One mile deep in the South Dakota badlands sits a tank holding seven tons of liquefied xenon. Scientists have turned an abandoned mine there into an underground laboratory, watching for something no one has ever seen before: dark matter.
The LUX-ZEPLIN experiment, one of the most sensitive cosmic detectors ever built, may have caught a first glimpse of it.
“I’ve been looking for dark matter interactions for the better part of 40 years now,” said Richard Gaitskill, Hazard Professor of Physics at Brown University. “So to finally be sort of breaking open the answer to that question — what is the dominant mass in the universe? — it’s incredibly exciting!”
The scientists behind the experiment presented their results at an astrophysics conference in Japan on Sept. 1.
Gaitskill, one of the project's co-founders and lead researchers, isn't using the word "discovery" just yet.
"Discovery in science is a very challenging endeavor," Gaitskill said. He said the team has spent the past three years double-checking the data, looking for other possible explanations.
How certain are they?
“About one in 200,” he said.
In other words, there’s a 199 out of 200 chance the detector caught a glimpse of dark matter. One out of 200, it’s an anomaly.
The problem he is trying to solve starts with what astronomers know about the Milky Way.
Our solar system is at the outer edge of the galaxy, about 20,000 light years from the center. Here at the edge, we are hurtling through space at roughly half a million miles an hour. By the ordinary rules of gravity, that's too fast. The stars and gas we can actually see don't add up to enough mass to hold the Milky Way together.
According to the math, our solar system should be flung off into oblivion. But it isn’t. Some mysterious force we can’t see is holding it together. Physicists have spent nearly a century trying to explain this. They’ve narrowed it down to two main explanations.
Either the theory of gravity itself is wrong (an idea known as Modified Newtonian Dynamics) or the universe contains far more matter than telescopes can detect. Gaitskill and most of his colleagues favor the second explanation.
“We gave that component of matter the name ‘dark matter’ because we can’t see it,” he said. It's invisible to every telescope ever built.
Scientists believe dark matter makes up most of the mass in the universe — about 95% of it. If they were able to detect dark matter, that would be a very big deal.
LUX-ZEPLIN is one of a handful of international experiments focused on this problem. It’s a scientific inquiry straight out of Indiana Jones.The quest for dark matter led to an abandoned mine outside Lead, South Dakota. The old Homestake Mine dates back to the Gold Rush.
Deep underground, the researchers are able to shield the detector from cosmic radiation that would otherwise drown out their experiment.
"We have a mile of rock to shield us from background noise," Gaitskill said.
At the bottom sits the detector: a vat of liquefied xenon. Seven tons of it, chilled and pressurized, ringed on each end by an array of light sensors assembled at Brown.
Think of the sensors as cosmic paparazzi, waiting in the dark for a single flash.
Xenon behaves a bit like its lighter cousin, neon, the gas that glows in old diner signs. Even the faintest brush from a passing particle can ignite a burst of light.
If dark matter really is streaming through the Earth, through this lab, through all of us, without our ever noticing, a xenon atom is one of the few things sensitive enough to catch it colliding with something. That something could be what theoretical physicists whimsically call a WIMP, a Weakly Interacting Massive Particle.
That's the theory, anyway.
Whether LUX-ZEPLIN has actually recorded one of those collisions, or just recorded some background noise, is the question Gaitskill and his team are still working through. Physics sets an unusually high bar for discoveries. Gaitskill's team isn't there yet.
The discovery announced this month may be the most promising indication yet of dark matter’s existence.
“This is why people are very excited within the field,” Gaitskill said. “The way it always starts, in terms of getting a new explanation or verifying a new model, it has to start with a single event.”
The scientists hope this may be it. If so, it could settle one of the oldest open questions in physics: what, exactly, is holding our universe together?
