  The NANOGrav Collaboration recently reported a strong evidence for a
stochastic common-spectrum process in the pulsar-timing data. We evaluate the
evidence of interpreting this process as mergers of super massive black hole
binaries and/or various stochastic gravitational wave background sources in the
early Universe, including first-order phase transitions, cosmic strings, domain
walls, and large amplitude curvature perturbations. We discuss the implications
of the constraints on these possible sources. It is found that the cosmic
string is the most favored source against other gravitational wave sources
based on the Bayes factor analysis.
