The universe is full of ‘impossible’ black holes and now scientists know why

International team Astrophysicists have found evidence that the universe recycles black holes, merging them to form larger ones. Gravitational waves recorded in recent years show that some of the heaviest black holes within star clusters show clear signs of being “second generation” black holes – the product of previous collisions – and therefore could not have arisen from the collapse of a massive star.

Impossible black holes

The evolutionary theory of stars explains that at the end of the life of the most massive stars, their cores compress until they form a point so dense that it curves spacetime to infinity. This is the classic black hole, 10 to 40 times the mass of the Sun. There are also supermassive black holes in the center of galaxies, with a mass of millions or billions of solar masses, and their origin is linked to processes that occurred in the first moments of the universe’s emergence.

Between these two extremes lies a contested category: black holes with masses between 40 and 100 solar masses. They are too heavy to be born after the death of a star, but they do not reach the dimensions needed to emerge from the collapse of a giant cloud of matter. Conventional stellar physics considers them impossible, but they appear frequently in detections.

A “normal” sized black hole, isolated in space.

Courtesy of the Space Telescope Science Institute’s Office of Public Outreach

Astrophysicists suggest that these massive black holes could form by the merger of two or more smaller, super-dense objects. The idea was plausible, but it needed proof. Until relatively recently, there was no way to obtain it.

Then gravitational wave detectors came to the scene. These instruments use lasers to measure the partial distortion of space-time caused by the collision of very dense objects. The first discovery, in 2015, confirmed a merger between black holes. Since then, each new signal has allowed better characterization of these structures and revealed that these collisions occur much more frequently than previously imagined.

Signature of the second generation

The study, published this month in the journal Nature Astronomy, analyzed a transient list of gravitational waves generated by the world’s three leading observatories. The database included 153 reliable detections of black hole mergers. Among them, 34 of them correspond to particularly heavy objects.

By comparing all the signals, the team identified two different groups. The lighter black holes, with a mass of about 40 solar masses, showed small parallel rotations, as expected for objects born from the collapse of a star. But from a certain point, around 45 solar masses, a completely different population emerged: heavier black holes, rotating rapidly and in chaotic directions — a statistical signature that can only arise when the object has already participated in a previous merger.

“This is the exact signature you would expect if black holes repeatedly merged into dense star clusters,” Isobel M. Romero Shaw, co-author of the paper, said in a statement from Cardiff University.

So far, researchers have not directly observed any of these “impossible” black holes. They do not appear in X-rays or the visible spectrum, unlike ultra-massive radiation. However, their collisions shake space-time, and this shaking reveals clumps that stellar physics cannot explain.

This study shows that the heaviest black holes are built rather than born. They arise from previous generations of collisions, which were accumulated in the densest environments in the universe.

This story originally appeared in WIRED en Español and was translated from Spanish.