Relative contribution of the hydrogen 2S two-photon decay and Lyman-alpha escape channels during the epoch of cosmological recombination

TL;DR

This paper analyzes the evolving roles of two-photon decay and Lyman-alpha escape in hydrogen recombination, revealing their impact on the CMB spectrum and the population dynamics of hydrogen energy levels during the epoch.

Contribution

It provides a detailed analysis of the time-dependent ratio of recombination channels and its influence on the Lyman-alpha line profile in the CMB spectrum.

Findings

The ratio of recombination via 2s two-photon decay to Lyman-alpha escape varies over time.

Overpopulation of the 2p level affects the recombination process near redshift 750.

The evolution of this ratio influences the shape of the Lyman-alpha line in the CMB spectrum.

Abstract

We discuss the evolution of the ratio in number of recombinations due to 2s two photon escape and due to the escape of Lyman-$\alpha$ photons from the resonance during the epoch of cosmological recombination, within the width of the last scattering surface and near its boundaries. We discuss how this ratio evolves in time, and how it defines the profile of the Lyman-$\alpha$ line in the spectrum of CMB. One of the key reasons for explaining its time dependence is the strong overpopulation of the 2p level relative to the 2s level at redshifts $z \la 750$.