Quantum Fluctuations in Open Pre-Big Bang Cosmology
  We solve exactly the (linear order) equations for tensor and scalar perturbations over the homogeneous, isotropic, open pre-big bang model recently discussed by several authors. We find that the parametric amplification of vacuum fluctuations (i.e. particle production) remains negligible throughout the perturbative pre-big bang phase. 
A. Ghosh, G. Pollifrone and G. Veneziano (CERN)