Challenges in Interpreting the NANOGrav 15-Year Data Set as Early Universe Gravitational Waves Produced by ALP Induced Instability

Understand

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 axion-like particle (ALP), a known source for gravitational waves.
• We find that relative to the previous analysis with the NANOGrav 12.5-year data set, the current 15-year data set favors parameter space with a relatively larger axion mass and decay constant.
• This favored parameter space is heavily constrained by $\Delta N_{\rm eff}$ and overproduction of ALP dark matter.