THE COSMOLOGY OF STRING THEORETIC AXIONS

String theory possesses numerous axion candidates. The recent realizat ion that the compactification radius in string theory might be large m eans that these states can solve the strong CP problem. This still lea ves the question of the cosmological bound on the axion mass. Here we explore two schemes for accommodating such light axions in cosmology. In the first, we note that in string theory the universe is likely to be dominated early on by the coherent oscillations of some moduli. The usual moduli problem assumes that these fields have masses comparable to the gravitino. We argue that string moduli are likely to be substa ntially more massive, eliminating this problem. In such cosmologies th e axion bound is significantly weakened. Plausible mechanisms for gene rating the baryon number density are described. In the second, we poin t out that in string theory, the axion potential might be much larger at early times than at present. In string theory, if CP violation is d escribed by a small parameter, the axion may sit sufficiently close to its true minimum at early times to invalidate the bounds. (C) 1997 El sevier Science B.V.