Scientists detect the most massive black hole merger to date, 225 times the mass of the Sun: "At the limits of what is currently possible."

The international LIGO-Virgo-KAGRA (LVK) collaboration has detected the merger of the most massive black holes ever observed with gravitational waves using the U.S. National Science Foundation (NSF)-funded LIGO observatories. The powerful merger produced a final black hole with a mass approximately 225 times that of our Sun. The signal, designated GW231123, was detected during the LVK network’s fourth observing cycle on November 23, 2023.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, made history in 2015 by directly detecting gravitational waves, ripples in space-time, for the first time. In that case, the waves emanated from a black hole merger that resulted in a final black hole with a mass 62 times that of our Sun. The signal was jointly detected by LIGO's twin detectors, one located in Livingston, Louisiana, and the other in Hanford, Washington.

Since then, the LIGO team has joined with partners from the Virgo detector in Italy and KAGRA (Kamioka Gravitational-Wave Detector) in Japan to form the KLV Collaboration. These detectors have collectively observed more than 200 black hole mergers in their fourth analysis cycle, and around 300 in total since the start of the first cycle in 2015.

Until 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 most recent event, GW231123, the 225-solar-mass black hole was created by the coalescence of black holes, each about 100 and 140 times the mass of the Sun.

In addition to their high masses, black holes also spin rapidly. "This is the most massive binary black hole system we've observed using gravitational waves and represents a real challenge to our understanding of black hole formation," notes Mark Hannam of Cardiff University and a member of the LVK Collaboration. "Black holes of this mass are forbidden in standard models of stellar evolution. One possibility is that the two black holes in this binary system formed through previous mergers of smaller black holes."

Dave Reitze, LIGO's executive director at Caltech, elaborates: "This observation demonstrates once again how gravitational waves uniquely reveal the fundamental and exotic nature of black holes throughout the universe."

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

"Black holes appear to spin very fast, close to the limit allowed by Einstein's theory of general relativity ," explains Charlie Hoy of the University of Portsmouth (UK) and a member of the LVK. "This makes modeling and interpreting the signal difficult. It's an excellent case study to further the development of our theoretical tools."

Researchers continue to refine their analyses and improve the models used to interpret these extreme events. "It will take years for the community to fully unravel this intricate pattern of signals and all its implications," confirms Gregorio Carullo of the University of Birmingham (United Kingdom) and a member of the LVK. "Although the most likely explanation remains a black hole merger, more complex scenarios could hold the key to deciphering their unexpected characteristics."

Gravitational wave detectors like LIGO, Virgo, and KAGRA are designed to measure tiny distortions in space-time caused by violent cosmic events. The fourth observation cycle began in May 2023, and additional observations from the first half of the cycle (through January 2024) will be published later this summer.

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