Old ideas and new twists in string cosmology

Some of the phenomenological implications of string cosmological models are reviewed, with particular attention to the spectra of the tensor, scalar and vector modes of the geometry. A class of self-dual string cosmological models is presented. These solutions provide an effective description of cold bounces, where a phase of accelerated contraction smoothly evolves into an epoch of decelerated Friedmann–Robertson–Walker expansion dominated by the dilaton. Some of the general problems of the scenario (continuity of the perturbations, reheating, dilaton stabilization,…) can be successfully discussed in this framework.