The secret to directly detecting dark matter could lie in the wind.
The mysterious substance continues to elude scientists even though it outweighs visible matter in the universe by about 8 to 1. All laboratory attempts to directly detect dark matter, seen only indirectly through the effect its gravity has on the movements of stars and galaxies have failed. unfulfilled
Those attempts have been based on the hope that dark matter has at least some other interaction with ordinary matter in addition to gravity (Serial number: 10/25/16). But a proposed experiment called Windchime, while decades away from completion, will try something new: It will search for dark matter using the only force it is 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 Division of Optical and Molecular Atomic Physics in Orlando, Florida. a breeze, the Windchime detector try to feel a dark matter ‘wind” blowing past Earth as the solar system rotates around the galaxy.
If the Milky Way is primarily a cloud of dark matter, as astronomical measurements suggest, then we should be cruising through it at around 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 notion that a collection of pendulums will swing in the breeze. In the case of backyard wind chimes, they can be metal rods or hanging chimes that tinkle in the moving air. For the dark matter detector, pendulums are sets of tiny, ultra-sensitive detectors that will be pulled by the gravitational forces they feel as chunks of dark matter pass by. Instead of air molecules bouncing off metal bells, the gravitational attraction of the particles that make up the dark matter wind would cause distinctive ripples as it passes through a billion sensors in a box that measures about a meter by side.
While it may seem logical to search for dark matter using gravity, no one has tried it in the nearly 40 years that scientists have been searching for dark matter in the laboratory. That’s because gravity is comparatively a very weak force and difficult to isolate in experiments.
“You’re looking for dark matter to [cause] a gravitational signal on the sensor,” says Carney, of the Lawrence Berkeley National Laboratory in California. “And you just ask. . . Could you see this gravitational signal? When you first estimate, the answer is no. It’s actually going to be unfeasibly difficult.”
That didn’t stop Carney and a small group of colleagues exploring the idea anyway in 2020. “Thirty years ago, it would have been crazy to propose it,” she says. “He’s still a little crazy, but he’s like crazy.”
Since then, the Project Windchime collaboration has grown to include 20 physicists. They have a Windchime prototype built with commercial accelerometers and are using it to develop the software and analysis that will lead to the final version of the detector, but it is a long way from the final design. Carney estimates that it could be a few more decades before sensors good enough to measure the gravity of even heavy dark matter are developed.
Carney bases the timeline on the development of the Laser interferometer Gravitational wave interferometeror LIGO, which was designed to search for gravitational waves from colliding black holes (Serial Number: 02/11/16). When LIGO was first conceived, he says, it was clear that the technology would have to be improved a hundred million times. Decades of development have resulted in an observatory that sees the sky in gravitational waves. With Windchime, “we’re in exactly the same boat,” she says.
Even in its final form, Windchime will only be sensitive to dark matter fragments that are roughly the mass of a fine speck of dust. That’s huge in the spectrum of known particles: more than a million trillion times the mass of a proton.
“There are a variety of very interesting dark matter candidates in [that scale] that are definitely worth looking for… including the primordial black holes of the early universe,” says Katherine Freese, a physicist at the University of Michigan in Ann Arbor who is not part of the Windchime collaboration. Black holes slowly evaporate, leaking mass back into space, she points out, which could leave many relics formed shortly after the Big Bang in the mass that Windchime could detect.
But if it never detects anything at all, the experiment still stands out from other dark matter detection schemes, 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 turns up nothing, says Hooper, “the amazing thing about [Windchime] … is that regardless of anything else you know about dark matter particles, they are not in this mass range.” With existing experimentsthe failure to detect anything could be due to wrong guesses about the forces affecting dark matter (Serial number: 7/7/22).
Windchime will be the only experiment imagined so far in which seeing nothing would definitively tell researchers what dark matter is not. However, with a bit of luck, it might discover a wind of tiny black holes, or even more exotic bits of dark matter, flying past as we zoom through the Milky Way.