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

**Authors:** Adeela Afzal, Gabriella Agazie, Akash Anumarlapudi, Anne M. Archibald, Zaven Arzoumanian, Paul T. Baker, Bence Bécsy, Jose Juan Blanco-Pillado, Laura Blecha, Kimberly K. Boddy, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Rand Burnette, Robin Case, Maria Charisi, Shami Chatterjee, Katerina Chatziioannou, Belinda D. Cheeseboro, Siyuan Chen  
**arXiv:** [2306.16219](https://arxiv.org/abs/2306.16219)  
**Date:** 2023-06-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2306.16219v1/

## Key points

- Investigation into the potential cosmological interpretations of a low-frequency gravitational-wave background found in NANOGrav's pulsar timing data set
- Various models explored: cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls
- All models except stable cosmic strings can reproduce the observed signal
- Comparisons made to standard interpretation of inspiraling supermassive black hole binaries (SMBHBs)
- Many cosmological models provide a better fit with Bayes factors ranging from 10 to 100 compared to SMBHBs
- Results heavily depend on modeling assumptions and should not be considered evidence for new physics at this stage
- Excluded parameter regions where predicted GW signal significantly exceeds NANOGrav signal identified
- Pulsar timing data used to constrain parameter space of these models
- Search for signals produced by ultralight dark matter (ULDM) and dark matter substructures in Milky Way conducted
- No evidence found for either signal, updated constraints reported
- Constraints on ULDM coupled to electrons, muons, or gluons outperform torsion balance and atomic clock constraints
- Detailed descriptions of authors' contributions and analysis methods provided
- NG15 data set used in study described
- Appendices include lists of parameters, prior ranges, recovered posterior ranges for each model, and median GW spectra based on recovered posterior distributions
- Paper expands on previous research by investigating various cosmological models to explain observed low-frequency gravitational-wave background in NANOGrav data
- Highlights importance of pulsar timing data in constraining parameter space for these models

## Summary

This paper presents an investigation into the potential cosmological interpretations of the positive evidence for a low-frequency gravitational-wave (GW) background found in the 15-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). The authors explore various models, including cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls, to explain this signal. They find that all these models, except stable cosmic strings of field theory origin, can reproduce the observed signal. Comparing these cosmological models to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many of them seem to provide a better fit with Bayes factors ranging from 10 to 100. However, it is important to note that these results heavily depend on modeling assumptions about the cosmic SMBHB population and should not be considered as evidence for new physics at this stage. The authors also 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 demonstrate how pulsar timing data can be used to constrain the parameter space of these models. Furthermore, this paper includes a search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. The authors find no evidence for either of these signals and report updated constraints on these models. In particular, they show that their constraints on ULDM coupled to electrons, muons, or gluons outperform torsion balance and atomic clock constraints. The paper provides detailed descriptions of each author's contributions to specific analyses and discusses their results accordingly. It also presents information about the NG15 data set used in this study and outlines general analysis methods employed. Additionally, it includes appendices with lists of parameters, prior ranges, and recovered posterior ranges for each model, as well as median GW spectra based on the recovered posterior distributions. Overall, this paper expands on previous research by investigating various cosmological models to explain the observed low-frequency gravitational-wave background in the NANOGrav data. It provides valuable insights into the potential origins of this signal and highlights the importance of pulsar timing data in constraining parameter space for these models.