Gravitational Waves from an Inflation Triggered First-Order Phase
  Transition
  Large excursion of the inflaton field can trigger interesting dynamics. One
important example is a first-order phase transition in a spectator sector which
couples to the inflaton. Gravitational waves (GWs) from such a first-order
phase transition during inflation, an example of an instantaneous source, have
an oscillatory feature. In this work, we show that this feature is generic for
a source in an era of accelerated expansion. We also demonstrate that the shape
of the GW signal contains information about the evolution of the early universe
following the phase transition. In particular, the slope of the infrared part
of the GW spectrum is sensitive to the evolution of the Hubble parameter when
the GW modes reenter the horizon after inflation. The slope of the profile of
the intermediate oscillatory part and the ultraviolet part of the GW spectrum
depend on the evolution of the Hubble parameter when the modes exit horizon
during the inflation and when they reenter the horizon during the reheating.
The ultraviolet spectrum also depends on the details of the dynamics of the
phase transition. We consider the GW signal in several models of evolution
during and after inflation, and compare them with the minimal scenario of
quasi-de Sitter inflation followed by radiation domination after a fast
reheating, and demonstrate that the shape of the GW can be used to distinguish
them. In this way, the GW signal considered in this paper offers a powerful
probe to the dynamics of the early universe which is otherwise difficult to
explore directly through CMB, large scale structure, big bang nucleosynthesis
(BBN), and other well-studied cosmological observables.
