Chemistry and Cooling of the Primordial Gas

Abstract.

H₂ and HD molecules provide the cooling needed for the fragmentation and collapse of the first structures in the universe. In this review, we describe the main chemical and physical processes occurring in the primordial gas after the recombination epoch. We also highlight the areas where improvements in the determination of reaction rates and excitation coefficients are necessary to reduce the remaining uncertainties in the predictions of the numerical models. Finally, the interaction of primordial molecules with the CBR and the role of H₂ and HD cooling in the early universe are discussed.