S-duality-invariant perturbations in string cosmology Original Research Article

We investigate the generation of curvature and isocurvature (dilaton, moduli and axion) perturbations in a general class of axion-dilaton-moduli models, including the pre-big bang scenario. Allowing for an arbitrary coupling constant λ between the dilaton field and the axion field, we exploit the SL(2,R) symmetry of the theory to obtain the spectral indices of the field perturbations in a pre-big bang type scenario. Axion field fluctuations about a homogeneous background field can yield a scale-invariant (Harrison-Zelʹdovich) spectrum. As an example we present a string-motivated case with SL(2, R) x SL(2, R) symmetry, where a second axion field arises from the compactification of the ten-dimensional theory to four dimensions.