RECFAST: Calculate the Recombination History of the Universe

**Abstract**: RECFAST calculates the recombination of H, HeI, and HeII in the early Universe; this involves a line-by-line treatment of each atomic level. It differs in comparison to previous calculations in two major ways: firstly, the ionization fraction x_e is approximately 10% smaller for redshifts <~800, due to non-equilibrium processes in the excited states of H, and secondly, HeI recombination is much slower than previously thought, and is delayed until just before H recombines. RECFAST enables fast computation of the ionization history (and quantities such as the power spectrum of CMB anisotropies which depend on it) for arbitrary cosmologies.

**Credit**: Seager, Sara; Sasselov, Dimitar D.; Scott, Douglas

**Site**: https://www.cfa.harvard.edu/~sasselov/rec/

http://adsabs.harvard.edu/abs/1999ApJ...523L...1S

**Bibcode**: 2011ascl.soft06026S

**ID**: ascl:1106.026