Most current efforts in the hunt for dark matter particles spend serious resources building detectors, with the aim of directly snagging WIMPs.
The mechanics of WIMP detectors tap into calculations that Freese cooked up in the 1980s. Her work predicted ways that WIMPs would scatter in theoretical collisions with atomic matter. By her estimate, one in ten billion WIMPs traveling through Earth would strike a detector nucleus so it leaves some sign of its existence. Dozens of teams of physicists have built such detectors with government and philanthropic support based on Freese’s predictions, which promise a deeper understanding of the universe, along with an abysmally low probability of success. In other words, you can look for them—but they’re going to be really hard to find.
Yet teams of scientists are holding out on vanishingly low odds of ever detecting WIMPs.
Underground and in the Dark
“If I didn’t have hope, I’d be working on something else,” insists Wolfgang Lorenzon, a professor in LSA’s Department of Physics.
Lorenzon first got involved in the hunt for dark matter particles with the PandaX experiment in China. “I think it’s probably the best place in the world to do a dark matter experiment, because it’s covered by 2.5 kilometers of marble,” he says. It’s the deepest laboratory in the world by a wide margin, though all WIMP detectors lie low underground, often buried beneath a mountain or stashed in a tunnel.
“All of these experiments have to go deep underground to get away from cosmic rays that otherwise would be bombarding the detector constantly,” says Freese. The mountain topping PandaX, in particular, has an almost crystalline marble structure that “expels a lot of the impurities that you don’t want,” says Lorenzon. Still, WIMPs are teeny-tiny and hard to come by. Interference by other small particles presents a confounding challenge, he says, “that plagues you all the time.”
Lorenzon and his lab now work with the LZ experiment, built 1,480 meters deep in an abandoned South Dakota gold mine. They’re building a device that will remove radon contaminants released by the experiment itself, including radon from the welded stainless steel tubing that transports gases through the detector.
A handful of dark matter detectors have some features in common. To try to catch WIMP particles, some research teams fill a giant tank with hundreds of pounds of liquid xenon, which nests within an even larger tank of ultrapure water to further shield against contaminating particles.
Harvesting xenon involves distilling it from the air as a liquid, keeping in mind that radioactive krypton also lurks there as another contaminant—a lingering residue in the atmosphere from historic nuclear weapon tests.
Xenon should work better than many other elements in WIMP detectors because its nucleus is so large; it offers the biggest practical target for the putative dark matter particle. If a WIMP strikes a xenon nucleus, the collision should produce a tiny spark of light that the detector then magnifies as a signal we can see.
But the hunt for dark matter particles, involving dozens of detectors internationally, has yielded nothing for decades. The one exception comes from an experiment called DAMA, housed beneath an Italian mountain. The detector has caught consistent WIMP hits for more than 15 years, but few in the scientific community accept the data. For one thing, instead of using xenon, the DAMA experiment operates with a proprietary crystal in its detector. Their exclusive access to the material makes comparison difficult among experiments — not to mention that it lends an unusual secrecy to their methods —and people wonder whether their results just reflect particle contamination. It doesn’t help that the DAMA team refuses to release its data.
Researchers continue building new detectors all over the world, some with similar crystal materials as DAMA, so they can validate DAMA’s results. Maybe soon, those sole signs of WIMPs may start to make more sense. But with all other detectors coming up empty for decades, how long will researchers look for WIMPs before they give up on them as the building blocks of dark matter?
The Gravity of the Situation
“The disquieting alternative is that a paradigm shift is required to make sense of the data,” Freese admits in her book, The Cosmic Cocktail. “Perhaps an entirely different way of looking at the world will replace the need for these invisible pieces of the universe.”
In other words, what if all these physicists excited over WIMPs turn out to be a bit too excited about the elusive particle?
To be sure, most proposals that explain the wonky speeds of celestial objects in outer orbits trace back to reasonable theory. But some scientists speculate: Could dark matter exist as clouds of several different particles at once? Does it act less like a particle and more like a wave? What if dark matter isn’t matter at all?
Here’s another possibility: What if gravity just works differently at the enormous scale of the universe? Isaac Newton’s laws first captured what we know about gravity, motion, inertia, and momentum. We take for granted the physics we experience every day, and now we even accept the revolutionary idea that those familiar forces don’t apply at the tiniest quantum scales. Would it be so discomfiting for Newton’s laws to transform once more at the opposite end of the range, at sizes that span galaxies?
Modified Newtonian Dynamics (MOND) suggests exactly that: Maybe we and Newton don’t fully understand gravity.