HyRec: A fast and highly accurate primordial hydrogen and helium recombination code

Abstract

We present a state-of-the-art primordial recombination code, HYREC, including all the physical effects that have been shown to significantly affect recombination. The computation of helium recombination includes simple analytic treatments of hydrogen continuum opacity in the He I ${2}^{1}{P}^{o}\ensuremath{-}{1}^{1}S$ line, the He I] ${2}^{3}{P}^{o}\ensuremath{-}{1}^{1}S$ line, and treats feedback between these lines within the on-the-spot approximation. Hydrogen recombination is computed using the effective multilevel atom method, virtually accounting for an infinite number of excited states. We account for two-photon transitions from $2s$ and higher levels as well as frequency diffusion in Lyman-$\ensuremath{\alpha}$ with a full radiative transfer calculation. We present a new method to evolve the radiation field simultaneously with the level populations and the free electron fraction. These computations are sped up by taking advantage of the particular sparseness pattern of the equations describing the radiative transfer. The computation time for a full recombination history is $\ensuremath{\sim}2$ seconds. This makes our code well suited for inclusion in Monte Carlo Markov chains for cosmological parameter estimation from upcoming high-precision cosmic microwave background anisotropy measurements.