Early-universe constraints on the electron mass

TL;DR

This paper explores how variations in the electron mass during the early Universe affect neutrino decoupling and Big Bang Nucleosynthesis, leading to constraints that support the constancy of the electron mass over cosmological timescales.

Contribution

It provides the first cosmological bounds on the electron mass during the early Universe, linking particle physics with cosmological observations.

Findings

Derived bounds on early-universe electron mass close to current laboratory value.

Supported the constancy of electron mass over cosmological timescales.

Constrained neutrino energy density and primordial element abundances.

Abstract

We investigate the impact of a nonstandard electron mass $m_e$ on early-Universe thermal history, focusing on neutrino decoupling and Big Bang Nucleosynthesis (BBN). In the standard cosmology, neutrino--electron interactions keep neutrinos in thermal contact with the electromagnetic plasma until shortly before $e^\pm$ annihilation. Varying $m_e$ shifts the decoupling epoch and the entropy transfer from $e^\pm$ annihilation, thereby modifying the neutrino energy density and the inferred effective number of relativistic species, $N_{\mathrm{eff}}$. Independently, during BBN the rates of charged-current weak processes, and hence the neutron-to-proton ratio, depend on $m_e$. By confronting BBN predictions for the primordial light-element abundances with observations and imposing cosmological constraints on $N_{\mathrm{eff}}$, we obtain a bound on $m_e$ in the early Universe of $m_e = 0.504^{+0.007}_{-0.006}$ MeV or $m_e=0.510\pm0.007$ MeV ($1\sigma$), depending on the considered nuclear reaction network (NACRE II or PRIMAT, respectively). The allowed range is close to the present laboratory value at the level of 1.4\%, thus supporting the constancy of the electron mass over cosmological timescales.