Is the NANOGrav signal a hint of dS decay during inflation?

Abstract

The NANOGrav collaboration recently has reported evidence for a stochastic common-spectrum process in its 12.5 yr dataset. We show that such a signal might be a hint of de Sitter (dS) decay during inflation. As suggested by the swampland conjectures, dS space is highly unstable if it exists at all. During inflation, the short-lived dS states will decay through a cascade of first-order phase transitions (PT). We find that the gravitational waves (GWs) yielded by such a PT will be “reddened” by subsequent dS expansion, which may result in a slightly red-tilt stochastic GW background at the low-frequency band, compatible with the NANOGrav 12.5 yr result.