Towards a complete treatment of the cosmological recombination problem, MNRAS 412

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 multilevel 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 Ne/Ne ∼ 0.9 per cent at z ∼ 920. Two-photon decay and the Raman scattering from higher levels (n> 3) result in small additional modifications, and precise results can be obtained when including their effect for the first three to five 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 run time, can be used in the analysis of future cosmic microwave background dat