Imprints of Cosmic Phase Transition in Inflationary Gravitational Waves
  We discuss the effects of cosmic phase transition on the spectrum of primordial gravitational waves generated during inflation. The energy density of the scalar condensation responsible for the phase transition may become sizable at the epoch of phase transition, which significantly affects the evolution of the universe. As a result, the amplitudes of the gravitational waves at high frequency modes are suppressed. Thus the gravitational wave spectrum can be a probe of phase transition in the early universe. 
Ryusuke Jinno, Takeo Moroi, Kazunori Nakayama
hep-ph astro-ph.CO