Early-universe constraints on the electron mass

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 ($1σ$), 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.

Figures (12)

• Figure 1: Evolution of the comoving temperatures $z_\gamma$, $z_{\nu_e}$, and $z_{\nu_\mu}$ as functions of $x'$.
• Figure 2: Evolution of the comoving energy density $\bar{\rho}$ as a function of $x'$.
• Figure 3: Evolution of $N_\mathrm{eff}$ as a function of $m_e$.
• Figure 4: Primordial abundances of ^4He, D, ^3He, and ^7Li as predicted by PRyMordial within the Standard Model (NACRE II compilation package used for the key processes), as a function of the cosmic baryon density. The cyan vertical band indicates the CMB constraint $\eta_{10} = 6.040 \pm 0.118$Yeh2022.
• Figure 5: Time evolution of primordial abundances for different values of $m_e$, using the NACRE II (top panel) and PRIMAT (bottom panel) compilations.