1999/09/16 by Sara Seager, Dimitar Sasselov, Dimitar D. Sasselov +2 · 5 citations
Physics and Astronomy · #Astrophysics and Star Formation Studies #Cosmology and Gravitation Theories #Dark Matter and Cosmic Phenomena #astro-ph
paper · pdf · doi:10.1086/312250
published as Astrophys.J.523:L1-L5,1999 · 5 pages, including 2 figures, uses emulateapj.sty, accepted for publication in ApJ Letters, the code RECFAST is available in fortran at http://www.astro.ubc.ca/people/scott/recfast.html and in C at http://cfa-www.harvard.edu/~seager/Rec/rec.html and http://cfa-www.harvard.edu/~sasselov/rec/
arxiv created 1999/09/16 · openalex publication_date 1999/09/20 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We have developed an improved recombination calculation of H, He I, and He II in the early universe that involves a line-by-line treatment of each atomic level. We find two major differences compared with previous calculations. First, the ionization fraction x e is approximately 10% smaller for redshifts ≲800 because of nonequilibrium processes in the excited states of H. Second, He I recombination is much slower than previously thought, and it is delayed until just before H recombines. We describe the basic physics behind the new results and present a simple way to reproduce our calculation. This should enable a fast computation of the ionization history (and of the quantities such as the power spectrum of cosmic microwave background anisotropies that depend on it) for arbitrary cosmologies, without the need to consider the hundreds of atomic levels used in our complete model.