Challenges in interpreting the NANOGrav 15-year dataset as early Universe gravitational waves produced by an ALP induced instability

In this paper, we study a possible early universe source for the recent observation of a stochastic gravitational wave background at the NANOGrav pulsar timing array. The source is a tachyonic instability in a dark gauge field induced by an axionlike particle (ALP), a known source for gravitational waves. We find that relative to the previous analysis with the NANOGrav 12.5-year dataset, the current 15-year dataset favors parameter space with a relatively larger axion mass and decay constant. This favored parameter space is heavily constrained by ΔNeff and overproduction of ALP dark matter. While there are potential mechanisms for avoiding the second problem, evading the ΔNeff constraint remains highly challenging. In particular, we find that the gravitational wave magnitude is significantly suppressed with respect to the gauge boson dark radiation, which implies that successfully explaining the NANOGrav observation requires a large additional dark radiation, violating the cosmological constraints. Satisfying the ΔNeff constraint will limit the potential contribution from this mechanism to the observed signal to at most a percent level.