Astronomers have recorded the most massive black hole merger till date. Each black hole is larger than a hundred suns, and their collision is the largest one detected yet. A team of astronomers discovered the event, called GW231123, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) — a pair of identical instruments located in Livingston, Louisiana, and Hanford, Washington — detected small, indistinct ripples in space-time produced by two black holes merging into each other. Physicists have named such ripples gravitational waves.
Albert Einstein predicted the presence of gravitational waves in 1915 as part of his theory of relativity. However, he thought that these gravitational waves were too weak to be ever discovered by human technology. Contrary to Einstein’s belief, LIGO detected them for the very first time when black holes collided in 2016. Since the first detection of gravitational waves, signs of about 300 black hole mergers have been picked up by LIGO and its sister instruments — Virgo in Italy, and KAGRA in Japan. According to Mark Hannam, head of Gravity Exploration Institute at Cardiff University in the UK and a member of the LIGO Scientific Collaboration, these detectors are the most sensitive measurement devices that humans have built. As a result, scientists are observing some of the most extreme and violent events through the smallest measurements.
GW 231123 is unique among the 300 black hole mergers, and not just because it is the largest of the collisions. The individual black holes are themselves exceptional because they lie in a range of masses where scientists don’t expect them to be produced from dying stars. The black holes are also spinning at speeds as fast as physically possible. This black hole merger poses a significant challenge to our understanding of black hole formation.
Gravitational waves are the solitary way that scientists use to detect a collision in a binary system in which two black holes orbit each other. Before astronomers could detect them with gravitational waves, scientists had doubts about the existence of black hole binaries since black holes don’t give off any light or any other electromagnetic radiation, so any kind of regular telescope is unable to detect their presence.
These gravitational waves are weak, and there are limitations to the information they can provide. For example, there’s confusion about the distance of GW231123 from Earth; it could be up to 12 billion light-years away. Hannam has greater confidence about the mass of the two black holes, which are believed to be approximately 100 and 140 times the mass of the sun.
Those numbers are puzzling because there are standard mechanisms where black holes form. Stars run out of fuel and die before collapsing. But there’s a range of masses where scientists think that it’s not possible for black holes to form in the usual manner. And the black holes from GW231123 are present in the middle of that mass gap. A question of how these black holes formed makes them interesting.
The “mass gap” Hannam is referring to starts at approximately 60 solar masses and extends to roughly 130. Since it is a theoretical range, meaning it has not been directly observed, knowledge about where this gap starts and where it ends is unclear. If black holes from GW231123 lie in this gap, then they likely didn’t form from stars collapsing, but in some other way. Hannam suggests that the two black holes were formed from previous mergers, rather than from the collapse of dying stars, which explains the mass gap.
In this scenario, black hole mergers occur in a cascade, producing more and more massive black holes. And since the black holes in GW231123 appear to be at masses that cannot be achieved through standard mechanisms, this suggests that another process is at work, involving successive mergers.
Gravitational waves are opening an exciting avenue into the study of black holes and revealing some truly amazing mysteries. Before the invention of gravitational wave astronomy, scientists could only detect black holes that were actively growing by pulling in material, producing a powerful light source. Gravitational waves, on the other hand, are revealing to us a different part of the black hole population that does not grow by pulling in material, but instead by merging with other black holes.
Another astonishing feature of these black holes is the speed at which they spin around each other. As of now, most black holes found through gravitational waves spin quite slowly. This suggests that GW231123 may have formed through a mechanism different from that of other observed mergers. It could also be a sign that existing models need to change. According to Hannam, high-speed spins are challenging to produce. However, this suggests that previous mergers have occurred, as scientists believe that previously merged black holes tend to spin faster.
“GW231123 challenges our models of gravitational wave signals, as it is complex to model such (fast) spins, and it stands out as an extraordinary event that is puzzling to interpret,” said Sophie Bini, a postdoctoral researcher at Caltech and a member of the LIGO-Virgo-KAGRA Collaboration. “What surprised me the most is how much there is still to learn about gravitational waves. I really hope that in the future we can observe other events similar to GW231123 to improve our understanding of such systems.”
The previous record-holder for the most massive black hole merger ever observed belonged to a black hole merger called GW190521. This merger was only 60% as big as GW231123. According to Hannam, scientists may observe larger mergers in the future, and with the advent of newer detection devices, black hole collisions can be observed with greater accuracy.
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