  Pulsar Timing Array collaborations have recently announced the discovery of a
stochastic gravitational wave background (SGWB) at nanohertz frequencies. We
analyze the GW signals from the domination of ultra-low mass primordial black
holes (PBHs) in the early universe and show that they can explain this recent
discovery. This scenario requires a relatively broad peak in the power spectrum
of scalar perturbations from inflation with a spectral index in a narrow range
of $1.45$ to $1.6$. The resulting PBH population would have mass around
$10^{8}$g, and the initial abundance $\beta_f$ lies between $10^{-10}$ and
$10^{-9}$. We find that this explanation is preferred by the data over the
generic model, assuming supermassive BHs as the source. These very light PBHs
would decay before Big Bang Nucleosynthesis (BBN); however, upcoming
third-generation terrestrial laser interferometers would be able to test the
model by observing the GW spectrum produced during the formation of the PBHs.
Also, the scalar power spectra associated with our scenario will be within the
reach of PIXIE probing CMB spectral distortions.
