Listening for the Sounds of Dark Matter

Dark matter makes up 85 percent of all the matter in the Universe, yet we cannot see it. But, could it be possible to hear this elusive “something?”

The first clues to the existence of dark matter floating between galaxies came in the 1930’s, from observations of clusters of galaxies by astronomer Fritz Zwicky. These findings were followed up four decades later by groundbreaking astronomer Vera Rubin, when she, along with fellow astronomer Kent Ford, detected the effects of dark matter on the rotational rates of galaxies. Observations conducted at Kitt Peak National Observatory in Arizona provided this first evidence of dark matter within galaxies.

Without the presence of dark matter, galaxies would break apart, and would not form into long-lived clusters. Although we cannot see dark matter (it does not radiate light, heat, or any other form of electromagnetic waves), we can see the effects its mass has on the objects we see in space. Dark matter is now believed to make up 85 percent of all matter in the Universe, nearly seven times as much as every star, planet, and all the gas and dust we see around us.

Two galaxy clusters racing away from each other, leaving behind gas collecting where dark matter was left behind.
Two galaxy clusters racing away from each other, leaving behind gas collecting where dark matter was left behind.
The presence of dark matter can be seen in this Hubble image of Abell 520, where a collision of two massive galaxy clusters left behind a well of dark matter. Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University).

One theory suggests dark matter may exist in the form of axion dark matter, and this idea, if correct, could explain many of the mysteries of this elusive “something” pervading space. This axion dark matter would permeate space as a wave, rather than discrete particles. Only recently, however, have researchers started to look for evidence of this axion dark matter. This quest has driven researchers and engineers to find new ways of proving — or disproving — this theory.

“Finding the axion is a bit like tuning a radio: you have to tune your antenna until you pick up the right frequency. Rather than music, experimentalists would be rewarded with ‘hearing’ the dark matter that the Earth is travelling through,” explains Dr. Alexander Millar of the Department of Physics at Stockholm University.

That’s a Crazy Radio, Daddy-O!

Axions were initially introduced to physics as a way to explain how the weak force (one of the four known forces of nature, like gravity and electromagnetism) could operate both forward and backwards in time, an important consideration in particle physics.

Wavy lines passing through a cylinder filled with thin wires.
Wavy lines passing through a cylinder filled with thin wires.
An illustration showing how a haloscope would work. Image credit: Alexander Millar/Stockholm University

A new study by Dr Millar and his team at Stockholm University suggests a new way of detecting an axion wave — by looking for a small electric field the wave would create as a magnetic field passed through it. This weak signal could then be amplified in a detector called a haloscope. These devices are designed to look for regular waves in plasma (the fourth state of matter, where subatomic particles flow freely, like water). This technique could allow researchers to detect an axion wave, potentially determining the basic nature of dark matter for the first time.

A 3D map of dark matter in the Universe, created by astronomers using the 870-megapixel Hyper Suprime-Cam on the 8.2-meter Subaru telescope. Video credit: HSC/Science Alert

“Without the cold plasma, axions cannot efficiently convert into light. The plasma plays a dual role, both creating an environment which allows for efficient conversion, and providing a resonant plasmon to collect the energy of the converted dark matter”, explains Dr. Matthew Lawson of the Department of Physics at Stockholm University.

In order to detect axion dark matter, investigators will need to tune the haloscope, looking for a signal which will only be heard when they find the correct match between the frequency of the plasma in the device, and the actual axion mass. The device would be tuned by altering the space between wires made from metamaterials, a class of materials designed to react in unnatural ways. Some metamaterials may even be used to design invisibility clocks, like those seen in Harry Potter.

“To produce such a plasma, the team proposes a cylindrical instrument, 60 cm [24 inches] across, containing an array of parallel thin wires in an external magnetic field. In this arrangement, the wires behave like a plasma whose characteristic frequency can be tuned by changing the spacing between the wires,” Sophia Chen reports in APS Physics.

A tunable plasma design would represent the greatest attempt ever made to directly “hear” dark matter, and potentially, unwrapping the answer to one of the biggest questions in the Universe.

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