Physicists have recorded the most massive black hole collision ever detected, with implications that could upend current models of black hole formation and growth.
The discovery, made by the Laser Interferometer Gravitational-Wave Observatory (LIGO), revealed two unusually heavy black holes—about 100 and 140 times the mass of the Sun—merging into a single black hole weighing roughly 225 solar masses. The event, named GW231123, was observed in November 2023 and is now the largest black hole merger ever seen through gravitational waves.
“This is incredibly exciting,” says Priyamvada Natarajan, a theoretical astrophysicist at Yale University who was not involved in the research. “These are black holes in the so-called forbidden mass range—ones we didn’t think should form this way.”
The findings were shared on 14 July at the GR-Amaldi gravitational waves conference in Glasgow, UK, and posted as a preprint on the arXiv server.
Black Holes in the ‘Forbidden’ Zone
LIGO identifies gravitational waves—tiny ripples in space-time—by detecting changes in the length of its massive L-shaped detectors caused by passing waves. These waves typically originate from extreme cosmic events, such as merging black holes or neutron stars.
Since its first detection in 2015, LIGO and its international partners—Virgo (Italy) and KAGRA (Japan)—have logged hundreds of black hole mergers. But none have matched the scale of GW231123.
“This event is about 50% more massive than the previous record holder,” says Mark Hannam, physicist at Cardiff University and a member of the LIGO–Virgo–KAGRA (LVK) Collaboration.
What makes this event especially puzzling is that both black holes fall into a predicted “mass gap” between 60 and 130 solar masses, where stellar evolution models suggest black holes shouldn’t form. Instead, stars this large should explode completely, leaving no remnant. That suggests the two black holes likely formed from earlier black hole mergers, part of a process known as hierarchical merging.
“It’s like four grandparents merging into two parents, then into one baby black hole,” says Alan Weinstein, physicist at Caltech and member of the LVK Collaboration.
Clues About Black Hole Growth
Adding to the mystery: the resulting black hole appears to be spinning at nearly 40 times per second—approaching the theoretical limit set by Einstein’s theory of general relativity.
This tells us something important about how black holes grow
Weinstein
The discovery provides a rare glimpse into the possible formation of intermediate-mass black holes, a missing link between stellar-mass black holes (a few to 100 solar masses) and supermassive black holes (millions of solar masses) that anchor galaxies. Despite decades of research, such intermediate black holes have remained elusive.
“This may be a sign that these mid-sized black holes help drive galaxy evolution,” says Hannam. “Little by little, we’re starting to catalog the full variety of black holes in the universe.”
Funding Uncertainty
However, continued breakthroughs may be at risk. Proposed funding cuts from the administration of U.S. President Donald Trump could force one of LIGO’s two U.S. observatories to shut down, undermining future detections.
Without both detectors operating in 2023, GW231123 might have gone unnoticed. “Seeing the same signal in both detectors is what confirmed it was real,” says Hannam.
Natarajan warns the proposed cuts would be “catastrophic.” “This discovery wouldn’t have happened with only one observatory running,” she says.
Fortunately, new facilities are on the horizon. An upgraded LIGO, plus additional detectors in India and elsewhere, promise to expand gravitational-wave astronomy dramatically.
“We’re just getting started,” says Hannam. “Thousands of black hole detections lie ahead, and we’re finally beginning to see the payoff from years of investment.”
