A physical characterization of the adiabatic vacuum in Robertson-Walker universe

As the red shift shows no sign of oscillatory behavior in an expanding universe we postulate that the energy _k of a scalar field does not oscillate in such a universe. In the massive case, expanding the differential equation for _k in mass powers, we find that the nonoscillatory solution coincides with the adiabatic solution up to them ⁶ order. We also demonstrate that this solution is unique for all the evolution of the universe with cosmological interest. Using the nonoscillatory solution as the one that defines the good vacuum state, we compute the particle and energy creation for a simple model of expanding universe. Both quantities turn out to be finite.