Discs, spikes and clouds


June 05, 2023

(Nanowerk News) The detection of gravitational waves generated by black hole mergers teaches us a lot about the nature of these extreme bodies. A team of researchers in Gianfranco Bertone’s group at the University of Amsterdam devised a new technique to extract information, not only about the black hole itself, but also about its surroundings.

In a new analysis led by Pippa Cole, and published in Natural Astronomy (“Distinguishing environmental effects on binary black hole gravitational waveforms”), former and current members of the research team have shown that with a future space-based gravitational wave detector, it is possible to distinguish the presence of a new disc of gas, dark matter, and light particles around a black hole. Artist's impression of the space-based LISA experiment detecting gravitational waves Artist’s impression of the space-based LISA experiment detecting gravitational waves. With future measurements like that, peeking into the backyard of a black hole would become possible. (Image: NASA)

A new window on the Universe

The direct detection of the first gravitational waves in 2015 has opened a new window on the universe, in particular enabling the observation of pair mergers of massive black holes. This young field of research has matured very quickly, and now dozens of black hole mergers have been observed. Observations are currently limited to the late stages of collapse – often only a few seconds – when the gravitational waves that are emitted are very strong. Fortunately, several new experiments are under construction that will allow researchers to observe pairs of black holes for much longer before a merger occurs, perhaps even for years.

When these much more precise measurements start coming in, researchers want to be ready and able to interpret them. Pippa Cole, postdoctoral researcher in Gianfranco Bertone’s group, and first author of the new publication, explains: “With current measurements, we can learn some facts about the black hole merger itself, but very little about the environment in which it merged. happen. The environment itself is very attractive. For example, it can teach us about one of the other recent mysteries in astrophysics: that of dark matter. Once we can use upcoming detectors like LISA to observe black hole mergers longer, it may become possible to make meaningful statements about their surroundings.”

Black hole environment

There are at least three types of interesting environments that can surround a black hole. The best known are accretion disks: disks of superheated gas swirling around black holes, as recently imaged by the Event Horizon Telescope. But there is another possibility.

A black hole can be surrounded by a cloud of ultralight particles, forming a structure that astronomers call a gravitational atom. And, finally, there could be dark matter, an elusive form of matter that seems to permeate the cosmos at all scales, but whose fundamental nature is still unknown. It is thought to accumulate around the black hole as it forms and grows, becoming a high-density configuration called a spike.

Cole: “The wonderful thing is that with the new observations, it will be possible to distinguish between all three situations – and to distinguish it from the case where the backyard of the black hole is just empty, where two black holes are circling. each other in a vacuum. We managed to develop a statistical technique, which, given enough data and a large enough mass difference between the two black holes, should be able to distinguish very clearly between the four scenarios.”

According to Cole and his collaborators, a next generation of experiments will be able to identify gravitational waves generated by a black hole merging in the presence of an environment, be it an accretion disk, gravitational atoms, or a dark matter spike. This opens up the possibility of searching for new ultralight particles, as well as dark matter candidates, with gravitational waves.

Bertone: “These are exciting times. We will soon enter a new era in physics and astronomy. Just as precision particle physics will allow us to seek new physics in particle accelerators on Earth, precision gravitational wave astronomy will soon allow us to search for dark matter and new particles in the Universe.”


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