Scientists say they detected the largest-ever merger of two black holes, forming one that is 225 times the mass of the sun, adding that the new discovery “pushes the limits of” how astronomers understand how black holes are created.
Two black holes combined to make the massive one, according to the LIGO-Virgo-KAGRA Collaboration, an international group that uses gravitational waves to detect black hole mergers and who identified the event. Gravitational waves occur when there are minute distortions in spacetime, caused by events like black hole mergers, the group said in a news release.
One of the black holes was about 103 times the mass of the sun. The other was about 137 times it. These large black holes may have been formed by even earlier mergers, Mark Hannam, a professor at Cardiff University and member of the LIGO Scientific Collaboration, said in the release.
Even within the merger, the black holes are rapidly spinning, scientists said. Their rotation speed is about 400,000 times Earth’s rotation speed, the collaboration said in a graphic. They are moving at about 80% to 90% of the maximum possible speed.
LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet) 
“The black holes appear to be spinning very rapidly — near the limit allowed by Einstein’s theory of general relativity,” said Dr. Charlie Hoy, a gravitational-wave astrophysicist, LIGO member and postdoctoral research fellow at the University of Portsmouth, in the news release. “That makes the signal difficult to model and interpret.”
The massive black hole has been dubbed GW231123. Its unusual size and behavior is challenging scientists’ understanding of black hole formation, Hannam said. Previously, the largest known black hole that came from a merger was about 140 times the mass of the sun.
GW231123’s discovery is also opening doors for new avenues of research, astronomers said. The behavior and size of the black hole “pushes the limits of” current theoretical models and existing gravitational-wave detection technology, the LIGO-Virgo-KAGRA Collaboration said.
“It will take years for the community to fully unravel this intricate signal pattern and all its implications,” Dr. Gregorio Carullo, a LIGO member and an assistant professor at the University of Birmingham’s Institute for Gravitational Wave Astronomy, said in a statement. “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!”
The black hole was discovered in November 2023, during an observation period by the LIGO-Virgo-KAGRA Collaboration. The observation period began in May 2023, and the first part of the period ended in January 2024.
More information about GW231123 and other black holes discovered by the collaboration will be presented this month at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves, two conferences presented jointly in Glasgow, Scotland. Data from the observation window will be published later in the summer. The data used to detect and study the massive black hole will also be made available for other researchers to use, the collaboration said.
