“`html

The LIGO-Virgo-KAGRA (LVK) Consortium has identified the collision of the heaviest black holes ever detected through gravitational waves utilizing the US National Science Foundation (NSF)-financed LIGO observatories. This intense merger resulted in a final black hole that is approximately 225 times more massive than our Sun. The signal, designated GW231123, was recorded during the fourth observational run of the LVK network on November 23, 2023.

LIGO, the Laser Interferometer Gravitational-wave Observatory, entered the history books in 2015 when it achieved the first-ever direct observation of gravitational waves, disturbances in space-time. In that instance, the waves originated from a black hole merger that produced a final black hole 62 times the mass of our Sun. The signal was detected collaboratively by the twin detectors of LIGO—one situated in Livingston, Louisiana, and the other in Hanford, Washington.

Since then, the LIGO team has collaborated with partners at the Virgo detector in Italy and KAGRA (Kamioka Gravitational Wave Detector) in Japan to establish the LVK Consortium. These instruments have collectively identified over 200 black hole mergers in their fourth run, and approximately 300 overall since the initiation of the first run in 2015.

Prior to this, the largest black hole merger—stemming from an event in 2021 designated GW190521—had a cumulative mass of 140 times that of the Sun.

In the recent GW231123 event, the black hole with a mass of 225 solar masses was formed through the merging of black holes approximately 100 and 140 times the mass of the Sun.

Alongside their significant masses, the black holes are spinning at an accelerated rate.

“This is the most massive black hole binary we have detected via gravitational waves, and it poses a substantial challenge to our understanding of black hole formation,” states Mark Hannam from Cardiff University and a member of the LVK Consortium. “Black holes of this size are prohibited according to standard stellar evolutionary models. One possibility is that the two black holes in this binary emerged from earlier mergers of smaller black holes.”

Dave Reitze, the executive director of LIGO at Caltech, comments, “This observation once again shows how gravitational waves are uniquely exposing the fundamental and exotic nature of black holes across the universe.”

A record-setting system

The substantial mass and exceedingly rapid rotation of the black holes in GW231123 challenge the boundaries of both gravitational-wave detection technology and current theoretical frameworks. Accurately deriving information from the signal necessitated the application of models that account for the complex dynamics of highly spinning black holes.

“The black holes seem to be rotating at an extremely high speed—close to the limit proposed by Einstein’s theory of general relativity,” elaborates Charlie Hoy from the University of Portsmouth and a member of the LVK. “This complicates the modeling and interpretation of the signal. It’s an exemplary case study for advancing the development of our theoretical tools.”

Researchers are persistently refining their analysis and enhancing the models employed to interpret such extreme occurrences. “It will take years for the community to fully decode this intricate signal pattern and all its implications,” states Gregorio Carullo from the University of Birmingham and a member of the LVK. “While the most probable explanation remains a black hole merger, more intricate scenarios may be crucial to understanding its unexpected characteristics. Exciting times are ahead!”

Exploring the boundaries of gravitational-wave astronomy

Gravitational-wave detectors like LIGO, Virgo, and KAGRA are engineered to detect minute fluctuations in space-time caused by cataclysmic cosmic events. The fourth observational run commenced in May 2023, with further observations from the initial half of the run (up to January 2024) expected to be published later in the summer.

“This event pushes our instruments and data-analysis capabilities to the limits of what is currently achievable,” remarks Sophie Bini, a postdoctoral researcher at Caltech and member of the LVK. “It’s a compelling demonstration of how much we can learn from gravitational-wave astronomy—and how much more remains to be explored.”

GW231123 will be showcased at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves, hosted jointly at the GR-Amaldi meeting in Glasgow, Scotland, UK, from July 14–18, 2025. The calibrated data utilized to detect and analyze GW231123 will be made accessible for further research through the Gravitational Wave Open Science Center (GWOSC).

The LIGO-Virgo-KAGRA Consortium

LIGO is financed by the NSF and managed by Caltech and MIT, which conceived and constructed the project. The financial backing for the Advanced LIGO project was primarily provided by the NSF, with significant contributions from Germany (Max Planck Society), the UK (Science and Technology Facilities Council), and Australia (Australian Research Council). Over 1,600 scientists worldwide are engaged in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are detailed at my.ligo.org/census.php.

The Virgo Collaboration currently comprises around 880 members from 152 institutions across 17 diverse (predominantly European) nations. The European Gravitational Observatory (EGO) accommodates the Virgo detector near Pisa in Italy and is funded by the Centre national de la recherche scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the National Institute for Subatomic Physics (Nikhef) in the Netherlands. A complete list of the Virgo Collaboration groups can be accessed at: www.virgo-gw.eu/about/scientific-collaboration/. More details can be found on the Virgo website at www.virgo-gw.eu.

KAGRA is the laser interferometer with a 3-kilometer arm length located in Kamioka, Gifu, Japan. The host institution is the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo, with the project co-hosted by the National Astronomical Observatory of Japan (NAOJ) and the High Energy Accelerator Research Organization (KEK). The KAGRA collaboration consists of over 400 members from 128 institutions in 17 countries/regions. General information about KAGRA is available at gwcenter.icrr.u-tokyo.ac.jp/en/. Research resources can be found at gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.

“`