NANOGrav Hints to Primordial Black Holes as Dark Matter

The NANOGrav Collaboration has recently published a strong evidence for a stochastic common-spectrum process that may be interpreted as a stochastic gravitational wave background. We show that such a signal can be explained by second-order gravitational waves produced during the formation of primordial black holes from the collapse of sizeable scalar perturbations generated during inflation. This possibility has two predictions: i) the primordial black holes may comprise the totality of the dark matter with the dominant contribution to their mass function falling in the range (10^{−15}÷10^{−11})M⊙ and ii) the gravitational wave stochastic background will be seen as well by the LISA experiment.