NANOGrav signal as mergers of Stupendously Large Primordial Black Holes
  We give an explanation for the signal detected by NANOGrav as the stochastic
gravitational wave background from binary mergers of primordial "Stupendously
Large Black Holes" (SLABs) of mass $M\sim(10^{11}-10^{12})M_{\odot}$, and
corresponding to roughly $0.1\%$ of the dark matter. We show that the stringent
bounds coming from $\mu$ distortions of the CMB can be surpassed if the
perturbations resulting in these BHs arise from the non-Gaussian distribution
of fluctuations expected in single field models of inflation generating a spike
in the power spectrum. While the tail of the stochastic background coming from
binaries with $M\lesssim 10^{11}M_{\odot}$ could both fit NANOGrav and respect
$\mu$ distortions limits, they become excluded from large scale structure
constraints.
