Is the NANOGrav signal a hint of dS decay during inflation?
  As suggested by the swampland conjectures, de Sitter (dS) space might be
highly unstable if it exists at all. During inflation, the short-lived dS
states will decay through a cascade of the first-order phase transition (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 GWs background at low-frequency band, compatible with the NANOGrav
12.5-yr result.
