Disks, Nails, and Clouds: A Look at the Backside of a Black Hole

The first direct detection of gravitational waves in 2015 opened a new window in the universe, making it possible to observe the merger of particularly large black hole pairs. This young field of research has developed very rapidly and dozens of black hole mergers have been observed today.

Current observations are limited to the final stages of collapse, where the propagating gravitational waves are extremely strong, usually lasting only a few seconds. Fortunately, several new experiments are on the way that will allow researchers to observe black hole pairs for much longer than they merge, potentially for years.

When these more precise measurements start coming in, researchers want to be ready and able to interpret them. Pippa Cole, PhD student in Gianfranco Bertone’s group and first author of a new paper published Nature Astronomyexplains: “With current measurements, we can learn a few facts about the black hole mergers themselves, but very little about the environment. “When we can use a new detector like LISA to observe black hole mergers for much longer, it becomes possible to make meaningful statements about our environment.”

Environments of black holes

There are at least three different types of interesting environments surrounding black holes. The most famous is the so-called accretion disk, a disk of very hot gas orbiting a black hole similar to those recently imaged by the Event Horizon telescope. But there are other possibilities.

A black hole may be surrounded by a cloud of ultralight particles that form what astronomers call a gravitational atom. And finally, it could be dark matter, an elusive form of matter that seems to permeate the cosmos at all scales, but whose fundamental nature is unknown. It is expected to accumulate around black holes as they form and grow in high-density configurations called clusters.

Cole says, “The great thing is that the new observations will make it possible to distinguish all three states, and also separate them from the case where the black hole’s backyard is empty, where two black holes spiral around each other in a vacuum. There’s enough data and there’s enough data and the difference between two black holes. “Given a large enough mass difference, we were able to develop statistical methods that should be able to distinguish all four scenarios very clearly.”

According to Cole and colleagues, next-generation experiments will be able to identify gravitational waves produced by merging black holes in the presence of a medium, whether it’s an accretion disk, a gravitational atom, or a dark matter explosion. This opens up the possibility of searching for new ultralight particles as well as dark matter candidates using gravitational waves.

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