Towards a complete treatment of the cosmological recombination problem

TL;DR

This paper introduces an advanced model for cosmological hydrogen recombination, incorporating detailed two-photon and Raman processes, to improve the accuracy of CMB analysis and develop a new computational tool, CosmoRec.

Contribution

It presents a comprehensive multi-level recombination model with explicit radiative transfer solutions, advancing beyond previous approximations for better CMB data analysis.

Findings

2s-1s Raman scattering delays recombination by ~0.9% at z~920

Higher-level two-photon and Raman effects cause small additional modifications

Inclusion of 3-5 shells yields precise recombination results

Abstract

A new approach to the cosmological recombination problem is presented, which completes our previous analysis on the effects of two-photon processes during the epoch of cosmological hydrogen recombination, accounting for ns-1s and nd-1s Raman events and two-photon transitions from levels with n>=2. The recombination problem for hydrogen is described using an effective 400-shell multi-level approach, to which we subsequently add all important recombination corrections discussed in the literature thus far. We explicitly solve the radiative transfer equation of the Lyman-series photon field to obtain the required modifications to the rate equations of the resolved levels. In agreement with earlier computations we find that 2s-1s Raman scattering leads to a delay in recombination by DN_e/N_e~0.9% at z~920. Two-photon decay and Raman scattering from higher levels (n>3) result in a small additional modifications, and precise results can be obtained when including their effect for the first 3-5 shells. This work is a major step towards a new cosmological recombination code (CosmoRec) that supersedes the physical model included in Recfast, and which, owing to its short runtime, can be used in the analysis of future CMB data from the Planck Surveyor.