Dark matter is one of the most mysterious mysteries of science. In a new study, astronomers were looking for clouds of hypothetical ultralight particles that could gather around black holes and declare themselves by sending gravitational waves.
All the ordinary (baryonic) matter that we see and interact with every day is only about 15 percent of all matter in the universe. Everything else is connected with dark matter, a mysterious substance that seems to interact with ordinary matter only through its gravitational influence.
What is dark matter, what properties it may have, where it can be found and how to detect it — all this is the subject of constant discussion and research.
One of the candidates for a dark matter particle is ultralight bosons. Bosons are a class of particles that includes photons and the legendary Higgs boson, but some models suggest that as yet undiscovered versions may exist with extremely low mass. If there are any, they can help close one of the biggest holes in our understanding of the universe.
"It's almost impossible to detect ultralight bosonic particles on Earth," says Lilly Sun, co—author of the study. "Particles, if they exist, have extremely low mass and rarely interact with other matter—which seems to be one of the key properties of dark matter."
In a new study, a team of astronomers studied the sky in search of a signal that could generate clouds of ultralight bosons. Since dark matter mainly interacts through gravity, astronomers turned to gravitational waves-ripples in the fabric of space-time itself.
Since 2015, dozens of gravitational wave signals have been detected, usually occurring during collisions between massive objects such as black holes and neutron stars. But they can also occur due to more "subtle" phenomena generating a much longer and softer wave at certain frequencies.
Scientists claim that ultralight bosons can gather in clouds around rapidly rotating black holes, where they "pull" an object and slow down its rotation. Eventually, the cloud itself begins to shrink as bosons annihilate into other particles that generate gravitational waves picked up by a certain kind of detectors.
"We believe that black holes trap a huge number of bosonic particles in their powerful gravitational field, creating a cloud that rotates with them," says Lilly Sun. "This process has been going on for millions of years and continues to generate gravitational waves that rush through space. ...By searching for the gravitational waves emitted by these clouds, we will be able to track these elusive bosonic particles and possibly crack the code of dark matter."
To check for such signals, the researchers examined data collected during the third observation session using the Advanced LIGO detector. Unfortunately (as expected), no such signal was detected, but this does not completely exclude the hypothesis. Instead, the fact of non-detection imposes restrictions on the type of bosons that can be involved, as well as on the age and distance to the clouds. Since the cloud shrinks with age, the gravitational wave signals for old clouds will be much weaker than for young clouds.
"We have learned that a certain type of bosonic clouds younger than 1,000 years is unlikely to exist anywhere in our galaxy, and clouds up to 10 million years old are unlikely to exist within about 3,260 light-years from Earth," the scientists say.
More sensitive detectors in the future will be able to detect weaker signals from older clouds or younger, more distant ones. Or, of course, scientists may be wrong, and dark matter may be something completely different.
The study is available on the arXiv preprint server.
PHOTO: Scientists claim that ultralight bosons can gather in clouds around rapidly rotating black holes, where they "pull" an object and slow down its rotation (Roman Grigoriev)

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