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

## TL;DR

This paper investigates how variations in electron mass affect neutrino decoupling, entropy flow, and Big Bang Nucleosynthesis, highlighting the sensitivity of early universe physics to fundamental particle properties and QED effects.

## Contribution

It provides a detailed analysis of the influence of electron mass and QED effects on early universe processes, emphasizing their impact on cosmological observables and particle physics constraints.

## Key findings

- Electron mass variations significantly alter neutrino decoupling dynamics.
- QED effects on plasma state introduce uncertainties in BBN predictions.
- Neutrino-to-photon energy ratios affect CMB and primordial element abundances.

## Abstract

We present calculations showing how electron rest mass influences entropy flow, neutrino decoupling, and Big Bang Nucleosynthesis (BBN) in the early universe. To elucidate this physics and especially the sensitivity of BBN and related epochs to electron mass, we consider a parameter space of rest mass values larger and smaller than the accepted vacuum value. Electromagnetic equilibrium, coupled with the high entropy of the early universe, guarantees that significant numbers of electron-positron pairs are present, and dominate over the number of ionization electrons to temperatures much lower than the vacuum electron rest mass. Scattering between the electrons-positrons and the neutrinos largely controls the flow of entropy from the plasma into the neutrino seas. Moreover, the number density of electron-positron-pair targets can be exponentially sensitive to the effective in-medium electron mass. This entropy flow influences the phasing of scale factor and temperature, the charged current weak-interaction-determined neutron-to-proton ratio, and the spectral distortions in the relic neutrino energy spectra. Our calculations show the sensitivity of the physics of this epoch to three separate effects: finite electron mass, finite-temperature quantum electrodynamic (QED) effects on the plasma equation of state, and Boltzmann neutrino energy transport. The ratio of neutrino to plasma component energy scales manifests in Cosmic Microwave Background (CMB) observables, namely the baryon density and the radiation energy density, along with the primordial helium and deuterium abundances. Our results demonstrate how the treatment of in-medium electron mass (i.e., QED effects) could translate into an important source of uncertainty in extracting neutrino and beyond-standard-model physics limits from future high-precision CMB data.