The most massive black hole merger detected to date using gravitational waves has been observed by the internationally supported LIGO-Virgo-KAGRA (LVK) Collaboration, operating with data from the LIGO Hanford and Livingston Observatories.
On November 23, 2023, during its fourth observing run, the LVK network picked up the GW231123 signal generated by the cosmic event when two black holes—measuring 100 and 140 times the mass of the Sun, respectively—merged to form a single supermassive black hole estimated at 225 solar masses.
The intensity of the event led to a complex and difficult-to-interpret signal, as both black holes had developed extraordinarily rapid spins. The formal announcement of the discovery was made at the GR-Amaldi meeting in Glasgow, UK, held July 14–18, 2025, ahead of a paper expected to be published later this summer.
Observing Cosmic Calamities
LVK is a global collaboration of three gravitational wave detectors, with LIGO representing the United States, while Virgo is based in Italy, and KAGRA operates in Japan. These powerful instruments allow astronomers to study some of the universe’s most extreme events by detecting the ripples in spacetime they produce. The data for this black hole merger came from LVK’s fourth observing run (O4), which began in May 2023. A report summarizing observations through January 2024 is scheduled for release later this summer.
Gravitational wave analysis is currently the most effective method for detecting black hole mergers. To date, scientists have identified around 300 such events, several of which came from the ongoing O4 run. Prior to the GW231123 signal, the most massive black hole merger on record involved a combined mass of just 140 solar masses.
An Extraordinary Black Hole Merger
“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 Professor Mark Hannam, from Cardiff University and a member of the LIGO Scientific 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.”
In addition to their mass, the black holes’ spin rates were exceptionally high. These factors combined to push the limits of the instruments used to detect gravitational waves. Even after the signal was captured, interpreting the data required complex theoretical modeling.
“The black holes appear to be spinning very rapidly—near the limit allowed by Einstein’s theory of general relativity,” explains Dr Charlie Hoy at the University of Portsmouth. “That makes the signal difficult to model and interpret. It’s an excellent case study for pushing forward the development of our theoretical tools.”
Continuing to Pursue Black Hole Mergers
“This event pushes our instrumentation and data-analysis capabilities to the edge of what’s currently possible,” says Dr Sophie Bini, a postdoctoral researcher at Caltech. “It’s a powerful example of how much we can learn from gravitational-wave astronomy—and how much more there is to uncover.”
While the event is the most massive of its kind ever recorded, it may only remain that way for now. The LVK team’s research opens new possibilities for recording such extreme events, and they plan to push their analysis techniques and theoretical models even further to detect more such events. Additionally, a significant amount of work remains to be done in investigating the data from this complex event itself.
“It will take years for the community to fully unravel this intricate signal pattern and all its implications,” said Dr Gregorio Carullo, Assistant Professor at the University of Birmingham.
“Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features,” Carullo added, expressing that based on such discoveries, he expects there will likely be “Exciting times ahead!”
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.