The LIGO-Virgo-KAGRA consortium has detected the merger of black holes with record-breaking masses.

The LIGO-Virgo-KAGRA (LVK) consortium has announced the detection of the merger of the most massive black holes ever observed through gravitational waves, made by the LIGO observatories located in Hanford and Livingston, USA, funded by the U.S. National Science Foundation. The merger resulted in the formation of a black hole over 240 times more massive than the Sun. The signal, designated GW231123, was observed on November 23, 2023, during the fourth observation campaign (O4) of the global LVK detector network.

The two merging black holes had masses of approximately 100 and 140 times that of the Sun. Besides their large mass, they are also characterized by rapid rotation, making the signal emitted by the system exceptionally difficult to interpret and suggesting a complex formation history for this system.

The discovery of such a massive and rapidly spinning system poses a challenge not only for our data analysis techniques – says Ed Porter, a researcher from the Laboratory of Astroparticle and Cosmology (APC) CNRS in Paris – but will have a profound impact for years to come on theoretical studies of black hole formation channels and gravitational wave modeling. In fact, current stellar evolution models do not allow for the existence of such massive black holes. These black holes might have formed as a result of previous mergers of smaller black holes.

So far, the LVK collaboration has published detection results of gravitational waves from around 100 black hole mergers. The most massive binary system was the source of signal GW190521, with a total mass of "only" 140 times that of the Sun.

Exploring the frontiers of gravitational wave astronomy

The large mass and extremely rapid rotation of the black holes in GW231123 push the boundaries of both gravitational wave detection technology and current theoretical models. Extracting accurate information from the signal required the use of theoretical models that account for the complex dynamics of rapidly rotating black holes.

This event pushes our capabilities in instrumentation and data analysis to the limits of what is currently possible – says Dr. Sophie Bini, a postdoctoral researcher at Caltech, formerly at the University of Trento. – It’s a powerful example of how much we can learn from gravitational wave astronomy – and how much still remains to be discovered.

Gravitational wave detectors such as LIGO in the United States, Virgo in Italy, and KAGRA in Japan are designed to measure tiny oscillations in spacetime caused by violent cosmic events like black hole mergers. The fourth observation campaign began in May 2023, and observations from its first half (until January 2024) will be published in late summer this year.

Thanks to the longest uninterrupted observation period to date and increased detector sensitivity, the fourth LIGO-Virgo-KAGRA observation campaign provides priceless new insight into our understanding of the universe – says Viola Sordini, a researcher at the Institute of Nuclear Physics (IPNL) CNRS in Lyon. – This exciting discovery opens a new season of results, with many more expected this summer and a continuation of discoveries over the next two years. Once these findings are announced, the data will be made publicly available to support the wider scientific community and open science.

GW231123 will be presented at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves, which will be held jointly as the GR-Amaldi meeting in Glasgow, United Kingdom, from July 14–18, 2025.

The LIGO-Virgo-KAGRA Consortium

The LIGO project is funded by the National Science Foundation (NSF) and managed by Caltech and MIT, which developed and implemented the project. Financial support for the Advanced LIGO project was provided by the National Science Foundation (NSF), while Germany (Max Planck Society), the United Kingdom (Science and Technology Facilities Council), and Australia (Australian Research Council) made significant contributions. More than 1,600 scientists from around the world participate in the project through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of other partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration currently includes around 1,000 members from 175 institutions across 20 different countries (mainly European), including researchers from Poland who are members of the Virgo-Polgraw team. The European Gravitational Observatory (EGO) operates the Virgo detector near Pisa, Italy, and is funded by the Centre National de la Recherche Scientifique (CNRS) in France, the National Institute for Nuclear Physics (INFN) in Italy, the National Institute for Subatomic Physics (Nikhef) in the Netherlands, the Research Foundation – Flanders (FWO), the Belgian Fund for Scientific Research (F.R.S.–FNRS), and the Polish Ministry of Science and Higher Education. A list of the groups forming the Virgo Collaboration can be found at https://www.virgo-gw.eu/scientific-collaboration/. More information is available on the Virgo project website: https://www.virgo-gw.eu.

KAGRA is a laser interferometer with 3-kilometer-long arms, located in the town of Kamioka in Gifu Prefecture, Japan. The managing institution is the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo, and the co-organizers of the project are the National Astronomical Observatory of Japan (NAOJ) and the High Energy Accelerator Research Organization (KEK). The KAGRA collaboration includes more than 400 members from 128 institutes across 17 countries/regions. Public information about the KAGRA project can be found on the website https://gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are available at http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.

ACC Cyfronet AGH is a member of the Polish Virgo Project Consortium and is carrying out the Virgo-PL project.