Radiative transfer effects in primordial hydrogen recombination
  The calculation of a highly accurate cosmological recombination history has
been the object of particular attention recently, as it constitutes the major
theoretical uncertainty when predicting the angular power spectrum of Cosmic
Microwave Background anisotropies. Lyman transitions, in particular the
Lyman-alpha line, have long been recognized as one of the bottlenecks of
recombination, due to their very low escape probabilities. The Sobolev
approximation does not describe radiative transfer in the vicinity of Lyman
lines to a sufficient degree of accuracy, and several corrections have already
been computed in other works. In this paper, the impact of some previously
ignored radiative transfer effects is calculated. First, the effect of Thomson
scattering in the vicinity of the Lyman-alpha line is evaluated, using a full
redistribution kernel incorporated into a radiative transfer code. The effect
of feedback of distortions generated by the optically thick deuterium
Lyman-alpha line blueward of the hydrogen line is investigated with an analytic
approximation. It is shown that both effects are negligible during cosmological
hydrogen recombination. Secondly, the importance of high-lying, non overlapping
Lyman transitions is assessed. It is shown that escape from lines above
Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected
without loss of accuracy. Thirdly, a formalism generalizing the Sobolev
approximation is developed to account for the overlap of the high-lying Lyman
lines, which is shown to lead to negligible changes to the recombination
history. Finally, the possibility of a cosmological hydrogen recombination
maser is investigated. It is shown that there is no such maser in the purely
radiative treatment presented here.
