This amazing discovery challenges theories of stellar evolution and black hole formation.
Two black holes merge in this 2016 simulation based on LIGO data. The event, which occurred 1.3 billion years ago, was the first confirmed detection of gravitational waves. Credit: SXS Project (black-holes.org)
- Scientists detected the merger of two extremely massive black holes.
- This merger created the most massive black hole ever observed via gravitational waves.
- The black holes were spinning very fast, challenging current scientific understanding.
- This discovery questions how such large black holes formed.
On November 23, 2023, a groundbreaking astronomical event unfolded as scientists from the LIGO-Virgo-KAGRA Collaboration detected the most massive black hole merger ever observed. The finding was detailed in a July 13, 2025, press release shared by the University of Birmingham. This cosmic collision occurred when two colossal black holes, approximately 100 and 140 times the mass of our Sun, combined to form a new black hole 225 times more massive than the Sun.
Designated GW231123, the event was picked up during the fourth observing run of the LIGO-Virgo-KAGRA (LVK) network, which includes the LIGO observatories in the U.S., the Virgo detector in Italy, and Japan’s KAGRA. “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 Professor Mark Hannam of Cardiff University, a member of the LIGO Scientific Collaboration.
What sets GW231123 apart?
What sets this merger apart is its immense scale and the rapid spinning of the black holes involved. “The black holes appear to be spinning very rapidly — near the limit allowed by Einstein’s theory of general relativity,” according to Dr Charlie Hoy at the University of Portsmouth. Unlike typical black holes that form from the remnants of massive stars after supernova explosions, the progenitors in this merger defy traditional models of stellar evolution. This raises fundamental questions about how such massive black holes came to exist in the first place.
As astronomer and science communicator Philip Plait explains, “Because the biggest black hole you can get from a supernova is maybe a few dozen times the mass of the Sun, 100 and 140 solar masses are way above that limit.” Plait, who earned his Ph.D. in astronomy from the University of Virginia in 1994, is widely known for his Bad Astronomy blog and science outreach.
One possibility, he notes, is that each of these giants was forged through earlier black hole mergers, a scenario known as hierarchical merging. Another, more speculative, explanation is that these black holes formed from unusually massive stars born in the early universe. Either way, the detection adds a new level of complexity to existing models and underscores the need to rethink how the universe builds its heaviest gravitational monsters.
How are gravitational waves detected?
When two black holes orbit each other, they slowly spiral inward, emitting faint gravitational waves that grow stronger as their dance tightens, culminating in a powerful burst of ripples that race across the universe and wash over Earth. These ripples are distortions in the very fabric of space-time, caused by the accelerating masses of the black holes as they draw closer and orbit faster.
As energy radiates away in the form of gravitational waves, the black holes lose orbital momentum, pulling them ever nearer until, in their final orbits, they whirl at nearly the speed of light. The last moments before merging produce the strongest waves, brief but intense signals detectable by highly sensitive instruments.
Observatories such as LIGO, Virgo, and KAGRA use laser interferometry to measure the minute stretching and squeezing of space-time distortions, smaller than a thousandth the width of a proton, to capture these extraordinary events despite their faintness by the time they reach Earth.
Gravitational-wave astronomy is still a young field, just a decade past its first detection, but it’s already reshaping our understanding of the cosmos. Each new signal reveals not only strange and powerful events but also the limitations of our current models. With GW231123, we’re reminded that black holes still hold many secrets, and that we’re only beginning to scratch the surface of what these ripples in spacetime can teach us about how the universe works and where it came from.
