A wind chime-based dark matter detector seems odd enough to work

Dark matter physicists may have one of the most frustrating jobs in science. Their work deals with something that, according to almost all models of the universe, must exist. But we have never found any direct evidence for dark matter. Where other scientists can capture their subjects in a laboratory and perform experiments on it, dark matter scholars have nothing more than a tantalizing array 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 studying dark matter he could also be forgiven for feeling a little more anxious lately. They’ve done a series of costly experiments aimed at finding some of the leading dark matter candidates he showed up empty-handed.

“It’s kind of an open season now,” said Daniel Carney, a theoretical physicist at the University of Maryland, the National Institute of Standards and Technology and Fermilab. “Physicists are really trying to think of new ways to search for dark matter and new types of dark matter that might be around.”

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

As simple as it may seem, it’s an approach that has never been attempted before, largely because the design of such an experiment involves such exquisite calibrations that it seems almost unlikely. But Carney and a small group of scientists have begun work on a prototype that they believe could someday lead to a detector that can detect the tiny gravitational pull of a particle that we can neither see nor hear.

The detector has a simple design – imagine a box full of tiny beads hanging or suspended in mid-air – but the theory behind its construction amounts to a fundamental rethinking of the search for dark matter.

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

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

But where that mass comes from is an open question. Physicists have proposed numerous theories of dark matter, such as a class of new particles known as weakly interacting massive particles or WIMPs. For years, WIMPs have been a prime candidate for dark matter, and physicists have devised elaborate experiments to capture them. These included giant pools of liquid xenon, meant to emit a flash of light if a WIMP passed.

But, nearly 15 years later, physicists are still waiting for that flash. And a series of alternative theories for dark matter, which comes from theoretical particles called axions, either from primeval black holes, or simply that our understanding of gravity is wrong – they failed to provide any concrete insights.

This is largely why Carney proposes to bring research back to the basic intuition that dark matter must have mass.

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

Their proposed project looks like something like a wind chime, according to Carney. A billion tiny sensors would remain motionless in an enclosed space, monitored by an extremely precise network of lasers capable of measuring motions less than a fraction of a proton’s diameter.

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 detector specs are still a little suspended. Sensors could be hung on thin strings or levitated by magnets. Or they could 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 particles passing through would exert a small gravitational pull on the sensors, causing them to oscillate in a recognizable way. Carney compares dark matter to the wind that makes the bars of a music box vibrate, making them vibrate.

But if the dark matter is wind, capturing it would be like detecting a sigh in the middle of a hurricane. Passing cars, footsteps, actual gusts of wind – all would have made the sensors wobble too, making it extremely difficult to spot the passage of a tiny particle.

For this reason, gravity wouldn’t be anyone’s first choice when it comes to finding dark matter, said Rafael Lang, a Purdue physicist and another member of the collaboration.

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

However, 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 of using gravity to search for it.

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

Scientists are partly following a path set by another experiment, the LIGO collaboration first detected gravitational waves in 2015. That detector also relies on very precise measurements of objects for its observations. Lasers that bounce back and forth between mirrors track their position with extraordinary precision, enough to detect the minute stretch and contraction of spacetime that occurs when a gravitational wave ripples.

LIGO, explained Lang, has shown that it is possible to perform the kind of ultra-precise measurements necessary for the operation of the proposed detector. That experiment must also take into account all kinds of potentially destructive noise, including ocean waves, seismic activity, and even gas molecules bouncing off mirrors. Despite everything, LIGO is able to keep the mirrors still enough to detect movements less than 1 / 10,000 of the diameter of a proton.

The Windchime collaboration tracker should be even more accurate. The detector should be so precise that even quantum fluctuations, those caused on very small scales by fundamental uncertainty in the position of a subatomic particle, could cancel out the detector’s sensitivity, as Carney describes in a recent article in Physical Review D. Quantum noise is also a factor in LIGO, and the experiment devised a few ways to address it, including using a form of light that has been manipulated to quell quantum fluctuations. But to be even more specific, Carney said, it will take years or even decades of further work.

At the moment, the collaboration with Windchime is in the early stages of building a simple prototype of the detector. This first demonstration of the concept should be sensitive enough, Carney thinks, to perhaps sense the passage of a bowling ball. Subsequent versions of the detector will greatly increase sensitivity, moving from the realm of human leisure to subatomic particles and beyond.

Even if the detector is built, its search may reveal nothing. Potential candidates for dark matter have masses that span about 90 orders of magnitude, a huge band that covers everything from subatomic particles to stars. Their detector will be able to look for particles with masses covering only two or three orders of magnitude centered around one hundred thousandth of a gram.

However, that range covers a few different proposed explanations for dark matter, including those with the whimsical name dark quark nuggets or the remains 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 had ever dreamed of such a device,” said Lang. “And now we’re starting to build it.”

Nathaniel Scharping is a Milwaukee-based science writer. Follow him on Twitter @NathanielScharp.