Over the last few years, gravitational waves have revolutionised physics. Sensitive instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo and KAGRA have spotted these ripples in spacetime emanating out from collisions between black holes and neutron stars billions of light-years away.
But can gravitational waves find dark matter? Now, researchers think that gravitational wave detectors may be able to spot a possible source of the elusive material that makes up 85% of the universe.
“Gravitational-wave discoveries not only provide information about mysterious compact objects in the universe, like black holes and neutron stars, they also allow us to look for new particles and dark matter,” says astrophysicist Lilli Sun from the Australian National University (ANU) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
Sun co-authored a new international study that appears on the preprint server ArXiv. The paper offers a new lead for dark matter: clouds of ultralight subatomic particles called bosons, predicted by theories that reach beyond the standard model of particle physics.
“It is almost impossible to detect these ultralight boson particles on Earth,” explains Sun.
“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. Dark matter is material that cannot be seen directly, but we know that dark matter exists because of the effect it has on objects that we can observe.”
But gravitational wave detectors could help astrophysicists determine whether these hypothetical particles exist. Theory predicts that they could be swirling in clouds around rapidly spinning black holes.
“We believe these black holes trap a huge number of boson particles in their powerful gravity field, creating a cloud co-rotating with them,” Sun says. “This delicate dance continues for millions of years and keeps generating gravitational waves that hurtle through space.”
We haven’t spotted any of these types of gravitational waves yet, but that’s not for lack of trying.
This new study describes the first all-sky search for gravitational-wave signals emitted by these ultralight boson clouds, using the LIGO interferometer in the US.
“The gravitational-wave signal frequency depends primarily on the mass of the boson, and weakly on the mass and spin of the black hole,” they write in their paper.
They therefore searched for signals within a frequency range of 20–610Hz. This is fairly low in terms of gravitational-wave frequencies – LIGO can detect events up to 10,000Hz, and other tech is pushing detection up to much higher frequencies, megahertz (1,000,000Hz) and above.
While no signals were found, they did constrain the parameters of boson clouds that might exist within our own galaxy, the Milky Way. The strength of any gravitational wave signal depends on the age of the boson cloud – older ones send out weaker signals.
“The boson cloud shrinks as it loses energy by sending out gravitational waves,” Sun says.
“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.”
And the team are full of ideas about how to improve their search method, from increasing computational efficiency to covering a wider frequency band.
“This, together with the expected detector improvements in upcoming runs of Advanced LIGO, Advanced Virgo and KAGRA detectors, will significantly increase the chance of detecting gravitational radiation from these interesting sources,” they conclude in their paper.