Time-dependent corrections to the Ly αescape probability during cosmological recombination

Abstract

We consider the effects connected with the detailed radiative transfer during the epoch of cosmological recombination on the ionization history of our Universe. We focus on the escape of photons from the hydrogen Lyman α resonance at redshifts 600 z 2000, one of two key mechanisms defining the rate of cosmological recombination. We approach this problem within the standard formulation, and corrections due to two-photon interactions are deferred to another paper. As a main result we show here that within a non-stationary approach to the escape problem, the resulting correction in the free electron fraction, Ne, is about ∼1.6–1.8% in the redshift range 800 z 1200. Therefore the discussed process results in one of the largest modifications to the ionization history close to the maximum of Thomson-visibility function at z ∼ 1100 considered so far. We prove our results both numerically and analytically, deriving the escape probability, and considering both Lyman α line emission and line absorption in a way different from the Sobolev approximation. In particular, we give a detailed derivation of the Sobolev escape probability during hydrogen recombination, and explain the underlying assumptions. We then discuss the escape of photons for the case of coherent scattering in the lab frame, solving this problem analytically in the quasi-stationary approximation and also in the time-dependent case. We show here that during hydrogen recombination the Sobolev approximation for the escape probability is not valid at the level of ΔP/P ∼ 5–10%. This is because during recombination the ionization degree changes significantly over a characteristic time Δz/z ∼ 10%, so that at percent level accuracy the photon distribution is not evolving along a sequence of quasi-stationary stages. Non-stationary corrections increase the effective escape by ΔP/P ∼ +6.4% at z ∼ 1490, and decrease it by ΔP/P ∼− 7.6% close to the maximum of the Thomson-visibility function. We also demonstrate the crucial role of line emission and absorption in distant wings (hundreds and thousands of Doppler widths from the resonance) for this effect, and argue that the final answer probably can only be given within a more rigorous formulation of the problem using a two- or multi-photon description.