Molecular processes in the early Universe

Abstract

Molecular processes first took place in the Universe in the recombination era as the expanding Universe cooled adiabatically and recombined when the cooling radiation field ran out of photons energetic enough to cause photoionization. The formation of neutral helium heralded the dawn of chemistry as the neutral atoms participated in processes of radiative association to form molecular ions. Dissociative recombination of the molecular ions produced neutral atoms and accelerated the conversion of the ionized plasma into a neutral gas. The subsequent chemistry involved hydrogen, deuterium, helium and lithium created earlier in a brief period of nucleosynthesis. With the continued expansion the Universe became cold and dark and chemistry came to a temporary end until the formation and gravitational collapse of the first distinct cosmological objects. Heavy elements were made, a new source of radiation – starlight – appeared and a richer chemistry was initiated.