A Correct Treatment of the Stimulated Process in Cosmic Recombination

Abstract

In previous studies on cosmic recombination, the stimulated-recombination rate was calculated assuming thermal equilibrium between the excited and ionized states of neutral hydrogen atoms. After pointing out that this assumption was not correct, we present a correct method for treating their transitions. Numerical calculations show that: (i) the epoch of recombination is not greatly altered by this revision, but that (ii) the residual ionization degree after a “freeze-out” becomes smaller by a factor of 3.