  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.
