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

Abstract

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.We also discuss the underlying relation and discrepancy between these two scenarios with a disformal transformation.

Constraints on inflation with null energy condition violation from advanced LIGO and advanced Virgo's first three observing runs

“…The violation of the NEC has profound implications for the generation of primordial GWs in the early Universe. It has been shown that NEC violation can result in a notable JCAP06(2024)028 amplification of the primordial tensor power spectrum at certain scales [40,41]. This amplification manifests as a blue tilt in the tensor power spectrum, indicating an enhanced production of high-frequency GWs.…”