Dark matter axions in models of string cosmology

Axions are produced during a period of dilaton-driven inflation by amplific ation of quantum fluctuations. We show that for some range of string cosmol ogy parameters and some range of axion masses, primordial axions may consti tute a large fraction of the present energy density in the universe in the form of cold dark matter. Due to the periodic nature of the axion potential energy density fluctuations are strongly suppressed. The spectrum of primo rdial axions is not thermal, allowing a small fraction of the axions to rem ain relativistic until quite late. (C) 1998 Elsevier Science B.V. All right s reserved.