What is the source of the PTA GW signal?
John Ellis, Malcolm Fairbairn, Gabriele Franciolini, Gert Hütsi, Antonio J. Iovino, Marek Lewicki, M. Raidal, Juan Urrutia, Ville Vaskonen, Hardi Veermäe
Abstract
The most conservative interpretation of the nHz stochastic gravitational wave background (SGWB) discovered by NANOGrav and other pulsar timing array (PTA) collaborations is astrophysical, namely that it originates from supermassive black hole (SMBH) binaries. However, alternative cosmological models have been proposed, including cosmic strings, phase transitions, domain walls, primordial fluctuations, and ``audible'' axions. We perform a multimodel analysis (MMA) to compare how well these different hypotheses fit the NANOGrav data, both in isolation and in combination with SMBH binaries, and address the questions: Which interpretations fit the data best, and which are disfavored? We also discuss experimental signatures that can help discriminate between different sources of the PTA GW signal, including fluctuations in the signal strength between frequency bins, individual sources, and how the PTA signal extends to higher frequencies.