Dark matter physicists may have one of the most frustrating jobs in science. Their work deals with something that must, by almost all models of the universe, exist. But we’ve never found any direct evidence for dark matter. Where other scientists can capture their subjects in a lab and perform experiments on it, scholars of dark matter are left with nothing but a tantalising 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 studying dark matter might also be forgiven for feeling a little more anxious lately. A number of expensive experiments meant to find some of the leading candidates for dark matter have turned up empty-handed.
“Now it’s sort of open season,” said Daniel Carney, a theoretical physicist at the University of Maryland, the National Institute of Standards and Technology, and Fermilab. “Physicists are really scrambling to think of new ways to look for dark matter and new types of dark matter that could be around.”
Carney thinks he might have a potential solution. The one thing we do 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, it’s an approach that’s never before been attempted, in large part because designing such an experiment involves calibrations so exquisite they seem almost improbable. But Carney and a small group of scientists have begun work on a prototype they say could one day lead to a detector capable of pinpointing the minute gravitational pull of a particle we can neither see nor feel.
The detector is simple in design — picture a box full of tiny beads hanging or suspended in midair — but the theory behind its construction amounts to a fundamental rethinking of the search for dark matter.
Astronomers first found hints of dark matter over a century ago, from observations of how stars moved around the Milky Way. Since then, more evidence has stacked up. Much of it boils down to the fact that, at large scales, things in the universe move in ways that the laws of gravity can’t account for. Galaxies rotate so quickly they should fly apart; similarly, clusters of galaxies don’t move according to our current understanding of gravity. Other lines of evidence come from the way that galaxies bend light around them, and from how the cosmic microwave background (leftover light from the Big Bang) radiates energy.
Five-sixths of the universe’s stuff seems to be missing, and we just can’t find it. It’s called “dark matter,” and scientists have gone looking for it with some of the world’s largest, most expensive experiments.Read more
It all adds up to the fact that the universe should have a lot more mass than we can see. Visible matter accounts for around 5% of the mass of the universe — dark matter should make up around five times as much.
But where that mass comes from is very much an open question. Physicists have proposed numerous theories for dark matter, such as a class of new particle known as Weakly Interacting Massive Particles, or WIMPs. For years, WIMPs were one of the leading candidates for dark matter, and physicists devised elaborate experiments to catch them. These included giant pools of liquid xenon, meant to give off a flash of light should a WIMP pass through.
But, nearly 15 years later, physicists are still waiting for that flash. And a number of alternative theories for dark matter — that it comes from theoretical particles called axions, or from primordial black holes, or simply that our understanding of gravity is wrong — have also failed to yield any concrete insights.
That’s a large part of the reason why Carney proposes stripping the search back to the basic insight that dark matter has to have mass.
“It’s the simplest approach, actually,” he said. “Literally the only thing you know about it is that it gravitates; it attracts normal matter gravitationally.”
Their proposed design resembles something like a wind chime, according to Carney. A billion tiny sensors would hang motionless in an enclosed space, monitored by an extremely precise network of lasers able to measure movements of 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 specifics of the detector are still somewhat up in the air. The sensors might hang from thin strings or be levitated by magnets. Or, they might use accelerometers, similar to the ones in our phones but far 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, jiggling them in a recognisable way. Carney likens dark matter to the wind that stirs the bars of a wind chime, setting them vibrating.
But if dark matter is the wind, catching it would be akin to detecting a sigh in the middle of a hurricane. Passing cars, footfalls, actual wind gusts — they would all jiggle the sensors, too, making picking out the passage of a tiny particle exceedingly difficult.
For this reason, gravity wouldn’t be anyone’s first choice when it comes to finding dark matter, said Rafael Lang, a physicist at Purdue and another member of the collaboration.
“Oh, it’s a horrible way, because gravity is so weak,” he said. “It’s incredibly difficult. It’s really, really bad. Anything else is better than gravity.”
Still, Lang said, the gravitational detector intrigued him more than almost any other dark matter project he’s seen, enough to overcome his reservations about the fundamental flaws of using gravity to search for it.
“It’s thinking big,” Lang said. “It’s going to be very difficult, but I think it’s very, very exciting.”
The scientists are in part following a trail blazed by another experiment, the LIGO collaboration that first detected gravitational waves in 2015. That detector also relies on very precise measurements of objects for its observations. Lasers bouncing back and forth between mirrors track their position with extraordinary accuracy, enough to detect the minute stretching and contracting of spacetime that occurs when a gravitational wave ripples through.
LIGO, Lang explained, showed that it’s possible to do the kind of ultra-precise measurements necessary for their proposed detector to work. That experiment must also account for all kinds of potentially disruptive noise, including ocean waves, seismic activity, and even molecules of gas bouncing off the mirrors. Despite all that, LIGO is able to keep the mirrors steady enough to pick out movements smaller than 1/10,000th the diameter of a proton.
The Windchime collaboration’s detector would need to be more accurate still. The detector would need to be so precise that even quantum fluctuations, those caused at very small scales by the fundamental uncertainty in the position of a subatomic particle, could throw off the detectors sensitivity, as Carney details in a recent paper in Physical Review D. Quantum noise is a factor at LIGO, too, and the experiment has devised a few ways of dealing with it, including using a form of light that’s been manipulated to quell quantum fluctuations. But to get even more precise, Carney said, will take years or even decades of further work.
At the moment, 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 maybe sense a passing bowling ball. Later versions of the detector will ramp up the sensitivity dramatically, moving from the realm of human leisure to subatomic particles and beyond.
Even if the detector does get built, its search may turn up nothing at all. The potential candidates for dark matter have masses spanning about 90 orders of magnitude, a huge swath 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 a hundred-thousandth of a gram.
Still, that range covers a few different proposed explanations for dark matter, including the whimsically named dark quark nuggets or the remnants of primordial black holes passing through the detector.
Quark nuggets or no, a dark-matter-detecting wind chime would be an entirely new type of experiment for scientists, offering the tantalising promise of new discoveries.
“Until last year, no one had dreamt of such a device,” Lang said. “And now we’re starting to build it.”