Adiabatic CMB perturbations in pre-big bang string cosmology

Abstract

We consider the pre-big bang scenario with a massive axion field which starts to dominate energy density when oscillating in an instanton-induced potential and subsequently reheats the universe as it decays into photons, thus creating adiabatic CMB perturbations. We find that the fluctuations in the axion field can give rise to a nearly flat spectrum of adiabatic perturbations with a spectral tilt $\hat{I}”n$ in the range $-0.1\lesssim \hat{I}”n\lesssim 1$.

15 pages, 1 figure; Version to appear in Nucl.Phys.B. Figure improved, discussion and one reference added