It’s a 20-minute elevator ride down to the Sanford Underground Research Facility in Lead, South Dakota, where the former Homestake gold mine will soon hold the LZ Dark Matter Experiment in its cavernous belly. Most scientists, wedged in shoulder-to-shoulder, don’t mind the slow descent through 4,850 feet of dirt and rock — any faster, and they might pass out the way researchers do up at SNOLAB (another underground physics lab in Ontario), where every month or so, someone faints on the way down.
“At first you’re like, oh my gosh, I’m under-ground, there’s a lot of rock above me,” says assistant professor and astrophysicist Kimberly Palladino, who’s helping head up the team of nine UW-Madison scientists and engineers tapped to work on the LZ experiment, a collaborative U.S. Department of Energy-funded project comprised of more than 200 of the best minds from 31 institutions around the world. “After a week, it’s just where your office is.”
With the LZ experiment, researchers are competing in a global race to finally “see” dark matter — arguably the most captivating piece of the still-unfolding puzzle of how our universe was formed. Physicists theorize that matter as we know it — walls, floors, your feet on the ground — accounts for only five percent of the total floating around and through us at all times. The other 95 percent is composed of either dark energy or dark matter, thus far only visible by its gravitational effect on what we can see — sort of like the way negative space helps define the shapes in a painting.
Einstein first predicted the dark energy phenomenon within his general theory of relativity in 1916, calling it the “cosmological constant.” But he predicated his ‘constant’ on the notion that the universe is static. When he learned the universe was actually moving, Einstein called the cosmological constant the “biggest blunder of my life”. However, Einstein was on to something, and later scientists picked up the thread.
In the 1930s, Swiss astrophysicist Fritz Zwicky proposed the concept of dark matter to explain a “mystery mass” that had to be helping hold the luminous parts of the universe together. But it wasn’t until the 1970s that American astrophysicist Vera Rubin was credited with confirming dark matter as the impetus for spiral galaxies spinning faster than scientists had previously assumed from their studies of visible stars.
Newer observations have confirmed that this dark matter knitting galaxies and galaxy clusters together cannot be made up of ordinary atoms. All of this, plus the astounding revelation that the overall expansion of the universe is even speeding up, have brought many researchers to double down on efforts to define the enigmatic dual mysteries of dark energy and dark matter they’re certain not only exist, but explain every-thing. They just have to prove they’re there.
“Even physicists don’t know what this stuff is. I’ve never seen it,” laughs physics professor Duncan Carlsmith (left). And yet, he may devote the rest of his career to trying.
Two years ago, Carlsmith switched from particle physics to astrophysics because of this project, which aims to detect dark matter using 10 metric tonnes of liquid xenon in a cylinder about the height of a human body, buried deep beneath the ground so as to avoid interference from cosmic rays and anything radioactive, which is in everything, even fingerprints. That inner cylinder is insulated inside a larger one that keeps the liquid xenon cool, and scientists hope that dark matter particles, whatever they are (theoretically an as-of-yet uncategorized form of Weakly Interacting Massive Particle, or WIMP) will interact with the xenon in a measurable way.
With the LZ experiment, researchers are competing in a global race to finally ‘see’ dark matter — arguably the most captivating piece of the still-unfolding puzzle of how our universe was formed.
Palladino explains it like this: If xenon atoms get hit, some become ionized (releasing an electron) and others excited (forming a molecule with a mate). Then, as they relax, they give off ultraviolet light. Palladino imagines dark matter like a thermal wind of particles scattering like billiard balls. Though a direct hit is probably rare, hopefully a few xenon atoms will catch a glancing, measurable blow.
Palladino worked for three years at Stanford University’s National Accelerator Laboratory on the LUX detector (a predecessor to the LZ detector) before she was recruited by Carlsmith to LZ and UW-Madison.
“As an undergraduate, I studied the Cosmic Microwave Background, and I thought, well, they’re just measuring the same thing better and better. I want to do something, you know, crazy,” says Palladino (left), who moved to Madison in the summer of 2016 and had a baby shortly thereafter with the husband she first met at the Johns Hopkins Center for Talented Youth in upstate New York when they were kids. “I want to be involved in finding something for the first time.”
Carlsmith’s motivation is similar: “This is an outstanding mystery, and we know it’s there,” he says, reflecting that when he was a grad student, the Quark Model, foundational to his particle physics work today, was only a hypothetical “nobody” believed. “When I look back in history and go, you know, what those people were doing, playing around? It changed the world,” says Carlsmith. “I want to be a part of that. I want to contribute.”
Dark matter is just mysterious and compelling enough to attract the likes of Carlsmith and Palladino, as well as an influx of student interest. But a long-term experiment like LZ will likely run at least 15 years in one form or another; the LZ detector itself won’t even finish construction until 2020. Though the Department of Energy refers to this schedule as a “fast track,” for the professors and researchers that are in it for the long haul, it can be disconcerting to think that decades of work may lead to discoveries that unfold beyond their lifetimes.
But that’s okay, says Palladino, who was only 10 years old when she first announced to her family that she would be a physicist. “You should question how the world works and be curious about it,” she says.
Should the LZ experiment work, the celebration will be thrilling — and short-lived. Findings demand replication on a larger and larger scale, and discoveries only lead to more questions. And, yet, as Carlsmith points out, when you can understand a thing, you can gain control over it, even put it to good use.
“We understood light,” he says, pointing out the relatively new phenomenon known as electricity, “and then we had lamps. And lasers. And fiber optic communication systems. It’s through understanding physics that all that became possible.”
The search continues
Other experiments, including the Wisconsin IceCube Particle Astrophysics Center projects IceCube, the High-Altitude Water Cherenkov Observatory (HAWC) and Cherenkov Telescope Array (CTA), are searching for the signatures of dark matter annihilation events as independent and indirect methods to investigate the nature of dark matter.
In addition, UW-Madison scientists are working at the Large Hadron Collider, searching for evidence that dark matter is produced during high-energy particle collisions. This combination of efforts provides the best opportunity yet for uncovering more about the nature of dark matter, and with it the evolution and structure of our universe.