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The LIGO-Virgo-KAGRA (LVK) Collaboration has detected the merger of the most massive black holes ever observed with gravitational waves using the LIGO observatories. The powerful merger produced a final black hole approximately 225 times the mass of our sun. The signal, designated GW231123, was detected during the fourth observing run of the LVK network on November 23, 2023.

LIGO, the Laser Interferometer Gravitational-wave Observatory, made history in 2015 when it made the first-ever direct detection of gravitational waves, ripples in space-time. In that case, the waves emanated from a black hole merger that resulted in a final black hole 62 times the mass of our sun. The signal was detected jointly by the twin detectors of LIGO, one located in Livingston, Louisiana, and the other in Hanford, Washington.

Since then, the LIGO team has teamed up with partners at the Virgo detector in Italy and KAGRA (Kamioka Gravitational Wave Detector) in Japan to form the LVK Collaboration. These detectors have collectively observed more than 200 black hole mergers in their fourth run, and about 300 in total since the start of the first run in 2015.

Before now, the most massive black hole merger—produced by an event that took place in 2021 called GW190521—had a total mass of 140 times that of the sun.

In the more recent GW231123 event, the 225-solar-mass black hole was created by the coalescence of black holes each approximately 100 and 140 times the mass of the sun.

In addition to their high masses, the black holes are also rapidly spinning.

"This is the most massive black hole binary we've observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation," says Mark Hannam of Cardiff University and a member of the LVK Collaboration.

"Black holes this massive are forbidden through standard stellar evolution models. One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes."

Dave Reitze, the executive director of LIGO at Caltech, says, "This observation once again demonstrates how gravitational waves are uniquely revealing the fundamental and exotic nature of black holes throughout the universe."

A record-breaking system

The high mass and extremely rapid spinning of the black holes in GW231123 push the limits of both gravitational-wave detection technology and current theoretical models. Extracting accurate information from the signal required the use of models that account for the intricate dynamics of highly spinning black holes.

"The black holes appear to be spinning very rapidly—near the limit allowed by Einstein's theory of general relativity," explains Charlie Hoy of the University of Portsmouth and a member of the LVK. "That makes the signal difficult to model and interpret. It's an excellent case study for pushing forward the development of our theoretical tools."

Researchers are continuing to refine their analysis and improve the models used to interpret such extreme events. "It will take years for the community to fully unravel this intricate signal pattern and all its implications," says Gregorio Carullo of the University of Birmingham and a member of the LVK. "Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features. Exciting times ahead!"

Probing the limits of gravitational-wave astronomy

Gravitational-wave detectors such as LIGO, Virgo, and KAGRA are designed to measure minute distortions in space-time caused by violent cosmic events. The fourth observing run began in May 2023, and additional observations from the first half of the run (up to January 2024) will be published later in the summer.

"This event pushes our instrumentation and data-analysis capabilities to the edge of what's currently possible," says Sophie Bini, a postdoctoral researcher at Caltech and member of the LVK. "It's a powerful example of how much we can learn from gravitational-wave astronomy—and how much more there is to uncover."

GW231123 will be presented at the held jointly at the GR-Amaldi meeting in Glasgow, Scotland, UK, July 14–18, 2025. The calibrated data used to detect and study GW231123 will be made available for other researchers to analyze through the Gravitational Wave Open Science Center (GWOSC).