Wind chime-based dark matter detector looks odd enough to work

Dark matter physicists can have one of the most frustrating jobs in science. Their work deals with something that must, according to almost every model of the universe, exist. But we never found direct evidence of dark matter. Where other scientists can capture their subjects in a lab and perform experiments there, dark matter scientists are left with nothing but a tantalizing set of clues. It’s like studying ghosts – if ghosts were real and also made up a quarter of the matter in the known universe.

Scientists who study dark matter could also be forgiven for feeling a little more anxious lately. A number of expensive experiments designed to find some of the main dark matter candidates have arrived empty-handed.

“Now it’s kind of an open season,” said Daniel Carney, theoretical physicist at the University of Maryland, the National Institute of Standards and Technology and Fermilab. “Physicists are really scrambling to find new ways to search for dark matter and new types of dark matter that might exist.”

Carney thinks he might have a potential solution. The only thing we know about dark matter is that it exerts a gravitational pull. So why don’t we look for it that way?

As simple as it sounds, this is an approach that has never been attempted before, in large part because the design of such an experiment involves calibrations so exquisite that it almost seems unlikely. But Carney and a small group of scientists have started work on a prototype that they believe could one day lead to a detector capable of locating the minute of gravitational attraction of a particle that we cannot see or feel.

The detector is simple in design – imagine a box full of tiny pearls hanging or suspended in the air – but the theory behind its construction comes down to fundamentally rethinking the search for dark matter.

Astronomers first discovered clues of dark matter over a century ago, from observations of how stars moved around the Milky Way. Since then, more evidence has accumulated. Much of it boils down to the fact that on a large scale things in the universe move in ways that the laws of gravity cannot explain. The galaxies are spinning so fast that they should fly away; similarly, galaxy clusters do not move according to our current understanding of gravity. Other evidence comes from how galaxies bend light around them and how the cosmic microwave background (leftover light from the Big Bang) radiates energy.

This all adds up to the fact that the universe is expected to have a lot more mass than we can see. Visible matter is about 5% of the mass of the universe – dark matter is expected to be about five times as much.

But where this mass comes from is a very open question. Physicists have come up with many theories about dark matter, such as a class of new particle known as low interacting massive particles, or WIMPs. For years, WIMPs have been one of the main candidates for dark matter, and physicists have designed elaborate experiments to catch them. These included giant pools of liquid xenon, intended to emit a flash of light when a WIMP passed.

But, almost 15 years later, physicists are still waiting for this flash. And a number of alternative theories for dark matter – whether it comes from theoretical particles called axions, or primordial black holes, or just that our understanding of gravity is wrong – have also failed to provide concrete information.

This is largely the reason why Carney proposes to reduce research to the basic view that dark matter must have mass.

“It’s the simplest approach, actually,” he says. “Literally the only thing you know about it is that it gravitates; it attracts normal matter by gravity.

Their proposed design looks something like a wind chime, according to Carney. A billion tiny sensors would be suspended motionless in an enclosed space, monitored by an extremely precise array of lasers capable of measuring movements of less than a fraction of the diameter of a proton.

Carney is part of the aptly named Windchime Collaboration, a newly formed group of 19 scientists from various institutions dedicated to exploring the potential of a gravitational dark matter detector.

The specifics of the detector are still somewhat unresolved. The sensors can be suspended from thin ropes or be lifted by magnets. Or, they can use accelerometers, similar to those on our phones but much more sensitive, to monitor changes in position.

Because we know that dark matter gravitates, any dark matter particle passing through would exert a tiny gravitational pull on the sensors, shaking them in a recognizable way. Carney compares dark matter to the wind that makes the bars of a wind chime vibrate.

But if dark matter is wind, catching it would be like detecting a sigh in the middle of a hurricane. Passing cars, footsteps, actual gusts of wind – they all shook all the sensors too, making it extremely difficult to detect a passing tiny particle.

Because of this, gravity wouldn’t be anyone’s first choice for finding dark matter, said Rafael Lang, a physicist at Purdue and another member of the collaboration.

“Oh, that’s a horrible way, because gravity is so low,” he said. “It’s incredibly difficult. It’s really, really bad. Everything else is better than gravity.

Still, Lang said, the gravitational detector intrigued him more than almost any other dark matter project he has seen, enough to overcome his reservations about the fundamental flaws in using gravity to search for it.

“He thinks big,” Lang said. “It’s going to be very difficult, but I think it’s very, very exciting.

Scientists are partly following a lead drawn by another experiment, the LIGO collaboration which first gravitational waves detected in 2015. This detector also relies on very precise measurements of objects for its observations. Lasers bouncing between mirrors track their position with extraordinary precision, enough to detect the minute stretch and contraction of space-time that occurs when a gravitational wave propagates.

LIGO, Lang explained, has shown that it is possible to make the kind of ultra-precise measurements needed to operate the proposed detector. This experiment must also account for all kinds of potentially disruptive noises, including ocean waves, seismic activity, and even gas molecules bouncing off mirrors. Despite all this, LIGO is able to keep mirrors stable enough to detect movements smaller than 1 / 10,000 of the diameter of a proton.

The Windchime Collaboration detector needs to be even more precise. The detector should be so precise that even quantum fluctuations, those caused at very small scales by the fundamental uncertainty of the position of a subatomic particle, could disrupt the sensitivity of the detectors, as Carney details in a recent article in Physical Review D. Quantum noise is also a factor at LIGO, and the experiment has come up with several ways to deal with it, including using a form of light that has been manipulated to muffle quantum fluctuations. But to be even more specific, Carney said, it will take years, if not decades, of extra work.

Currently, the Windchime collaboration is in the early stages of building a simple prototype of the detector. This first proof of concept should be sensitive enough, Carney thinks, to perhaps smell a passing bowling ball. Later versions of the detector will dramatically increase sensitivity, moving from the realm of human recreation to subatomic particles and beyond.

Even if the detector is built, its search may not reveal anything at all. Potential dark matter candidates have masses spanning around 90 orders of magnitude, a huge band that covers everything from subatomic particles to stars. Their detector will be able to search for particles whose masses cover only two or three orders of magnitude centered around one hundred thousandth of a gram.

Still, this lineup covers a few different explanations offered for dark matter, including the fancy name dark quark nuggets or the remnants of primordial black holes passing through the detector.

Quark nuggets or not, a wind chime detecting dark matter would be an entirely new type of experiment for scientists, offering the tantalizing promise of new discoveries.

“Until last year, no one dreamed of such a device,” Lang said. “And now we’re starting to build it.”

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Nathaniel Scharping is a science writer from Milwaukee. Follow him on twitter @NathanielScharp.