U of A particle physicist, Marie-Cécile Piro, spends most of her days over two kilometres underground. Over five Empire State Buildings’ worth of earth, rock, and dirt separates her from the rest of civilization. There, deep in the earth’s crust, Piro is in search of what many believe is the glue that holds the world’s billions of galaxies together: dark matter.
Despite having permeated throughout popular culture, from classic video-games like Metroid to the popular science fiction series Star Trek, the truth is, researchers still aren’t sure dark matter exists. Across the globe, physicists like Piro are racing to be the first ones to find it. At the SNO (Sudbury Neutrino Observatory) underground laboratory in Sudbury, Ontario, Piro is hedging her bets on three of the world’s largest dark matter detectors. Over the next decade, she will be patiently waiting for the rare chance that a dark matter particle collides with one of her machines, sparking the faintest echo of light and sound that will prove its existence.
Our lives are occupied by different types of matter, from the air we breathe to the electrons that power our technologies. But across the cosmos, these things that physicists can feel, touch, and observe, amount to only 15 per cent of all matter in the universe. The rest is dark matter, both invisible and untouchable.
“This is a new form of matter,” Piro said. “It’s not composed of the electrons, protons, or neutrons you learn about in high school. There’s nothing to compare it to, at least not yet.”
The hunt for dark matter didn’t begin until the 1970s, when American astronomer, Vera Rubin, proved that the stars and planets orbiting our neighboring galaxies were moving at speeds hundreds of times faster than predicted. Like swinging a yo-yo in a circle, the faster you spin, the harder it becomes to hold on to. In space, the string holding the stars and planets in their orbits is the immense gravity created by the weight of the galaxies themselves.
But with the stars and planets spinning at over 100 kilometres a second, this string should have snapped, with stars and planets thrown into deep space. Physicists believe it’s dark matter that’s creating a force strong enough to hold the galaxies together. Many astronomers also believe it’s this same force that catalyzed the original formation of galaxies after the Big Bang. By pulling dust, gas, and other ordinary matter together, stars, moons, and planets began to form.
Without dark matter, the Milky Way – our own home galaxy – likely wouldn’t exist, and the universe would have no signs of life at all.
But unlike regular matter, dark matter doesn’t reflect, emit, or absorb light. To map out its location, Piro says astronomers have to look at the way light behaves as it moves around it. Because of the immense gravitational forces dark matter creates, the space around it becomes deformed, and light passing through becomes distorted – similar to how images can bend when examined through a magnifying glass.
By measuring this distortion, astronomers have mapped out where dark matter might be located across millions of galaxies. But knowing where it is hasn’t made figuring out what it’s made of any easier. Despite numerous global efforts, the particles that make up dark matter remains as elusive as ever. Piro describes it as seeing a shadow, but not being able to tell what casts it.
“We’re trying to find something we can’t see,” Piro said. “You just know something is there, but you don’t know what it can be.”
The prevailing theory is that dark matter is made of WIMPs, or weakly interacting massive particles. They’re “massive” because they create so much gravity, but are “weakly interacting” because they can’t usually be touched by regular matter or even light. As the earth travels around the Milky Way, dark matter particles fly through our planet and our bodies every second, but are both unseen and unfelt because they don’t interact well with the regular matter that makes us up.
Piro says this enigmatic property of dark matter can be best observed by watching the Bullet Cluster – two colliding galaxies 3.7 billion light years away. As the regular matter like gas and dust collide, they mixed and ignited into a firework display of energy and light. But when cosmologists looked at the moving dark matter, they saw most of the mass passing through the cluster unfazed.
To detect the undetectable, particle physicists like Piro need their machines to reach an almost unimaginable level of sensitivity, where even trace levels of radioactive uranium or thorium that are present in all our day to day objects can accidentally trigger the sensors. To these detectors, the earth’s surface is an incredibly noisy place.
“We’re surrounded by particles,” Piro explained. “Particles coming from cosmic rays, from the sun, from the atmosphere, even from the impurities in the materials we use to build the machines. All of this can trigger our detector.”
To protect her work from this constant pattern of regular matter, Piro performs her experiments in the SNO underground laboratory located in the subterranean Creighton Nickel Mine. It’s the second deepest underground research facility in the world and one of only a handful that are suitable for dark matter research.
To reach SNOLAB, Piro starts by gearing up into mining coveralls, water-proof boots, and a hard hat before boarding an open cage elevator. Piro and several dozen others are then pulled into a still operational nickel mine at up to 40 kilometres per hour, about three to four times faster than a standard elevator. While adjusting to the much higher atmospheric pressure of the underground tunnels, she then walks 1.8 kilometres in 30-degree Celsius heat before reaching the laboratory entrance.
Down there, Piro is working on three different dark matter detectors.
The first one is the DEAP-3600, a giant acrylic sphere filled with 3600 kilograms of liquid argon chilled to -188 degrees Celsius, a temperature too cold for even modern spacesuits to withstand. The second is the PICO-500. While still under construction, the PICO looks more like a traditional water tank, but is instead filled with super-heated fluoropropane. And the third is NEWS-G, which uses a blend of gaseous neon and methane.
To understand how the detectors work, imagine being a batter at a baseball game. Similar to striking a baseball, when a dark matter particle collides with a molecule of liquid or gas in the detector, a sound can be heard. In a typical game, you can usually see the ball coming. But when you’re dealing with invisible dark matter, it’s more like playing a game blindfolded. Chances are you’re going to miss. But if you let that detector run every day for a couple years, you might just get lucky.
“Dark matter is everywhere,” Piro said. “It’s just that it interacts very weakly with regular matter, so we’re expecting only one to three hits a year.”
By working on multiple detectors, Piro can maximize her chances of finding dark matter. The different liquid or gas in each machine also gives each one a different competitive edge. NEWS-G for instance is more sensitive to lightweight WIMP particles, while PICO is better at weeding out erroneous signals like gamma rays which can occasionally slip through the kilometers of rock above the lab.
She hopes all three will eventually detect a signal of a similar size and weight. That’s when she’ll know she’s found it.
Piro began searching for dark matter as a college student in her twenties when she was first recruited onto the PICASSO project, the PICO project’s earlier incarnation. Now over a decade later, Piro believes we’re closer than ever to unmasking the dark matter enigma. Within the next ten years Piro hopes to have an answer. If dark matter is made of WIMPs, we’ll find it. If not, then we might have spent the last 20 years barking up the wrong tree.
“Either we detect it or we don’t,” Piro said. “There’s nothing else we can do.”
That’s not to say the hunt will end, but rather physicists might have to reevaluate what they’re hunting for. In the world of particle physics, Piro says anything is possible. Instead of a particle, dark matter may work more like a vector field, similar to how magnetic fields function. Or perhaps it’s a particle with unconventional traits, making it hard to measure through collisions.
For instance, in solid state physics where atoms are cooled to temperatures nearing absolute zero, Piro says molecules can begin to move and act in mysterious ways. Even “frozen” solids can start to behave like liquids.
Exploring these possibilities will require decades more research and a whole new generation of detectors. But regardless of what we find, Piro says she’ll never stop searching.“What I’m trying to do is to solve the mystery of the universe and our story just isn’t complete without it.”