Somewhere in the distant universe, two colossal black holes — each so massive they should never have existed — spiralled into one another and became something even more impossible: a single black hole 225 times the mass of our Sun.
And scientists in Glasgow helped hear it happen.
The detection, announced by the LIGO-Virgo-KAGRA (LVK) collaboration and now supported by a landmark study published in Nature, represents one of the most significant breakthroughs in gravitational wave astronomy since the field began barely a decade ago. For the first time, researchers have confirmed that "intermediate-mass" black holes — objects that sit in a theoretically forbidden zone between the small and the stupendously large — are not only real, but are being built before our eyes through the violent collisions of their smaller cousins.
The mass gap: a cosmic mystery
For decades, astrophysicists have known about two kinds of black hole. Stellar black holes, formed when massive stars collapse, can reach up to about 40 or 50 times the mass of the Sun. At the other extreme sit supermassive black holes — the monsters at the centres of galaxies, weighing millions or even billions of solar masses.
But between those two categories lay an awkward gap. Stellar theory predicted that stars massive enough to produce black holes between roughly 50 and 130 solar masses would instead tear themselves apart in cataclysmic explosions called pair-instability supernovae — effectively forbidding black holes of that size from forming through normal stellar death.
The question that nagged at physicists was simple: if nature forbids them, where are the intermediate-mass black holes?
Glasgow's instruments heard the answer
The answer came through gravitational waves — ripples in the very fabric of spacetime, generated when massive objects collide. These waves are detected by LIGO, an extraordinary pair of observatories in the United States whose laser interferometers can measure a distortion in space smaller than one-thousandth the width of a proton. Put another way: if LIGO's laser arms stretched from here to the nearest star, the instruments could detect a change in length smaller than the width of a human hair.
That astonishing sensitivity exists in no small part because of work done in Glasgow. The University of Glasgow's Institute for Gravitational Research (IGR) designed and built the fused silica suspension systems that hold LIGO's mirrors in place — an engineering feat that required pushing the boundaries of materials science and precision manufacturing. Without Glasgow's suspensions, LIGO simply could not achieve the sensitivity required to detect these cosmic whispers.
"The ALUK project was a complete success: all the suspension systems were supplied, installed and working by 2013," the IGR states. Glasgow also pioneered signal recycling, another essential technology built into LIGO's design.
Catching the impossible
The signal designated GW231123, detected on 23 November 2023 during LIGO's fourth observing run, was staggering. Two black holes — roughly 100 and 140 solar masses respectively, both squarely within the "forbidden" mass gap — merged to produce a final black hole of approximately 225 solar masses. Both were spinning at rates approaching the theoretical maximum allowed by Einstein's general relativity.
"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 LVK collaboration. "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."
That explanation — hierarchical merging, in which smaller black holes collide to build ever-larger ones — is precisely what a separate analysis of the full GWTC-4 catalogue, published in Nature, now supports at the population level. The study found the lower boundary of the pair-instability gap sits at roughly 44 solar masses, and that a distinct subpopulation of rapidly spinning black holes appears to be populating the gap from below, consistent with objects assembled through successive mergers.
What it means
The implications are profound. Intermediate-mass black holes may be the missing link in understanding how supermassive black holes — the titans at galactic centres — first grew to their enormous size. If nature can build 225-solar-mass black holes through mergers, the pathway from stellar remnants to galaxy-anchoring behemoths becomes clearer.
"This observation once again demonstrates how gravitational waves are uniquely revealing the fundamental and exotic nature of black holes throughout the universe," said Dave Reitze, executive director of LIGO at Caltech.
The event was presented at the GR-Amaldi conference in Glasgow in July 2025 — a fitting venue, given the city's pivotal role in making these detections possible.
For Glasgow's gravitational wave scientists, the message is clear: the universe has been hiding something extraordinary in that forbidden gap. And thanks in part to instruments they built with their own hands, we can finally hear it.
