On the Creation of Scalar Particles in a Flat Robertson-Walker   Space-time
  The problem of particle creation from vacuum in a flat Robertson-Walker space-time is studied. Two sets of exact solutions for the Klein Gordon equation are given. Then the canonical method based on Bogoliubov transformation is applied to calculate the pair creation probability and the density number of created particles. Some particular cosmological models such as radiation dominated universe and Milne universe are discussed. For the both cases the vacuum to vacuum transition probability is calculated and the imaginary part of the effective action is extracted. 
S. Haouat and R. Chekireb
hep-th gr-qc