Low-energy Graceful Exit in Anisotropic String Cosmology Backgrounds

We discuss the possibility of a smooth transition from the pre-to the post-big bang regime, in the context of the lowest-order string eeective action (without higher-derivative corrections), taking into account with a phenomenological model of source the repulsive gravitational eeects due to the back-reaction of the quantum uctuations outside the horizon. We determine a set of necessary conditions for a successful and realistic transition, and we nd that such conditions can be satissed (by an appropriate model of source), provided the background is higher-dimensional and anisotropic.