S-duality invariant perturbations in string cosmology

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-Zel’dovich) spectrum. As an example we present a string-motivated case with SL(2,R)×SL(2,R) symmetry, where a second axion field arises from the compactification of the ten–dimensional theory to four dimensions. Electronic address: [email protected] Electronic address: [email protected] Electronic address: [email protected]