  Very recently, several pulsar timing array collaborations, including CPTA,
EPTA, and NANOGrav, reported their results from searches for an isotropic
stochastic gravitational wave background (SGWB), with each finding positive
evidence for 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 observations show that the current
new datasets do not display a strong preference for any specific SGWB source
based on Bayesian analysis.
