TOPICAL REVIEW: Atomic and molecular processes in the early Universe

When our universe was about 380,000 years old, it underwent a period of recombination and produced the first neutral atoms. This event, the recombination epoch, represents a transition, from when the matter and radiation are strongly coupled to when they evolve independently. The universe after this point is optically thin, that is photons are most likely to cross the universe without interactions. The matter, no longer coupled to the radiation, begins to collapse and form stars, galaxies and clusters of galaxies. This structure formation is controlled by atomic and molecular processes.In this paper we review the atomic physics that is most relevant to this early period in the universe, when the first collapsed objects were just beginning to form. We start by reviewing the conditions of the early universe. Then we discuss the formation of the first atoms molecular ions and molecules. Finally, we discuss how these processes are important in the collapse of the first objects.