The Role of H2 Molecules in Cosmological Structure Formation

We review the relevance of H2 molecules for structure formation in cosmology. Molecules are important at high redshifts, when the first collapsed structures appear with typical temperatures of a few hundred Kelvin. In these chemically pristine clouds, radiative cooling is dominated H2 molecules. As a result, H2 ``astro-chemistry'' is likely to determine the epoch when the first astrophysical objects appear. We summarize results of recent three-dimensional simulations. A discussion of the effects of feedback, and implications for the reionization of the universe is also given.