Black Hole Dances in Globular Clusters ✎ express

A fisherman's net is stretched across the depths of space. When a massive star dies, it leaves behind a black hole — a sinker in this net. Usually, such sinkers weigh from a few to tens of suns. But for GW231123, the two sinkers turned out to be excessively heavy: each fell into the "pair-instability gap" (the mass range where a star should explode completely, leaving no black hole). How did nature bypass this prohibition?

Scientists built a virtual universe in a computer to find the answer. They ran a simulation of galaxy formation and traced the fate of millions of binary stars. The usual path, two stars living together and dying together, did not work. In one scenario, the pairs were too wide: the black holes orbited each other at distances greater than a thousand solar radii and did not have time to merge within the age of the universe. In another scenario, suitable pairs were born only in environments nearly devoid of metals (elements heavier than helium). But such places are too rare to explain the event rate.

So the researchers turned to a third path: globular clusters. These are dense swarms of hundreds of thousands of stars, where black holes can collide repeatedly. Like in a crowded queue where everyone jostles their neighbor, they merge, forming ever more massive objects. Such a hierarchy of mergers naturally produces black holes of the required mass. The rapid spin detected by the LIGO-Virgo-KAGRA detectors also points to this mechanism: after each merger, the spin grows. The model showed that the peak of such mergers occurs at an epoch when the universe was 1–2 billion years old, just the time of vigorous globular cluster formation.

Now the same net that holds the sinkers becomes the arena for their dance. Black holes don't just fall into it: they collide, grow, and circle again until they merge into an object that should not have existed. GW231123 is the first catch whose weight directly points to this hidden mechanism.