The NANOGrav 15-year Data Set: Search for Signals from New Physics

TL;DR

This paper analyzes 15 years of NANOGrav pulsar timing data, exploring cosmological sources of gravitational waves, comparing models to black hole binaries, and setting new constraints on ultralight dark matter.

Contribution

It investigates cosmological interpretations of the gravitational-wave background, compares their fit to astrophysical models, and constrains dark matter models using pulsar timing data.

Findings

Cosmological models can reproduce the observed GW signal.

Many models fit better than SMBHBs, with Bayes factors 10-100.

No evidence found for ultralight dark matter signals.

Abstract

The 15-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low-frequency gravitational-wave (GW) background. In this paper, we investigate potential cosmological interpretations of this signal, specifically cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls. We find that, with the exception of stable cosmic strings of field theory origin, all these models can reproduce the observed signal. When compared to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many cosmological models seem to provide a better fit resulting in Bayes factors in the range from 10 to 100. However, these results strongly depend on modeling assumptions about the cosmic SMBHB population and, at this stage, should not be regarded as evidence for new physics. Furthermore, we identify excluded parameter regions where the predicted GW signal from cosmological sources significantly exceeds the NANOGrav signal. These parameter constraints are independent of the origin of the NANOGrav signal and illustrate how pulsar timing data provide a new way to constrain the parameter space of these models. Finally, we search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. We find no evidence for either of these signals and thus report updated constraints on these models. In the case of ULDM, these constraints outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons.