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

TL;DR

This paper explores mechanisms involving NEC violation and diminishing GW speed during inflation to produce enhanced primordial gravitational waves, using effective field theory and phase modeling.

Contribution

It introduces a unified framework for stable NEC-violating and $c_T$-diminishing scenarios during inflation within Horndeski theories, with detailed spectrum calculations.

Findings

Derived a template for the primordial GW spectrum in NEC-violating scenarios.

Analyzed the relation and differences between the two scenarios via disformal transformation.

Demonstrated stability of both scenarios within effective field theory.

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.