A New Calculation of the Recombination Epoch
Abstract
We have developed an improved recombination calculation of H, He I, and He II in the early universe that involves a linebyline 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 September 1999
 DOI:
 10.1086/312250
 arXiv:
 arXiv:astroph/9909275
 Bibcode:
 1999ApJ...523L...1S
 Keywords:

 ATOMIC PROCESSES;
 COSMOLOGY: COSMIC MICROWAVE BACKGROUND;
 COSMOLOGY: THEORY;
 COSMOLOGY: EARLY UNIVERSE;
 Atomic Processes;
 Cosmology: Cosmic Microwave Background;
 Cosmology: Theory;
 Cosmology: Early Universe;
 Astrophysics
 EPrint:
 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://cfawww.harvard.edu/~seager/Rec/rec.html and http://cfawww.harvard.edu/~sasselov/rec/