models.

Constraints from nucleosynthesis and atomic clocks are currently weak, but future data may be decisive.

Abstract

We summarize and explain the current status of time variations of the electron mass in cosmology, showing that such variations allow for significant easing of the Hubble tension, from the current $\sim5\sigma$ significance, down to between $3.4\sigma$ and $1.0\sigma$ significance, depending on the precise model and data. Electron mass variations are preferred by Cosmic Microwave Background (CMB) data in combination with the latest results on baryonic acoustic oscillations (BAO) and type Ia supernovae at a level of significance between $2\sigma$ and $3.6\sigma$ depending on the model and the data. This preference for a model involving an electron mass variation is neither tightly constrained from light element abundances generated during big bang nucleosynthesis nor from post-recombination observations using quasars and atomic clocks, though future data is expected to give strong evidence in favor of or against this model.