International team astrophysicists have found evidence that the Universe is recycling black holes, combining them to create even larger ones. Gravitational waves recorded in recent years show that some of the heaviest black holes in star clusters show clear signs of being “second generation” black holes – products of past collisions – and therefore could not have been formed by 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 shrink until they form a point so dense that it warps space-time infinitely. It is a classic black hole with masses 10 to 40 times greater than the Sun. In the centers of galaxies there are also supermassive black holes with millions or billions of solar masses, the formation of which is associated with processes occurring in the earliest moments of the existence of the Universe.
Between these two extremes lies a controversial category: black holes with masses ranging from 40 to 100 solar masses. They are too hefty to be born after the death of a star, but they do not reach the dimensions necessary to emerge from the collapse of a giant cloud of material. Conventional stellar physics considers them “impossible,” yet they often appear in detections.
Astrophysicists suggest that these massive black holes may be formed by the merger of two or more smaller, ultradense objects. The idea was plausible, but required evidence. Until relatively recently, it was not possible to obtain it.
Then gravitational wave detectors appeared on the scene. These instruments utilize lasers to measure micro-warping of space-time created by the collision of extremely dense objects. The first detection in 2015 confirmed the black hole merger. Since then, each fresh signal has allowed for better characterization of these structures and revealed that these collisions occur much more frequently than previously thought.
Second generation signature
The study, published this month in Natural astronomyanalyzed a transient catalog of gravitational waves generated by three leading observatories in the world. The database contained 153 credible cases of black hole mergers. Among them, 34 concerned particularly hefty items.
By comparing all the signals, the team identified two distinct populations. Lighter black holes, with masses up to about 40 solar masses, showed compact, aligned spins, as expected for objects created by the collapse of a star. But after a certain point, around 45 solar masses, a completely different population emerged: heavier black holes, spinning rapidly and in disordered directions – a statistical signature that can only arise if the object has already been involved in a previous merger.
“This is exactly the signature that would be expected if black holes repeatedly merged into dense star clusters,” said Isobel M. Romero-Shaw, co-author of the study, in her paper statement from Cardiff University.
So far, researchers haven’t directly observed any of these “impossible” black holes. They do not appear in X-rays or in the noticeable spectrum, unlike supermassive ones. However, their collisions vibrate space-time, and these vibrations reveal masses that stellar physics cannot explain.
This study shows that the heaviest black holes are created, not created. They are created as a result of collisions of previous generations, created in the densest environments of space.
This story originally appeared on WIRED in Spanish and was translated from Spanish.