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 the following $1\sigma$ bounds on $m_e$ in the early Universe: $m_e=0.505_{-0.007}^{+0.006}\mathrm{MeV}$ (for the NACRE II nuclear reaction network) or $m_e=0.509_{-0.004}^{+0.005}\mathrm{MeV}$ (for the PRIMAT nuclear reaction network). These bounds have been derived by adopting the recent determination of the primordial helium-4 abundance by the Large Binocular Telescope observations of 54 metal-poor H ii regions. If instead we adopt the Particle Data Book helium-4 abundance, the bounds are $m_e=0.503_{-0.015}^{+0.011}\mathrm{MeV}$ (NACRE II) or $m_e=0.521_{-0.007}^{+0.009}\mathrm{MeV}$ (PRIMAT) The obtained allowed ranges are close to the present laboratory value at the level of $\sim 0.4\%–2\%$, depending on the dataset and nuclear network, thus supporting the constancy of the electron mass over cosmological timescales.

Close

PrevNext

Reuse & PermissionsView figure in article

• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 

• Big bang nucleosynthesis
• Determination of fundamental constants
• Evolution of the Universe
• Electrons