Relativistic Electron Scattering and Big Bang Nucleosynthesis

TL;DR

This paper reveals that relativistic electron scattering in the early universe modifies nuclear velocity distributions during Big Bang nucleosynthesis, significantly affecting predicted light-element abundances and potentially indicating new physics.

Contribution

It introduces a novel correction to BBN reaction rates by accounting for relativistic electron scattering, challenging previous assumptions of Maxwell-Boltzmann distributions.

Findings

Modified reaction rates alter light-element abundance predictions.

Discrepancies between BBN predictions and observations are exacerbated.

Suggests possible need for new physics in early universe models.

Abstract

This paper is superseded by Arxiv:1911.07334. Big-bang nucleosynthesis (BBN) is a valuable tool to constrain the physics of the early universe and is the only probe of the radiation-dominated epoch. A fundamental assumption in BBN is that the nuclear velocity distributions obey Maxwell-Boltzmann statistics as they do in stars. In this letter, however, we point out that there is a fundamental difference between stellar reaction rates and BBN reaction rates. Specifically, the BBN epoch is characterized by a dilute baryon plasma for which the velocity distribution of nuclei is mainly determined by the dominant Coulomb scattering with mildly relativistic electrons. This modifies the nuclear velocity distributions and significantly alters the thermonuclear reaction rates, and hence, the light-element abundances. We show that this novel result alters all previous calculations of light-element abundances from BBN, and indeed exacerbates the discrepancies between BBN and inferred primordial light-element abundances possibly suggesting the need for new physics in the early universe.