Adiabatic CMB perturbations in pre-Big-Bang string cosmology Original Research Article

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 Δn in the range −0.1≲Δn≲1.