An international team of astrophysicists has found evidence that the universe recycles black holes, merging them to form even larger ones. Gravitational waves recorded in recent years show that some of the heaviest black holes within star clusters exhibit clear signs of being “second-generation” black holes—products of past collisions—and therefore could not have originated from the collapse of a massive star.
Impossible Black Holes
The evolutionary theory of stars explains that, at the end of the lives of the most massive stars, their cores compress until they form a point so dense that it curves space-time to infinity. This is the classic black hole, with masses 10 to 40 times that of the sun. There are also supermassive black holes, in the center of galaxies, with millions or billions of solar masses, whose origin is related to processes that occurred in the earliest 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 necessary dimensions to emerge from the collapse of a gigantic cloud of matter. Conventional stellar physics considers them “impossible,” yet they appear frequently in detections.
A “normal” sized black hole, isolated in space.
Courtesy of Space Telescope Science Institute Office of Public Outreach
Astrophysicists propose that these massive black holes could form by the merging of two or more smaller, ultradense objects. The idea was plausible, but it needed evidence. Until relatively 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 for a better characterization of 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 leading 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, up to about 40 solar masses, showed small, aligned spins, as 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 arise only 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 stellar clusters,” said Isobel M. Romero-Shaw, coauthor of the research, 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 in the visible spectrum, unlike supermassive ones. However, their collisions vibrate space-time, and that vibration reveals masses 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, assembled in the densest environments in the cosmos.
This story originally appeared in WIRED en Español and has been translated from Spanish.
