The secret to direct detection of dark matter could be blowing in the wind.
The mysterious substance continues to elude scientists despite outweighing visible matter in the universe by about 8 to 1. All laboratory attempts to detect dark matter directly—seen only indirectly through the effects of its gravity on the motions of stars and galaxies—have gone unfulfilled.
These attempts were based on the hope that dark matter has at least one other interaction with ordinary matter besides gravity (SN: 10/25/16). But a proposed experiment called the Windchime, although decades away from its realization, will try something new: it will search for dark matter using the only force it’s guaranteed to feel — gravity.
“The core idea is extremely simple,” says theoretical physicist Daniel Carney, who described the scheme in May at a meeting of the American Physical Society’s Department of Atomic, Molecular, and Optical Physics in Orlando, Florida. Like a wind chime on a porch rattling in with a breeze, the windchime detector would attempt to capture a dark matter “wind” blowing past Earth as the solar system whips around the galaxy.
If the Milky Way is mostly a cloud of dark matter, as astronomical measurements suggest, then we should be sailing through it at about 200 kilometers per second. This creates a dark matter wind, for the same reason you feel a wind when you stick your hand out the window of a moving car.
The windchime detector is based on the idea that a collection of pendulums swings in the wind. In the case of backyard wind chimes, it can be metal sticks or dangling bells that tinkle in moving air. For the dark matter detector, the pendulums are arrays of tiny, ultra-sensitive detectors that are nudged by the gravitational forces they feel as they pass bits of dark matter. Instead of air molecules bouncing off metal bells, the gravitational pull of the particles that make up the dark matter wind would cause distinctive ripples as it blows through about a billion sensors in a box about a meter on a side.
While it might seem logical to use gravity to look for dark matter, in the nearly 40 years that scientists have been studying dark matter in the lab, no one has tried it. This is because gravity is a very weak force in comparison and is difficult to isolate in experiments.
“You are looking for dark matter [cause] a gravitational signal in the sensor,” says Carney of Lawrence Berkeley National Laboratory in California. “And you just ask . . . Could I possibly see this gravitational signal? If this is your first time making the estimate, the answer is no. It’s actually going to be impractically difficult.”
However, that didn’t stop Carney and a small group of colleagues from exploring the idea in 2020 anyway. “Thirty years ago it would have been completely insane to propose this,” he says. “It’s still kind of crazy, but it’s like borderline insanity.”
Collaboration in the Windchime project has since grown to 20 physicists. They have a prototype Windchime built from commercial accelerometers and are using it to develop the software and analysis that will lead to the final version of the detector, but it’s a long way from the final design. Carney estimates that it could take a few more decades to develop sensors good enough to measure gravity even from heavy dark matter.
Carney bases the timeline on the development of the Laser Interferometer Gravitational-Wave Interferometer, or LIGO, designed to search for gravitational waves emanating from colliding black holes (SN: 2/11/16). When LIGO was first conceived, he said, it was clear that the technology needed to be improved a hundred million times over. Decades of development resulted in an observatory that views the sky in gravitational waves. At Windchime, “we’re in exactly the same boat,” he says.
Even in its final form, Windchime will only be sensitive to bits of dark matter, roughly the mass of a fine speck of dust. This is enormous in the spectrum of known particles – more than a million trillion times the mass of a proton.
“There are a number of very interesting candidates for dark matter [that scale] definitely worth looking for…including primordial black holes from the early Universe,” says Katherine Freese, a physicist at the University of Michigan at Ann Arbor who is not part of the Windchime collaboration. Black holes are slowly evaporating and leaking mass back into space, she notes, which could leave behind many relics formed shortly after the Big Bang in the mass that Windchime was able to spot.
But if it doesn’t detect anything at all, the experiment still stands out from other dark matter detection systems, says Dan Hooper, a physicist at Fermilab in Batavia, Illinois, who is also not affiliated with the project. That’s because it would be the first experiment that could completely rule out some types of dark matter.
Even if the experiment doesn’t come up, Hooper says, “The amazing thing about it [Windchime] … is that, regardless of everything else you know about dark matter particles, they are not in that mass range.” With existing experiments, a failure to detect anything may instead be due to erroneous assumptions about the forces acting on Act Dark Matter (SN: 7/7/22).
Windchime will be the only experiment presented so far where seeing nothing would definitely tell researchers what dark matter isn’t. With any luck, however, it could reveal a wind of tiny black holes or even more exotic bits of dark matter sweeping by as we orbit the Milky Way.