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We present calculations showing how electron rest mass influences entropyflow, neutrino decoupling, and Big Bang Nucleosynthesis (BBN) in the earlyuniverse. To elucidate this physics and especially the sensitivity of BBN andrelated epochs to electron mass, we consider a parameter space of rest massvalues larger and smaller than the accepted vacuum value. Electromagneticequilibrium, coupled with the high entropy of the early universe, guaranteesthat significant numbers of electron-positron pairs are present, and dominateover the number of ionization electrons to temperatures much lower than thevacuum electron rest mass. Scattering between the electrons-positrons and theneutrinos largely controls the flow of entropy from the plasma into theneutrino seas. Moreover, the number density of electron-positron-pair targetscan be exponentially sensitive to the effective in-medium electron mass. Thisentropy flow influences the phasing of scale factor and temperature, thecharged current weak-interaction-determined neutron-to-proton ratio, and thespectral distortions in the relic neutrino energy spectra. Our calculationsshow the sensitivity of the physics of this epoch to three separate effects:finite electron mass, finite-temperature quantum electrodynamic (QED) effectson the plasma equation of state, and Boltzmann neutrino energy transport. Theratio of neutrino to plasma component energy scales manifests in CosmicMicrowave Background (CMB) observables, namely the baryon density and theradiation energy density, along with the primordial helium and deuteriumabundances. Our results demonstrate how the treatment of in-medium electronmass (i.e., QED effects) could translate into an important source ofuncertainty in extracting neutrino and beyond-standard-model physics limitsfrom future high-precision CMB data.

Insights into neutrino decoupling gleaned from considerations of the role of electron mass