Solution to the graceful exit problem in pre-big-bang cosmology - art. no.084004

We examine the string cosmology equations with a dilaton potential in the c ontext of the pre-big-bang scenario with the desired scale factor duality, and give a generic algorithm for obtaining solutions with appropriate evolu tionary properties. This enables us to find pre-big-bang type solutions wit h suitable dilaton behavior that are regular at t = 0, thereby solving the graceful exit problem. However, to avoid fine-tuning of initial data, an "e xotic" equation of state is needed that relates the fluid properties to the dilaton held. We discuss why such an equation of state should be required for reliable dilaton behavior at late times.