Gravitational waves, ripples in space-time caused by violent cosmic events, travel at the speed of light in all directions and eventually fade away like ripples in water. But some events are so disruptive and extreme that they create perturbations in space-time that look more like powerful waves than small ripples, with enough energy to reach our own detectors here on Earth.
Today, the LIGO Collaboration announced the detection of the most colossal black hole merger known to date, with the end product being a giant black hole more than 225 times the mass of the Sun. Much of this signal, designated GW231123, contradicts known models of stellar evolution, leaving physicists puzzling over how such a merger was possible at all.
LIGO, or the Laser Interferometer Gravitational-Wave Observatory, made physics history in 2015 by detecting gravitational waves for the first time by capturing the cosmological echo of two colliding black holes. Since this Nobel Prize-winning discovery, the LIGO Collaboration, an international partnership between LIGO and Virgo and KAGRA in Italy and Japan, respectively, has continued to closely monitor the galaxy. The Collaboration has detected numerous signals from neutron stars, supernovae, and about 300 black hole mergers.
But GW231123, which was first observed on November 23, 2023, seems to be an unprecedented black hole merger beast. Two enormous black holes, 137 and 103 times the mass of the Sun, have managed to hold together despite their enormous combined mass, rotating at 400,000 times the speed of the Earth’s rotation to form an even larger black hole. To put its size in perspective, the previous record holder for such a merger, GW190521, is about 140 times the mass of the Sun.
Given the gravitationally chaotic nature of the black hole environment, with its pushes and pulls, it is surprising that this merger was stable enough for the resulting gravitational waves to reach LIGO, which detected signals lasting 0.1 seconds. Such episodes should be “forbidden” according to standard models of evolution, said Mark Hannam, a LIGO member and physicist at the University of Cardiff, in a statement.
“One possibility is that the two black holes in this binary were formed by a previous merger of smaller black holes,” he suggested. “This is the most massive binary black hole we have observed with gravitational waves, and it poses a real challenge to our understanding of black hole formation.”
“It appears that black holes are spinning very fast – almost to the limit allowed by Einstein’s theory of relativity,” explained Charlie Hoy, a LIGO member and physicist at the University of Portsmouth in England, in the same release. “This makes the signal challenging to model and interpret. This is a great example to push forward the development of our theoretical tools.”
The scientists will present their findings about GW231123 next week at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves, which will be held jointly as part of the GR-Amaldi meeting in Glasgow, UK. After that, the data will be published for public discussion, which will kick off the race to solve the mystery of GW231123 – although it is unlikely that we will get a clear answer in the near future.
“It will take years for the community to fully unravel this complex signal structure and all its implications,” added Gregorio Carullo, also a LIGO member and a physicist at the University of Birmingham, England. “While black hole mergers remain the most likely explanation, more complex scenarios may hold the key to deciphering its unexpected features. Exciting times ahead!”
Physicists first thought about gravitational waves in the late 19th century, but it was Albert Einstein who made the idea popular. As one of the few observational methods that does not require light to “see” cosmic phenomena, gravitational waves have an unparalleled potential to help humanity uncover many of the mysteries of black holes, ancient stars, and even dark matter. So, exciting times are ahead!
