Entropy generation and inflation in wave collision induced pre-big-bang   cosmology
  We study inflation and entropy generation in a recently proposed pre-big-bang model universe produced in a collision of gravitational and dilaton waves. It is shown that enough inflation occurs provided the incoming waves are sufficiently weak. We also find that entropy in this model is dynamically generated as the result of the nonlinear interaction of the incoming waves, before the universe enters the phase of dilaton driven inflation. In particular, we give the scaling of the entropy produced in the collision in terms of the focusing lengths of the incoming waves. 
A. Feinstein, K.E. Kunze and M.A. Vazquez-Mozo