💥 Black holes colliding like dominoes

The most massive black holes in the universe could be born in chaos. By scrutinizing the ripples of spacetime, scientists have distinguished two categories of black holes. The lighter ones would come from the collapse of massive stars, while the heavier ones would be the result of chain collisions of black holes in very dense star clusters, a process far more violent and intense than imagined until today.

To reach this conclusion, the researchers used data from the gravitational wave detectors LIGO, Virgo, and KAGRA. These instruments capture the tiny vibrations of spacetime caused by cataclysmic events, such as the merger of black holes.


By analyzing 153 mergers, the team from Cardiff University was able to trace the evolution of these objects. The lead author of the study, Fabio Antonini, specifies that gravitational astronomy is no longer limited to counting mergers: it is beginning to reveal how and where black holes grow.

These results reveal a clear separation between low-mass black holes and high-mass ones. The former rotate slowly on their own axis, a typical sign of birth by stellar collapse. The latter, on the other hand, have fast, randomly oriented rotations. This signature matches exactly what is expected if black holes merge repeatedly in a dense cluster.

Furthermore, the study also confirms the existence of a theoretical "mass gap" around 45 solar masses. Beyond this threshold, the most massive stars would not produce a black hole when they die. They would be completely destroyed by a supernova before a black hole could form. Black holes exceeding this mass cannot therefore come from a single star. Their origin would necessarily be the result of hierarchical mergers, like those observed in globular clusters.

Future observations from gravitational detectors should refine this picture. By tracking ever more mergers, astronomers hope to better understand the fate of the most massive stars and the role of globular clusters in the making of giant black holes.