Solar-Mass Primordial Black Holes Explain NANOGrav Hint of Gravitational
  Waves
  The NANOGrav collaboration for the pulsar timing array (PTA) observation
recently announced evidence of an isotropic stochastic process, which may be
the first detection of the stochastic gravitational-wave (GW) background. We
discuss the possibility that the signal is caused by the second-order GWs
associated with the formation of solar-mass primordial black holes (PBHs). This
possibility can be tested by future interferometer-type GW observations
targeting the stochastic GWs from merger events of solar-mass PBHs as well as
by updates of PTA observations.
