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

Abstract

The 15 yr 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.

Inflationary interpretation of the stochastic gravitational wave background signal detected by pulsar timing array experiments

“…In June 2023 the NANOGrav, EPTA, PPTA, and CPTA PTA experiments reported evidence for excess red common-spectrum signals in their latest datasets (NANOGrav 15-year, EPTA DR2 complemented with InPTA DR1, PPTA DR3, and CPTA DR1), with interpulsar correlations following the HD pattern [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Considering that all four signals are consistent with each other, and the fact that NANOGrav achieved the most precise determination of the spectral index (which not all experiments managed to infer), for concreteness and simplicity I will focus on the NANOGrav 15-year signal in the remainder of this work, but I expect my main conclusions to broadly carry over to the results of the other three PTA experiments as well.…”

“…Very recently, in June 2023, various PTA experiments, including NANOGrav, EPTA, PPTA, and the Chinese Pulsar Timing Array (CPTA), in the case of EPTA including also data from the Indian PTA (InPTA), reported on the analyses of their latest datasets, which all confirm the presence of excess red common-spectrum signals, with strain amplitude of order O(10 −15 ) at the reference frequency f = 1 yr −1 [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Importantly, all analyses report evidence (with varying strength) for HD correlations, which point to a genuine GW origin for the signals, in turn making these the first convincing detections of a SGWB signal in the nHz range.…”

The NANOGrav 15 yr Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries

“…Given the large measurement and modeling uncertainties of the GWB spectrum, detecting an individual binary would also be the most reliable indicator that the background is indeed due to binaries instead of other cosmological sources (Afzal et al 2023). These prospects motivate the continuous improvement of our search algorithms such as making them more efficient , searching for multiple binaries simultaneously (see, e.g., Babak & Sesana 2012;Bécsy & Cornish 2020), searching for a correlated background along with the individual binary, searching for eccentric binaries (see, e.g., Taylor et al 2016;Susobhanan et al 2020;Susobhanan 2022), and many more.…”

NANOGrav results and dark first order phase transitions

“…As displayed, there exist several FOPTs with T * ≃ 1 ÷ 100 MeV and α ≃ 0.1 ÷ 0.3 that are compatible with the NANOGrav excess of a stochastic spectrum. The degeneracy in the (T * , α) parameter space has also been obtained in the recent Monte Carlo analysis by the NANOGrav collaboration [5] and allows to accommodate the results by the PPTA and EPTA collaborations which report a different best fit within 2σ.…”

“…Fresh batches of pulsar timing (PTA) data have recently been released by various collaborations, reporting an excess of a stochastic spectrum which appears to be compatible with a gravitational wave background (GWB) at frequencies around f ∼ 1 ÷ 10 nHz. Among these, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently released the data collected in the first 15 yrs of activity [1][2][3][4][5]. In addition to previous analyses [6][7][8][9], the recent NANOGrav 15-year data also reports the evidence for quadrupolar correlations that follow the signature described by Hellings and Downs [10] and is then conclusive with regard to the nature of the detection.…”