This is an article from the fall 2017 issue of LSA Magazine. Read more stories from the magazine.
Every year, Mario Mateo brings a trash can lid to Chile and looks at the stars.
At least, the object looks like the round metal lid of a trash can. But it’s a bit stranger than that. The big metal disc — about the size of a snow sled — has been studded with a constellation of holes drilled through the surface. Red, blue, maroon, and teal doodles use tangled lines to connect some of the holes, circle several, and label others with cryptic names and reference numbers. People at the airport might take one look at his gear and mistake Mateo for something other than a tenured professor — given his old T-shirt, beat-up sneakers, bushy white mustache that matches the tufts of hair orbiting his head, and unique luggage — but one of those leaky metal lids hangs on the wall of his office in LSA’s Department of Astronomy.
The “plug plates,” Mateo calls them, have drastically increased the number of stars he can view and measure through a telescope: an unprecedented 256 celestial objects at a time. He and his grad students precisely map the pinpoints of light they want to observe in the Chilean sky, then drill holes at those points on the disc. Braids of fiber optic cables pierce the back of the plate, where Mateo plugs each of the holes with its own wire by hand. With the help of a telescope, the fiber optics collect light from each target star glinting through the atmosphere.
Photograph by Natalie Condon
Over repeated observations, Mateo learns things about the stars he sees —their chemistry, temperature, size, and motion. He gets a sense of how each star moves through space, and how quickly. After tracking thousands of stars over the years, Mateo has seen for himself what some scientists have puzzled over for a while: That something weird is going on.
Stars at the outer edge of small galaxies orbit the galactic center much faster than they should. Galaxies, too, orbit each other, and Mateo finds that distant galaxies also spin more quickly than expected around our own galaxy, the Milky Way. Imagine spinning a yo-yo around your head so fast that the toy snaps off its string and flings away. Peripheral stars move so fast that they actually should detach from their home galaxy like wild projectiles.
But they don’t.
As early as the 1930s, astronomers came to assume that some mysterious thing must grip celestial objects in their outer orbits, keeping stars from busting loose. They gave the hypothetical thing a name: dark matter.
More weird supporting evidence for that idea popped up, and the imaginative explanation took hold: Dark matter fills the universe as invisible mass and skews our view of other objects. About fifteen percent of matter in the universe must be the atomic stuff we know, see, sit on, and eat; the rest is dark matter and doesn’t really interact with us at all, except through gravity. If all that is true, then dark matter originally seeded today’s galaxies, our planet, and the life we know.
Dark matter has become the most convincing explanation to astrophysicists —the thing that almost all of them are searching for, but nobody’s ever found.
Dark Matter in Particular
“It’s 85 percent of all matter,” says theoretical physicist Katherine Freese about dark matter in the universe. “We’ve got to find out what the hell it is, obviously.”
In the past, astronomers found the planet Neptune when the familiar planet Uranus seemed to wobble outside of its expected orbit. Tracing the weird wobble to its cause revealed the once invisible planet Neptune, whose gravity tugs at Uranus in ways we can predict. “Neptune was ‘dark matter’ at one point, right?” says Mateo. “It was causing Uranus to move in a way that nobody could see.”
What if other giant objects in space, too dim to notice with telescopes, have enough gravitational pull to cause the unexpectedly high speeds of outer stars? Freese ran the numbers and nixed the idea. The universe just doesn’t hold enough atomic matter to account for all that gravity.
Now, she’s betting big on theoretical particles. Freese and many others imagine dark matter as a substance made of Weakly Interacting Massive Particles (WIMPs). Depending on the mass of these particles — which we don’t yet know — Freese says that their density on Earth would be about one WIMP per coffee cup. Billions travel from outer space and through every person on Earth every second, but direct hits should happen much less frequently.
In theory, a WIMP can strike the nucleus of a cell or substance with enough oomph to jiggle atoms at the smallest scales. Freese calculates that collisions could happen as rarely as once a month. “For a 150-pound human,” she writes, “the number of hits could be as frequent as one every minute!” No need to call your doctor — experts predict no negative health effects of the hypothetical particle striking your cells.
If dark matter really is matter made of WIMPs, we could catch the particles in a few ways. Massive particle smashers like the Large Hadron Collider might incidentally create a WIMP, but no sign of the mystery particle has shown up yet. Satellites in outer space could detect the byproducts of WIMPs, but while some surveys have announced exciting leads, all have rung as false alarms so far.
You can get a clear view of the Milky Way in the dark sky over Chile, where Mario Mateo looks through his telescope. Some of these stars and galaxies actually should careen out of their orbit, out of control, through outer space. Weirdly, many of them don’t. Courtesy of European Southern Observatory / P. Horálek.
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.
LUX, the enormous underground dark matter experiment housed in an old, abandoned gold mine in South Dakota. LUX soon will be replaced by a beefed-up version: the LZ experiment. Courtesy of Sanford Underground Research Facility / Matthew Kapust.
“I was shocked to see how many things that I just assumed dark matter would be better at, MOND was pretty good at, too,” says astronomy alumnus Stacy McGaugh (Ph.D. 1992). His work takes the mathematical predictions of Newton at the galactic scale and tweaks them just a bit; he can pretty reliably predict what often surprises an observer at a telescope. “There are many, many situations in which those predictions come true,” he says. “Quantitatively, accurately, amazingly.”
Then again, he says, “There are some situations in which they don’t.”
McGaugh regrets the “dark matter” label that early astronomers applied to their strange observations. He thinks the term itself may have set a precedent that blocks us from weighing other options to explain what we see in outer space.
“I realized that it was a linguistic hang-up. We’d called it the ‘dark matter’ problem, and so that framed how we thought about it, in terms of invisible mass,” says McGaugh. “I’m concerned that we’re hung up on something conceptual.”
“If I were some sort of alien being that lived a billion years, and I wanted to travel to a distant galaxy, I’d use MOND to get to that galaxy,” says Mateo. “And MOND would actually get me there! I couldn’t use the dark matter model and actually get from here to there predictably.”
“MOND as it’s written now is not a complete answer,” says McGaugh. “But I think it has to be part of the final solution. We need a satisfactory explanation of why MOND gets the predictions right.”
“And so,” he prompts, “Which thread of evidence are you going to believe?”
Freese believes that these discoveries will happen in her lifetime, and she’s prepared to throw away her own predictions if the data point in another direction.
“We start on faith — that’s called theory,” she says, “and then nature is what it is, and you’ve got to say, ‘Okay.’”
“I refuse to believe that nature’s given us a problem that is impossible to address,” says Mateo. “Physics and astronomy have long progressed without having a good fundamental physical explanation for dark matter, but we’re not getting closer to it, and that’s unusual. That’s what’s weird.”
“If you’re in this kind of business, then that’s something you have to be comfortable with,” Lorenzon says. “Some people may need to have a result at the end of the day, otherwise they don’t feel the work that they’ve done is valuable. I don’t feel that way.”
Whatever the answers that resolve this stubborn conundrum, Freese stays optimistic.
“When there are great advances in fundamental science, it always affects human existence in some way we can’t foresee,” she says. “I would say that’s true here, too.”