The Universe Is Full of ‘Impossible’ Black Holes. Now Scientists Know Why
An international team astrophysicists have discovered that the universe recycles black holes and merges 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 – products of past collisions – and therefore cannot arise 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 to form a point so dense that it bends space-time to infinity. This is the classic black hole, with masses 10 to 40 times those of the sun. There are also supermassive black holes, at the centers of galaxies, with millions or billions of solar masses, whose origin is linked to processes occurring in the first moments of the universe.
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 necessary to emerge from the collapse of a gigantic cloud of matter. Conventional stellar physics considers them “impossible”, but they appear frequently in detections.
Astrophysicists propose that these massive black holes could form by the merger of two or more smaller, ultradense objects. The idea was plausible, but it needed proof. Until recently, there was no way to obtain it.
Then gravitational wave detectors came on the scene. These instruments use lasers to measure the micro-distortion of space-time generated by the collision of extremely dense objects. The first detection, in 2015, confirmed a merger between black holes. Since then, each new signal has allowed us to better characterize these structures and revealed that these collisions occur much more frequently than previously imagined.
The second generation signature
The study, published this month in Nature Astronomy, analyzed a transient catalog of gravitational waves generated by the world’s three major observatories. The database included 153 reliable detections of black hole mergers. Among them, 34 corresponded to particularly heavy objects.
By comparing all the signals, the team identified two distinct populations. The lighter black holes, reaching around 40 solar masses, had small, aligned spins, as would be expected for objects born from the collapse of a star. But from a certain point, around 45 solar masses, a completely different population appeared: heavier black holes, spinning rapidly and in chaotic directions – a statistical signature that can only appear when the object has already participated in a previous merger.
“This is the exact signature we would expect if black holes repeatedly merged into dense stellar clusters,” Isobel M. Romero-Shaw, co-author of the research, said in a Cardiff University statement.
Until now, researchers have not directly observed any of these “impossible” black holes. They do not appear in X-rays or the visible spectrum, unlike supermassive rays. However, their collisions cause space-time to vibrate, and this vibration reveals masses that stellar physics cannot explain.
This study shows that the heaviest black holes are formed rather than born. They come from previous generations of collisions, assembled in the densest environments of the cosmos.
This story originally appeared in WIRED in Spanish and has been translated from Spanish.
