Astrophysicists have conducted a census of invisible objects that betray their presence only through the trembling of space. They studied the fresh catalog GWTC-4 — a list of dozens of black hole mergers recorded by the gravitational wave detectors LIGO, Virgo, and KAGRA. Each such event is like the strike of a giant bell, sending ripples through the fabric of the cosmos.
Particular attention was drawn to intermediate-mass black holes — those from a hundred to several thousand solar masses. These are not ordinary cosmic residents; their origin is a puzzle. One hypothesis suggests that such giants are born from the collapse of very massive single stars. In that case, they should spin slowly, like a figure skater who never performed a spinning move. But the catalog contained no such slow heavyweights. Instead, all the candidates found were spinning fast — as if they had been spun up during assembly from several smaller holes in repeated collisions within dense stellar clusters.
The analysis also showed that the rate of mergers involving slow intermediate-mass holes does not exceed 0.077 per cubic gigaparsec per year. For comparison: if you take a cube with a side of a billion light-years, such an event occurs less than once every ten years. This is an important negative result: it tells us that the direct collapse scenario for single stars, if it works at all, is extremely rare. The main path to giants is hierarchical mergers, where black holes grow by consuming each other, and with each step they gain spin speed, like a rolling snowball.
A black hole with a mass of 65 suns is smaller in size than the distance from Moscow to Tula — its event horizon would fit within a couple of hundred kilometers.