Parity-violating primordial gravitational waves from null energy condition violation

TL;DR

This paper explores how null energy condition violation in early universe scenarios can lead to observable parity-violating primordial gravitational waves, highlighting their potential as probes of fundamental physics.

Contribution

It demonstrates that NEC violation can significantly enhance parity-violating effects in primordial GWs, especially from background fields, providing new insights into early universe physics.

Findings

Background fields significantly enhance parity violation at certain scales.

Spectator fields produce weaker parity-violating signals.

Primordial GWs can serve as probes for NEC-violating physics.

Abstract

We investigate the parity-violating effects in primordial gravitational waves (GWs) due to null energy condition (NEC) violation in two very early universe scenarios: bounce-inflation and intermediate NEC violation during inflation. In both scenarios, we numerically solve the power spectra of parity-violating primordial GWs generated by coupling the background field and the spectator field with the Nieh-Yan term, respectively. We find that the background field can significantly enhance parity-violating effects at scales corresponding to the maximum of the GW power spectra. In contrast, the parity-violating effects produced by the spectator show significantly weaker observability even if the coupling constant is large. Therefore, in NEC-violating scenarios, the significant observable parity-violating effects in primordial GWs primarily arise from the physics directly related to NEC violation. This result highlights the potential of primordial GWs as crucial tools for exploring NEC-violating and parity-violating physics.