Searching for the Music of Dark Matter

Galaxy and dark matter detector diagram

The illustration shows how a cavity optomechanical system is used as a dark matter sensor. The white dotted wave represents the dark matter signal. The arrow indicates that it pushes on the mirror, changing the length of the optical cavity. The length of the optical cavity is measured using an optical field, represented by the red wave.

Dalziel Wilson

Dalziel Wilson

Dalziel Wilson

Scientists are certain dark matter exists, yet after more than 50 years of searching, they're still unsure what it's made of.

Dalziel Wilson, a University of Arizona assistant professor in the James C. Wyant College of Optical Sciences, is senior author on a paper published in Physical Review Letters that describes a new way to look for the particles that might make up dark matter.

Wilson is an experimentalist in quantum optomechanics, and he makes mechanical devices and uses lasers to measure their response to stimuli at the quantum level. He is part of a small but growing group of scientists that think that by slightly reconfiguring optomechanics technology, they can make tabletop dark-matter detectors. By employing many of these devices in labs across the world, they hope to find direct evidence for the mysterious matter.

dark matter search device with reflective surface

A rendering of the device Dalziel Wilson proposes to use in the search for dark matter.

Dalziel Wilson

"The kind of device we want to use is called a nanomechanical resonator," Wilson said. "You can think of it like a miniature turning fork. It's a vibrating device, which, due to its small size, is very sensitive to perturbations from the environment."

Nanomechanical resonators have recently been used to detect and amplify weak signatures of the uncertainty principle in quantum mechanics, which says that the position and the velocity of an object cannot both be measured at the same time. Wilson and his peers reason that very weak signals produced by dark matter can also be detected and amplified using such devices.

Wilson's co-authors include UArizona optical sciences graduate student Mitul Dey Chowdhury, Haverford College's Daniel Grin and the University of Delaware's Jack Manley and Swati Singh.

Coming Together

A few years ago, the dark matter community decided it was necessary to rethink how to look for dark matter. So, they teamed up with researchers in different fields, including optomechanics.

If you add up all the things that emit light, such as stars, planets and interstellar gas, it only accounts for about 15% of the matter in the universe. The other 85% is known as dark matter. It doesn't emit light, but researchers know it exists by its gravitational effects. They also know it isn't ordinary matter, such as gas, dust, stars, planets and people, explained Singh, a quantum theorist and an alumnus of the UArizona College of Optical Sciences. 

"It could be made up of black holes, or it could be made up of something trillions of times smaller than an electron, known as ultralight dark matter," Singh said.

"It's fruitful to come at these problems both ways – theoretically and experimentally," Wilson said.

Theorists say that dark matter might not only interact with ordinary matter gravitationally, but also via a "dark photon force," but such interactions produce a very weak signal.

Fortunately, Wilson is in the game of detecting and amplifying weak signals.

His device, as described in his team's experimental proposal, is a thin piece of silica nitride glass that is stretched into a drum. It's 100 nanometers thick and a millimeter wide. Such extreme ratios of thinness to width make the drum very sensitive to inertial forces while at the same time decoupling it from other environmental disturbances.

"It's nanotechnology in the pursuit of quantum mechanics, coincidentally putting us in a regime where we can pursue dark matter searches," Wilson said. "We're not the first to realize this coincidence, but we're among the first to propose a concrete detector design. The idea is that we can take a technology that we've been building in the field of optomechanics for a while and functionalize it to search for dark matter."

Wilson and Singh teamed up because they believe the idea applies to many types of devices, and different people could search with their favorite. The sensor might be a drum – like Wilson's – or a string, a membrane, a levitated sphere, a cantilever or a pendulum.

Searching for a dark matter signal won't be easy, however. It will be like looking for a needle in a huge haystack.

"We're looking for that weak note, that little vibration," Wilson said. "We want to take a mechanical resonator that resonates at the same frequency as the note, just like a tuning fork, then let it ring up. That note we want to play is extremely weak, so we want to listen for long enough that it rings up to a tangible amplitude and with many different devices."

But scientists don't know exactly where to look, so they have to do what other people do when they search for dark matter, which is look at a certain frequency for a month, then change the tuning fork dimensions, look for another month and so on.

"This paper is sort of a call to arms," Wilson said. "Detecting dark matter is a daunting challenge. It's like fishing in the middle of the ocean. You have no idea what's biting or how deep the fish are, but the most important fish is the first; then you know what to look for. We just need to put enough lines in the water that we start to feel something."

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