Generating enhanced parity-violating gravitational waves during inflation with violation of NEC postprint

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 scale, 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. For a sufficiently large $P_{\rm T}$, a $\Delta\chi$ as small as a few percent can imprint a significant signal of parity violation in the GWs background, which might be detectable for pulsar timing arrays and space-based detectors in the future.