A New Calculation of the Recombination Epoch

Details A New Calculation of the Recombination Epoch A New Calculation of the Recombination Epoch

1999-09-15

arxiv.org/abs/astro-ph/9909275v2

Astrophys.J.523:L1-L5,1999

Astronomy and Astrophysics

Astrophysics

5 pages, including 2 figures, uses emulateapj.sty, accepted for publication in ApJ Letters, the code RECFAST is available in f

Scientific paper

10.1086/312250

We have developed an improved recombination calculation of H, HeI, and HeII in the early Universe which involves a line-by-line treatment of each atomic level. We find two major differences compared with previous calculations. Firstly, the ionization fraction x_e is approximately 10% smaller for redshifts <~800, due to non-equilibrium processes in the excited states of H. Secondly, HeI recombination is much slower than previously thought, and 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 fast computation of the ionization history (and quantities such as the power spectrum of CMB anisotropies which depend on it) for arbitrary cosmologies, without the need to consider the hundreds of atomic levels used in our complete model.

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