Different analyses of gravitational-wave observations are converging on evidence for a distinct population of massive black hole binaries produced through repeated mergers.

Throughout the Universe, pairs of orbiting black holes emit ripples in spacetime that propagate across the cosmos. These gravitational waves carry away orbital energy, causing the black holes to slowly spiral closer together. This process is extremely slow, but, in a minority of cases, it leads to a cataclysmic merger within the age of the Universe. Since the historic detection of gravitational waves in 2015 (see Viewpoint: The First Sounds of Merging Black Holes), the LIGO, Virgo, and KAGRA gravitational-wave detectors have advanced to the point of recording a signal from merging black holes every few days of operation, yielding a cumulative catalog of hundreds [1]. Understanding how, when, and where the Universe produces these extreme astrophysical collisions remains an open question, with implications spanning physical scales from the subatomic to the cosmological.

Now, two teams led, respectively, by Cailin Plunkett at MIT [2] and Sharan Banagiri at Monash University in Australia [3] present evidence that a subset of binary black hole observations can be connected to a particular origin story: that of hierarchical mergers, in which at least one member of the pair is not the remnant of a dead star but instead the product of an earlier black hole merger (Fig. 1). The fact that these and other analyses [4– 10], based on markedly different assumptions, converge on a similar conclusion strengthens the case that hierarchical mergers constitute an important contribution to the binary black hole population.

They make the universe’s most extreme gravitational laboratories

The one thing we all ‘know’ about black holes is that nothing escapes their ineluctable grasp.

That is mostly true – but since the 1970s, physicists have predicted that black holes could slowly lose energy in the form of thermal radiation.

This is Hawking radiation, and while it has been recreated in laboratory analogs, the mechanism whereby it siphons energy from a black hole, known as backreaction, has remained elusive.

Hawking radiation is a form of radiation emitted by black holes, as theoretically predicted by Stephen Hawking. It suggests that black holes do not merely swallow matter—as had previously been assumed—but also emit very faint radiation themselves. This radiation has not yet been observed in space; instead, researchers use models in the laboratory that mimic the behavior of black holes.

Although the effect of Hawking radiation is well known in astrophysics, the mechanism by which it arises in a gravitational context has not yet been fully elucidated. A scientist from Paderborn University along with an international team of researchers from the Weizmann Institute of Science in Israel and Cinvestav in Mexico is now shedding light on this mechanism using gravitational analogs in the laboratory.

The team has theoretically modeled the process by which Hawking radiation is generated in a nonlinear optical environment, identifying a simple, direct mechanism in the process. Furthermore, the team was able to observe in experiments that the radiation affects the system. The results have now been published in Nature.