Decoding the phases of early and late time reheating through imprints on
  primordial gravitational waves
  Primordial gravitational waves (GWs) carry the imprints of the dynamics of
the universe during its earliest stages. With a variety of GW detectors being
proposed to operate over a wide range of frequencies, there is great
expectation that observations of primordial GWs can provide us with an
unprecedented window to the physics operating during inflation and reheating.
In this work, we closely examine the effects of the regime of reheating on the
spectrum of primordial GWs observed today. We consider a scenario wherein the
phase of reheating is described by an averaged equation of state (EoS)
parameter with an abrupt transition to radiation domination as well as a
scenario wherein there is a gradual change in the effective EoS parameter to
that of radiation due to the perturbative decay of the inflaton. We show that
the perturbative decay of the inflaton leads to oscillations in the spectrum of
GWs, which, if observed, can possibly help us decipher finer aspects of the
reheating mechanism. We also examine the effects of a secondary phase of
reheating arising due to a brief epoch driven possibly by an exotic,
non-canonical, scalar field. Interestingly, we find that, for suitable values
of the EoS parameter governing the secondary phase of reheating, the GWs can be
of the strength as suggested by the recent NANOGrav observations. We conclude
with a discussion of the wider implications of our analysis.
