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

Quantum field theory in a de Sitter universe transiting to the radiation stage