  We study some physical aspects of quantum field theory in a two stage
universe starting from the inflationary de Sitter and transiting into the
radiation dominated stage. We look into the time evolution of the primordial
vacuum states, associated with the (i) comoving and (ii) Bunch-Davies modes. We
show how the power spectrum for a comoving observer, obtained from the
excitation of the aforementioned states defined in the de Sitter stage, changes
as the universe transits into the radiation stage. In addition, we also develop
a methodology to transfer the well known result of particle creation in the
static de Sitter frame, originating from the aforementioned vacuum states,
while the universe makes a transition to the next (radiation dominated) stage.
