The Chemistry of the Early Universe

Abstract

The process of molecule formation in the primordial gas is considered in the framework of Friedmann cosmological models from redshift z=1e4 to z=0. First, a comprehensive analysis of 87 gas phase reaction rates (both collisional and radiative) relevant in the physical environment of the expanding universe is presented and critically discussed. On this basis, calculations are carried out of the abundance of 21 molecular species as function of redshift evaluating consistently the molecular heating and cooling due to H2, HD and LiH molecules. One of the major improvements of this work is the use of a better treatment of H recombination that leads to a reduction of a factor 2-3 in the abundance of electrons and H+ at freeze-out, with respect to previous studies. We find that in the standard model, the residual fractional ionization at z=1 is 3.02e-4, and the main molecular species fractional abundances H2/H=1.1e-6, HD/H2=1.1e-3, HeH+/H=6.2e-13, LiH+/H=9.4e-18, and LiH/LiH+=7.6e-3. Finally, we provide accurate cooling functions of H2, HD and LiH in a wide range of density and temperature that can be conveniently used in a variety of cosmological applications.