Axion perturbation spectra in string cosmologies

We discuss the semi-classical perturbation spectra produced in the massless fields of the low energy string action in a pre big bang type scenario. Ax ion fields may possess an almost scale-invariant (Delta n approximate to 0) spectrum on large scales dependent upon the evolution of the dilaton and m oduli fields to which they are coupled. As an example we calculate the spec tra for three axion fields present in a truncated type IIB model and show t hat they are related with at least one of the fields having a scale-invaria nt or red (Delta n < 0) perturbation spectrum. In the simplest pre big bang scenario this may be inconsistent with the observed isotropy of the microw ave background. More generally the relations between the perturbation spect ra in low energy string cosmologies should reflect the symmetries of the th eory. (C) 1998 Elsevier science B.V. All rights reserved.