Initial conditions and the structure of the singularity in pre-big-bang cosmology

We propose a picture, within the pre-big-bang approach, in which the univer se emerges from a bath of plane gravitational and dilatonic waves. The wave s interact gravitationally breaking the exact plane symmetry and lead gener ically to gravitational collapse resulting in a singularity with Kasner-lik e structure. The analytic relations between the Kasner exponents and the in itial data are evaluated explicitly and it is shown that pre-big-bang infla tion may occur within a dense set of initial data. Finally, we argue that p lane waves carry zero gravitational entropy and thus are, from a thermodyna mical point of view, good candidates for the universe to emerge from.