NANOGrav signal as mergers of Stupendously Large Primordial Black Holes

TL;DR

This paper proposes that the NANOGrav gravitational wave signal originates from mergers of extremely large primordial black holes, which could account for a small fraction of dark matter, with the model consistent with cosmic microwave background constraints.

Contribution

It introduces a novel explanation for the NANOGrav signal involving supermassive primordial black holes formed from non-Gaussian inflationary fluctuations, surpassing previous bounds from CMB distortions.

Findings

Primordial black hole mergers can produce the observed NANOGrav signal.

Non-Gaussian inflationary models can generate the necessary black hole abundance.

The model respects constraints from CMB $$ distortions and large-scale structure.

Abstract

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.