Generating enhanced primordial GWs during inflation with intermittent violation of NEC and diminishment of GW propagating speed

Understand

We investigate both the null energy condition (NEC) violating scenario and the $c_T$-diminishing scenario for generating enhanced power spectrum of primordial gravitational waves (GWs) during inflation, where $c_T$ is the propagating speed of primordial GWs.

• Both of these two scenarios can be realized stably with theories beyond Horndeski, hence can be uniformly implemented within the framework of the effective field theory.
• We calculate the power spectrum of primordial GWs by assuming that the inflationary Universe undergoes three phases, where the violation of NEC or the diminishment of $c_T$ occurs in the intermediate phase.
• A template of the spectrum is given for the NEC-violating scenario.