Pre-Big-Bang in String Cosmology
  The duality-type symmetries of string cosmology naturally lead to a pre-big-bang phase of accelerated evolution as dual counterpart of the decelerated expansion of standard cosmology. We discuss several properties of this scenario, including the possibility that tracks of the pre-big-bang may be found either in the spectrum of relic gravitons or in the distortion they induce on the cosmic microwave background. 
M. Gasperini and G. Veneziano