Generating enhanced parity-violating gravitational waves during inflation with violation of the null energy condition

TL;DR

This paper explores how violating the null energy condition during inflation can enhance the parity violation effects in primordial gravitational waves, leading to potentially observable signatures with unique oscillatory features.

Contribution

It introduces a model where NEC violation during inflation amplifies parity violation in gravitational waves, with distinctive oscillatory signatures not seen in previous models.

Findings

Enhanced parity violation amplitude during NEC violation

Higher energy scale after NEC-violating phase increases GW power spectrum

Distinct oscillatory features in parity violation parameter

Abstract

A violation of the null energy condition (NEC) during inflation in a single-field inflation model will naturally enhance the amplitude of the parity violation effect (defined by $\Delta\chi$) of inflationary primordial gravitational waves (GWs), provided the inflaton is non-minimally coupled to a gravitational Chern-Simons term. After going through the NEC-violating phase, the Universe enters subsequent slow-roll inflation with a higher energy scale (i.e., a greater Hubble parameter $H$), which results in an enhanced nearly scale-invariant power spectrum (i.e., $P_{\rm T}$) of inflationary primordial GWs in the high-frequency band, while $P_{\rm T}$ remains consistent with observations in the frequency band of the cosmic microwave background. Therefore, the violation of NEC during inflation will amplify the observability (i.e., $P_{\rm T}\cdot\Delta\chi$) of the parity violation effect on small scales. Intriguingly, our model has particular oscillatory features on $\Delta\chi$ that may not be mimicked by others.