The NANOGrav 15-year Data Set: Search for Signals from New Physics (2306.16219v1)

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.

• The paper compares astrophysical and cosmological models of gravitational-wave backgrounds using Bayesian inference on pulsar timing data.
• It evaluates potential sources including inflationary waves, scalar-induced waves, cosmic strings, and domain walls with refined spectral analysis.
• The study employs MCMC methods to constrain new physics scenarios, paving the way for multi-messenger astrophysical investigations.

Overview of the NANOGrav 15-year Data Set Paper on Gravitational Waves and New Physics

The paper "The NANOGrav 15-year Data Set: Search for Signals from New Physics" presents an evaluation of a potential low-frequency gravitational-wave background (GWB) using pulsar timing data collected over 15 years by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). The paper explores several cosmological interpretations of the observed GWB signal, including cosmic inflation, scalar-induced gravitational waves (SIGWs), first-order phase transitions, cosmic strings, and domain walls. Here, I summarize the key findings, the methodology employed, and the implications for our understanding of early universe dynamics and physics beyond the Standard Model (BSM).

Methodology and Analysis

• Data Source and Processing: The paper analyzes a 15-year dataset featuring 67 millisecond pulsars, with observations extending from 2004 to 2020. The authors employ pulsar timing techniques to model the arrival times of radio pulses and disentangle them from systematic errors, noise, and potential signals of cosmological or astrophysical origin.
• Parameters and Models: For each hypothesized source of the GWB, detailed models are constructed that describe the expected gravitational wave signals. Each model's parameters are subject to Bayesian inference techniques that allow the authors to compare different theoretical predictions against the observed data.
• Statistical Methods: The analysis is conducted using Markov Chain Monte Carlo (MCMC) methods with a focus on evaluating Bayesian evidence for each model. The authors quantify the strength of evidence favoring each source through Bayes factors, comparing them against the canonical model of supermassive black hole binaries (SMBHB) in terms of fitting the GWB signal.

Key Results

• Supermassive Black Hole Binaries: The paper finds that purely astrophysical interpretations of the GWB, such as those originating from a population of inspiraling SMBHBs, provide a viable but not necessarily the best fit to the data. A mild tension is noted between the predicted and observed spectral shape, which other non-standard models might overcome.
• Cosmological Interpretations: Among the new physics scenarios explored:
  • Inflationary Gravitational Waves (IGWs): The analysis provides evidence that IGWs with a blue-tilted spectrum might offer a slightly improved fit over the SMBHB interpretation, particularly scenarios involving a low reheating temperature and a steep spectral index.
  • Scalar-induced Gravitational Waves (SIGWs): Models of SIGWs, such as those induced by enhanced scalar perturbations leading to primordial black hole formation, offer more promising fits, with some tension due to possible overproduction of black holes.
  • Cosmic Strings: While stable cosmic string models, assuming field theory origins, do not provide a compelling fit, cosmic superstrings with certain parameters offer a better alignment with the observed data.
  • Domain Walls and First-order Phase Transitions: These scenarios are plausible, with domain walls gaining additional support if they decay into dark radiation, potentially contributing a significant component to the GWB.

Implications and Future Directions

• Constraints on BSM Physics: The paper demonstrates how PTA data can be employed to constrain parameter spaces of BSM theories effectively. Certain cosmological scenarios can now be further investigated with tighter constraints applied based on these findings.
• Complementary Observations: The work highlights the potential for multi-messaging astrophysics, where PTA results need to be complemented by other cosmic datasets—such as from CMB experiments or direct gravitational wave detectors—to refine the constraints on cosmological models further.
• Continued Search for Deterministic Signals: Although no strong evidence for deterministic signals from ultralight dark matter or dark matter substructures was found, the methodologies developed herein offer paths forward for enriched searches in future datasets.

The NANOGrav collaboration's findings provide a fertile ground for theoretical exploration across models of the early universe, promising to tighten or, conversely, confirm constraints necessary to understand the origins of cosmic structure and the fundamental physics shaping it. As such, this extensive analysis opens new vistas in gravitational wave cosmology and the ongoing quest to understand the cosmos's first moments.