  Over the years, de Sitter spacetime has been a central focus, in studies
involving quantum fields, for its importance in the early and late expansion
stages of the universe. While de Sitter spacetime closely mimics
characteristics of the inflationary and dark energy dominated universe it does
not help to understand the radiation and matter dominated expansions. In this
review, we revisit some recent works, involving the author, which study
gravitational particle creation beyond the de Sitter stage. Specifically, we
present novel aspects of particle creation in the radiation dominated universe,
and then provide an analysis of time evolution of the primordial (Bunch-Davies)
vacuum state, its particle excitation and quantum versus classical behavior of
field modes, in a multi stage universe, comprising of, (i) the inflationary de
Sitter, (ii) the radiation dominated and, (iii) the late dark energy dominated
stages.
