We discuss the conditions under which pre-big-bang models can fit the observed large-scale anisotropy with a primordial spectrum of massive (Kalb-Ramond) axion fluctuations. The primordial spectrum must be sufficiently flat at low frequency and sufficiently steeper at high frequency. For a steep and/or long enough high-frequency branch of the spectrum the bounds imposed by COBE’s normalization allow axion masses of the typical order for a Peccei-Quinn-Weinberg-Wilczek axion. We provide a particular example in which an appropriate axion spectrum is obtained from a class of backgrounds satisfying the low-energy string cosmology equations.

• Received 9 June 1998

DOI:https://doi.org/10.1103/PhysRevD.59.043503