Intermittent null energy condition violations during inflation and primordial gravitational waves
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 Hinf1, the Universe goes through an NEC-violating period and then enters subsequent slow-roll inflation with a higher H (=Hinf2 Hinf1). The resulting primordial gravitational wave spectrum is nearly flat at the cosmic microwave background band, as well as at the frequency f 1/ 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.