What is the source of the PTA GW signal?

TL;DR

This paper compares astrophysical and cosmological models to explain the PTA-detected nanohertz gravitational wave background, analyzing their fit to data and potential signatures for source discrimination.

Contribution

It performs a multi-model analysis to evaluate and compare various hypotheses for the PTA GW signal, including both astrophysical and cosmological sources.

Findings

SMBH binaries provide the best fit to the data.

Cosmological models are generally disfavoured but not ruled out.

Distinct signatures can help differentiate between source types.

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 multi-model 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 disfavoured? 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.