Scientists detect most massive black hole merger to date

The LIGO-Virgo-KAGRA (LVK) Collaboration has detected the merger of the most massive black holes ever observed, resulting in a final black hole approximately 225 times the mass of our Sun.

The merger’s signal, designated GW231123, was detected during the fourth observing run of the LVK network on 23 November 2023, and presented this week at the 24th International Conference on General Relativity and Gravitation and 16th Edoardo Amaldi Conference on Gravitational Waves in Glasgow. Additional observations from the first half of the run (May 2023–January 2024) will be published in the coming weeks.

LIGO (the Laser Interferometer Gravitational-Wave Observatory) was designed to measure minute distortions in space-time, known as gravitational waves, that are caused by violent cosmic events such as black hole mergers. It made history in 2015 with the first ever direct detection of gravitational waves, which 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 Louisiana and the other in 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, which has collectively observed about 300 black hole mergers over the past 10 years. Before now, the most massive 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 GW231123 event, the twin LIGO observatories measured distortions of space-time from the collision of two black holes approximately 100 and 140 times the mass of the Sun. In addition to their high masses, the black holes also appear to be spinning rapidly, making the signal uniquely challenging to interpret and suggesting the possibility of a complex formation history.

“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,” said Mark Hannam, a member of the LVK Collaboration from Cardiff University. “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.”

With the high mass and extremely rapid spinning of the black holes in GW231123 pushing 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,” said LKV member Charlie Hoy, from 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.”

Researchers are now looking to refine their analysis and improve the models used to interpret such extreme events, with LVK member Gregorio Carullo, from the University of Birmingham, saying it will take years for the community to fully unravel this intricate signal pattern and all its implications.

“Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features,” Carullo said.

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

Image credit: iStock.com/Nazarii Neshcherenskyi