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

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., $P_{\mathrm{T}}$) of inflationary primordial GWs in the high-frequency band, while $P_{\mathrm{T}}$ 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., $P_{\mathrm{T}}\cdot \Delta \chi$) 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.

24 pages, 16 figures; new references added, updated to match the published version

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