O(d,d)-invariant collapse/inflation from colliding superstring waves
  Generalizing previous work, we study the collision of massless superstring plane waves in D space-time dimensions within an explicitly O(D-2,D-2)-invariant set of field equations. We discuss some general properties of the solutions, showing in particular that they always lead to the formation of a singularity in the future. Using the above symmetry, we obtain entire classes of new analytic solutions with non-trivial metric, dilaton and antisymmetric field, and discuss some of their properties of specific relevance to string cosmology. 
Valerio Bozza, Gabriele Veneziano