Recombination of H and He in Yang-Mills Gravity

TL;DR

This paper explores the recombination process in the early universe under Yang-Mills Gravity, revealing differences from standard models and proposing ways to test YMG through CMB observations.

Contribution

It analytically and numerically studies hydrogen and helium recombination in YMG, highlighting stronger helium effects and broader recombination temperature ranges compared to GR.

Findings

Primordial helium significantly affects free electron density in YMG.

Recombination occurs over a wider temperature range in YMG.

Last scattering temperature remains sharply around 2000 K.

Abstract

We investigate some aspects of the thermal history of the early universe according to Yang-Mills Gravity (YMG); a gauge theory of gravity set in flat spacetime. Specifically, equations for the ionization fractions of hydrogen and singly ionized helium during the recombination epoch are deduced analytically and then solved numerically. By considering several approximations we find that the presence of primordial helium and its interaction with Lyman series photons has a much stronger effect on the overall free electron density in YMG than it does in the standard, General Relativity (GR) based, model. Compared to the standard model recombination happens over a much larger range of temperatures, although there is still a very sharp temperature of last scattering around 2000 K. Since the ionization history of the universe is not directly observable we discuss how one may use it to predict the CMB power spectrum and thus test YMG. This topic will be explored in detail in an upcoming paper.