The second data release from the European Pulsar Timing Array

IV. Implications for massive black holes, dark matter, and the early Universe^⋆

¹ Institute of Astrophysics, FORTH, N. Plastira 100, 70013 Heraklion, Greece
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India
⁴ Department of Electrical Engineering, IIT Hyderabad, Kandi, Telangana 502284, India
⁵ Cosmology, Universe and Relativity at Louvain (CURL), Institute of Mathematics and Physics, University of Louvain, 2 Chemin du Cyclotron, 1348 Louvain-la-Neuve, Belgium
⁶ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁷ The Institute of Mathematical Sciences, C. I. T. Campus, Taramani, Chennai 600113, India
⁸ Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India
⁹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
¹⁰ Scuola Internazionale Superiore di Studi Avanzati, Via Bonomea 265, 34136 Trieste, Italy and INFN Sezione di Trieste
¹¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹² Department of Physical Sciences, Indian Institute of Science Education and Research, Mohali, Punjab 140306, India
¹³ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, Université d’Orléans/CNRS, 45071 Orléans Cedex 02, France
¹⁴ Observatoire Radioastronomique de Nançay, Observatoire de Paris, Université PSL, Université d’Orléans, CNRS, 18330 Nançay, France
¹⁵ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁶ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁷ INAF – Osservatorio Astronomico di Brera, via Brera 20, 20121 Milano, Italy
¹⁸ Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁹ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius, (CA), Italy
²⁰ Hellenic Open University, School of Science and Technology, 26335 Patras, Greece
²¹ Université de Genève, Département de Physique Théorique and Centre for Astroparticle Physics, 24 quai Ernest-Ansermet, 1211 Genève 4, Switzerland
²² CERN, Theoretical Physics Department, 1 Esplanade des Particules, 1211 Genéve 23, Switzerland
²³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
²⁴ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Navy Nagar, Colaba, Mumbai 400005, India
²⁵ Department of Physics, IIT Hyderabad, Kandi, Telangana 502284, India
²⁶ Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
²⁷ Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala 695547, India
²⁸ School of Mathematics and Physics, Faculty of Engineering and Physical Science, University of Surrey, Guildford GU2 7XH, UK
²⁹ School of Physics, Faculty of Science, University of East Anglia, Norwich NR4 7TJ, UK
³⁰ Sternberg Astronomical Institute, Moscow State University, Universitetsky pr., 13, Moscow 119234, Russia
³¹ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Muühlenberg 1, 14476 Potsdam, Germany
³² Gran Sasso Science Institute (GSSI), 67100 L’Aquila, Italy
³³ INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi, Italy
³⁴ National Centre for Radio Astrophysics, Pune University Campus, Pune 411007, India
³⁵ Kumamoto University, Graduate School of Science and Technology, Kumamoto 860-8555, Japan
³⁶ Università di Cagliari, Dipartimento di Fisica, S.P. Monserrato-Sestu Km 0,700, 09042 Monserrato, (CA), Italy
³⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³⁸ Department of Physical Sciences,Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
³⁹ Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata, Kolkata 741246, India
⁴⁰ School of Physics, Trinity College Dublin, College Green, Dublin 2 D02 PN40, Ireland
⁴¹ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁴² Department of Physics, St. Xavier’s College (Autonomous), Mumbai 400001, India
⁴³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁴⁴ E.A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁴⁵ Centre of Excellence for Data Science, Artificial Intelligence and Modelling (DAIM), University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁴⁶ Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
⁴⁷ Department of Physics, BITS Pilani Hyderabad Campus, Hyderabad, 500078 Telangana, India
⁴⁸ Joint Astronomy Programme, Indian Institute of Science, Bengaluru, Karnataka 560012, India
⁴⁹ Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA
⁵⁰ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
⁵¹ Raman Research Institute India, Bengaluru, Karnataka 560080, India
⁵² Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁵³ Kazan Federal University, 18 Kremlyovskaya, 420008 Kazan, Russia
⁵⁴ Department of Physics, IISER Bhopal, Bhopal Bypass Road, Bhauri, Bhopal, 462066 Madhya Pradesh, India
⁵⁵ Ollscoil na Gaillimhe – University of Galway, University Road, Galway H91 TK33, Ireland
⁵⁶ Center for Gravitation, Cosmology, and Astrophysics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
⁵⁷ Division of Natural Science, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
⁵⁸ International Research Organization for Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
⁵⁹ Laboratoire Univers et Théories LUTh, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
⁶⁰ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Leibniz Universität Hannover, Callinstrasse 38, 30167 Hannover, Germany
⁶¹ Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1 Building, Suite 214, Orlando, 32826-0650 FL, USA
⁶² Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁶³ Advanced Institute of Natural Sciences, Beijing Normal University, Zhuhai 519087, PR China
⁶⁴ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China

Received: 11 July 2023
Accepted: 20 November 2023

Abstract

The European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) collaborations have measured a low-frequency common signal in the combination of their second and first data releases, respectively, with the correlation properties of a gravitational wave background (GWB). Such a signal may have its origin in a number of physical processes including a cosmic population of inspiralling supermassive black hole binaries (SMBHBs); inflation, phase transitions, cosmic strings, and tensor mode generation by the non-linear evolution of scalar perturbations in the early Universe; and oscillations of the Galactic potential in the presence of ultra-light dark matter (ULDM). At the current stage of emerging evidence, it is impossible to discriminate among the different origins. Therefore, for this paper, we consider each process separately, and investigated the implications of the signal under the hypothesis that it is generated by that specific process. We find that the signal is consistent with a cosmic population of inspiralling SMBHBs, and its relatively high amplitude can be used to place constraints on binary merger timescales and the SMBH-host galaxy scaling relations. If this origin is confirmed, this would be the first direct evidence that SMBHBs merge in nature, adding an important observational piece to the puzzle of structure formation and galaxy evolution. As for early Universe processes, the measurement would place tight constraints on the cosmic string tension and on the level of turbulence developed by first-order phase transitions. Other processes would require non-standard scenarios, such as a blue-tilted inflationary spectrum or an excess in the primordial spectrum of scalar perturbations at large wavenumbers. Finally, a ULDM origin of the detected signal is disfavoured, which leads to direct constraints on the abundance of ULDM in our Galaxy.