Dark matter is one of the most puzzling mysteries of science, with a huge range of suspects being investigated. In a new study, astronomers have searched for clouds of hypothetical, ultralight particles that could congregate around black holes, and make themselves known by sending out gravitational waves.
All the matter we see and interact with everyday, from people to planets, only accounts for about 15 percent of the matter in the universe. The vast majority of it is tied up in dark matter, the enigmatic substance that only seems to interact with regular matter through its gravitational influence. What exactly dark matter is, what properties it may have, where it could be found and how we might detect it, are all subjects of ongoing speculation and investigation.
Ultralight bosons are one such candidate. Bosons are a class of particle that includes photons and the legendary Higgs boson, but some models suggest that undiscovered versions may exist with extremely tiny masses. If they do, they might help plug one of the biggest holes in our understanding of the cosmos.
"It is almost impossible to detect these ultralight boson particles on Earth," says Dr. Lilli Sun, co-lead author of the study. ”The particles, if they exist, have extremely small mass and rarely interact with other matter – which is one of the key properties that dark matter seems to have.”
So for the new study, the team searched the skies for the kind of signal that clouds of ultralight bosons could produce. Since dark matter mostly interacts through gravity, the astronomers turned to gravitational waves – ripples in the very fabric of spacetime.
Dozens of gravitational wave signals have been detected since 2015, usually produced during collisions between compact objects like black holes and neutron stars. But they could also come from more subtle phenomena, generating a much longer, more gentle wave at particular frequencies.
The team says that ultralight bosons could gather in clouds around rapidly rotating black holes, where they “drag” on the object and slow its spin. Eventually, the cloud itself starts to shrink as the bosons annihilate into other particles, which generates gravitational waves with a particular fingerprint detectors could pick up.
"We believe these black holes trap a huge number of boson particles in their powerful gravity field, creating a cloud co-rotating with them,” says Sun. “This delicate dance continues for millions of years and keeps generating gravitational waves that hurtle through space. By searching for gravitational waves emitted by these clouds we may be able to track down these elusive boson particles and possibly crack the code of dark matter.”
To check for these signals, the researchers examined data gathered during the third observing run of the Advanced LIGO detector. Unfortunately, no such signal was detected, but that doesn’t completely rule out the hypothesis. Instead, it puts constraints on the type of bosons that might be involved, and the age and distance of the clouds. Since the cloud shrinks with age, the gravitational wave signals would be much weaker for old clouds than young clouds.
"We learnt that a particular type of boson cloud younger than 1,000 years is not likely to exist anywhere in our galaxy, while clouds that are up to 10 million years old are not likely to exist within about 3,260 light-years from Earth,” says Sun.
More sensitive detectors in future could still find weaker signals from older clouds, or younger ones that are more distant. Or, of course, we might be going about this all wrong, and dark matter could be something else entirely.
The research is available on the preprint server ArXiv.
Source: Australian National University