HYREC-2: a highly accurate sub-millisecond recombination code

Understand

We present the new recombination code HYREC-2, holding the same accuracy as the state-of-the-art codes HYREC and COSMOREC and, at the same time, surpassing the computational speed of the code RECFAST commonly used for CMB-anisotropy data analyses.

• HYREC-2 is based on an effective 4-level atom model, accounting exactly for the non-equilibrium of highly excited states of hydrogen, and very accurately for radiative transfer effects with a correction to the Lyman-$\alpha$ escape rate.
• The latter is computed with the original HYREC, and tabulated, as a function of temperature, along with its derivatives with respect to the relevant cosmological parameters.
• This enables the code to keep the same accuracy as the original HYREC-2 over the full 99.7% confidence region of cosmological parameters currently allowed by Planck, while running in under one millisecond on a standard laptop.