The NANOGrav 15 yr Data Set: Search for Signals from New Physics
Authors: Afzal Adeela , Agazie Gabriella , Anumarlapudi Akash , Archibald Anne M. , Arzoumanian Zaven , Baker Paul T. , Bécsy Bence , Blanco-Pillado Jose Juan , Blecha Laura , Boddy Kimberly K. , Brazier Adam , Brook Paul R. , Burke-Spolaor Sarah , Burnette Rand , Case Robin , Charisi Maria , Chatterjee Shami , Chatziioannou Katerina , Cheeseboro Belinda D. , Chen Siyuan , Cohen Tyler , Cordes James M. , Cornish Neil J. , Crawford Fronefield , Cromartie H. Thankful , Crowter Kathryn , Cutler Curt J. , DeCesar Megan E. , DeGan Dallas , Demorest Paul B. , Deng Heling , Dolch Timothy , Drachler Brendan , von Eckardstein Richard , Ferrara Elizabeth C. , Fiore William , Fonseca Emmanuel , Freedman Gabriel E. , Garver-Daniels Nate , Gentile Peter A. , Gersbach Kyle A. , Glaser Joseph , Good Deborah C. , Guertin Lydia , Gültekin Kayhan , Hazboun Jeffrey S. , Hourihane Sophie , Islo Kristina , Jennings Ross J. , Johnson Aaron D. , Jones Megan L. , Kaiser Andrew R. , Kaplan David L. , Kelley Luke Zoltan , Kerr Matthew , Key Joey S. , Laal Nima , Lam Michael T. , Lamb William G. , W. Lazio T. Joseph , Lee Vincent S. H. , Lewandowska Natalia , Lino dos Santos Rafael R. , Littenberg Tyson B. , Liu Tingting , Lorimer Duncan R. , Luo Jing , Lynch Ryan S. , Ma Chung-Pei , Madison Dustin R. , McEwen Alexander , McKee James W. , McLaughlin Maura A. , McMann Natasha , Meyers Bradley W. , Meyers Patrick M. , Mingarelli Chiara M. F. , Mitridate Andrea , Nay Jonathan , Natarajan Priyamvada , Ng Cherry , Nice David J. , Ocker Stella Koch , Olum Ken D. , Pennucci Timothy T. , Perera Benetge B. P. , Petrov Polina , Pol Nihan S. , Radovan Henri A. , Ransom Scott M. , Ray Paul S. , Romano Joseph D. , Sardesai Shashwat C. , Schmiedekamp Ann , Schmiedekamp Carl , Schmitz Kai , Schröder Tobias , Schult Levi , Shapiro-Albert Brent J. , Siemens Xavier , Simon Joseph , Siwek Magdalena S. , Stairs Ingrid H. , Stinebring Daniel R. , Stovall Kevin , Stratmann Peter , Sun Jerry P. , Susobhanan Abhimanyu , Swiggum Joseph K. , Taylor Jacob , Taylor Stephen R. , Trickle Tanner , Turner Jacob E. , Unal Caner , Vallisneri Michele , Verma Sonali , Vigeland Sarah J. , Wahl Haley M. , Wang Qiaohong , Witt Caitlin A. , Wright David , Young Olivia , Zurek Kathryn M. ,   The NANOGrav Collaboration
Published: June 2023

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.