  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.
