Intermittent null energy condition violations during inflation and primordial gravitational waves

TL;DR

This paper explores how intermittent violations of the null energy condition during inflation can produce distinctive gravitational wave spectra, potentially explaining recent observations and predicting complex features in the stochastic background.

Contribution

It introduces a multi-stage inflation model with intermittent NEC violations, revealing novel gravitational wave signatures and their implications for cosmological observations.

Findings

Nearly flat gravitational wave spectrum at CMB and year-scale frequencies

Higher amplitude of gravitational waves consistent with NANOGrav data

Potential for a Great Wall-like spectrum from multiple NEC violations

Abstract

Primordial null energy condition (NEC) violation would imprint a blue-tilted spectrum on gravitational wave background (GWB). However, its implications on the GWB might be far richer than expected. We present a scenario, in which after a slow-roll (NEC-preserving) inflation with Hubble parameter $H\simeq H_{inf1}$, the Universe goes through an NEC-violating period and then enters subsequent slow-roll inflation with a higher $H$ ($=H_{inf2}\gg H_{inf1}$). The resulting primordial gravitational wave spectrum is nearly flat at the cosmic microwave background band, as well as at the frequency $f\sim 1/{\rm yr}$ but with higher amplitude (compatible with the recent NANOGrav result). It is also highlighted that for the multi-stage inflation if the NEC violations happened intermittently, we might have a Great Wall-like spectrum of the stochastic GWB at the corresponding frequency band.