Ultralow mass primordial black holes in the early Universe can explain the pulsar timing array signal
Nilanjandev Bhaumik, Rajeev Kumar Jain, Marek Lewicki
Abstract
Pulsar timing array collaborations have recently announced the discovery of a stochastic gravitational wave (GW) background at nanohertz frequencies. We analyze the GW signals from the domination of ultralow 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--1.6. The resulting PBH population would have mass around ${10}^{8}\text{ }\text{ }\mathrm{g}$, and the initial abundance ${\ensuremath{\beta}}_{f}$ lies between ${10}^{\ensuremath{-}10}$ and ${10}^{\ensuremath{-}9}$. We find that this explanation is preferred by the data over the generic model, assuming supermassive black holes as the source. These very light PBHs would decay before big bang nucleosynthesis; 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 cosmic microwave background spectral distortions.