Effect of neutrino rest mass on ionization equilibrium freeze-out

TL;DR

This paper explores how small neutrino rest masses influence the early universe's ionization freeze-out, potentially affecting cosmological measurements and offering a new way to probe neutrino mass with future CMB experiments.

Contribution

It introduces a novel effect of neutrino rest mass on ionization freeze-out and its implications for cosmological parameter inference.

Findings

Neutrino mass increases the expansion rate near photon decoupling.

This leads to an earlier, higher temperature freeze-out of ionization.

The effect could be detectable with next-generation CMB experiments.

Abstract

We discuss how small neutrino rest masses can increase the expansion rate near the photon decoupling epoch in the early universe, causing an earlier, higher temperature freeze-out for ionization equilibrium compared to the massless neutrino case. This yields a larger free-electron fraction. A larger ratio of the sound horizon to the photon diffusion length follows, implying a smaller inferred Neff. This neutrino-mass/recombination effect depends strongly on the neutrino rest masses. Though below current sensitivity, this effect could be probed by next-generation cosmic microwave background experiments, giving an observational handle of neutrino mass physics.