We have reviewed the chemistry and cooling behaviour of low-density (n < 10 4 cm −3 ) primordial gas and devised a cooling model wich involves 19 collisional and 9 radiative processes and is applicable for temperatures in the range (1K< T < 10 8 K). We derived new fits of rate coefficients for the photo-attachment of neutral hydrogen, the formation of molecular hydrogen via H − , charge exchange between H 2 and H + , electron detachment of H − by neutral hydrogen, dissociative recombination of H + 2 with slow electrons, photodissociation of H + 2 , and photodissociation of H 2 . Further it was found that the molecular hydrogen produced through the gas-phase processes, H + 2 + H → H 2 + H + , and H − + H → H 2 + e − , is likely to be converted into its para configuration on a faster time scale than the formation time scale. We have tested the model extensively and shown it to agree well with former studies. We further studied the chemical kinetics in great detail and devised a minimal model which is substantially simpler than the full reaction network but predicts correct abundances. This minimal model shows convincingly that 12 collisional processes are sufficient to model the H, He, H + , H − , He + , He ++ , and H 2 abundances in low density primordial gas for applications with no radiation fields.

Abstract.The comparison of quasar absorption spectra with numerically simulated spectra from hierarchical cosmological models of structure formation promises to be a valuable tool to discriminate among these models. We present simulation results for the column density, Doppler b parameter, and optical depth probability distributions for five popular cosmological models.