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

TL;DR

HYREC-2 is a new recombination code that achieves state-of-the-art accuracy comparable to existing codes while significantly improving computational speed, enabling efficient analysis of CMB data and related cosmological observables.

Contribution

HYREC-2 introduces a highly accurate, fast recombination code based on an effective 4-level atom model with precise radiative transfer effects, surpassing RECFAST in speed.

Findings

Runs in under one millisecond on a standard laptop.

Maintains accuracy within the 99.7% confidence region of Planck data.

No noticeable bias in cosmological parameters for high-precision experiments.

Abstract

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. Our code leads to no noticeable bias in any cosmological parameters even in the case of an ideal cosmic-variance limited experiment up to $\ell$ = 5000. Beyond CMB anisotropy calculations, HYREC-2 will be a useful tool to compute various observables that depend on the recombination and thermal history, such as the recombination spectrum or the 21-cm signal.