Gravitational wave sources for pulsar timing arrays

Very recently, the major pulsar timing array collaborations, including CPTA, EPTA, InPTA, NANOGrav, and PPTA, reported their results from searches for an isotropic stochastic gravitational wave background (SGWB), collectively representing positive evidence for a SGWB. In this work, we assessed the credibility of interpreting the Hellings-Downs correlated free-spectrum process of EPTA, PPTA, and NANOGrav as either the result of supermassive black hole binary mergers or various stochastic SGWB sources that originated in the early Universe, including first-order phase transitions, cosmic strings, domain walls, and large-amplitude curvature perturbations. Our results show that the current new datasets do not distinctly favor one specific SGWB source over the others based on Bayesian analysis. We also place constraints on new physics for the SGWB sources.