5 Questions About Dark Matter — the Universe’s Biggest Mystery
UT Austin theoretical astrophysicist Katherine Freese shares what we know (and don’t) about the invisible substance shaping the cosmos.
Costumes, spooky stories, ghosts and spirits — Halloween and mystery go hand in hand. It’s only fitting that Oct. 31 is also an international celebration of one invisible substance that scientists have been trying to understand for decades: dark matter.
Ordinary matter (any substance that has mass and occupies space) is the stuff that makes up stars, planets and people. Yet it accounts for only about 5% of the universe’s total matter. About 25% is dark matter, which helps hold galaxies together with its gravitational pull, like an invisible glue. The remaining 70% is dark energy — the name scientists give what’s causing the universe to expand at an accelerating rate over time (another scientific mystery that UT’s McDonald Observatory is leading the charge to uncover).
Like ordinary matter, dark matter has mass and takes up space. But what exactly is dark matter? What is dark energy? And what makes up the universe? These questions are central to decades of research conducted by Katherine Freese, director of the Weinberg Institute for Theoretical Physics and Kodosky Chair of Physics at The University of Texas at Austin.
Recently, the underground LUX-ZEPLIN experiment recorded a possible detection of dark matter: a single particle interaction that scientists find difficult to explain. Scott Kravitz, an assistant professor of physics at UT, served as the experiment’s analysis coordinator during most of the search. The finding isn’t enough to claim a discovery, but it adds momentum to a search Freese helped shape through pioneering work on how underground detectors could spot dark matter.
In honor of Dark Matter Day on Oct. 31, we asked Freese five questions about the mysterious matter.
Q: Your book, “The Cosmic Cocktail: Three Parts Dark Matter,” explores two of the universe’s biggest mysteries: dark matter and dark energy. Can you explain the difference between the two?
Matter feels gravitational attraction. Whether ordinary matter or dark matter, gravity works on both in a normal way. Dark energy, on the other hand, is causing the expansion of our universe to accelerate, meaning it has some kind of repulsive behavior that we don’t understand the origin of.
Q: Dark matter sounds kind of ghost-like: something we can’t see, but it affects the objects around it. How do we know it’s there, and how would our universe look different if dark matter didn’t exist?
The dark matter problem is now 90 years old, dating back to the 1930s. Scientists looked at a cluster made up of hundreds of galaxies and observed their motion, far from the center of the cluster. Scientists found that a galaxy was moving in a really rapid orbit in the cluster, so fast that it should have broken free because the gravitational pull of all the other visible galaxies wasn’t enough to keep it on its orbit. The scientists postulated that there must be additional matter in the cluster — dark matter — providing the gravity to keep the galaxy within the cluster. They called it “dark matter” because it’s dark; it neither emits nor absorbs light.
We would not exist without dark matter, because it is the gravitational attraction of dark matter that forms the initial proto-galaxies — smaller structures that later merged to make galaxies. Ordinary matter falls into those structures after they are formed by the dark matter.
According to numerical simulations, you follow these dark matter particles as they fall into each other, which first make small objects, maybe Earth-size objects, and then merge to make larger ones. And in the end, you get a filamentary structure of dark matter, and it’s at the nodes or intersections of these filaments where the galaxies are sitting. And these simulations match reality.
Q: In your recent “What’s Up With That?” interview with Neil deGrasse Tyson, you discussed experiments that search for dark matter deep underground. How do you catch something so elusive — and how would scientists know if they found it?
Believe it or not, I worked on this problem 40 years ago. We made predictions for the well-motivated possibility that dark matter is made of a particular type of particle, called the Weakly Interacting Massive Particles (WIMPs). Dark matter particles interact with ordinary particles and among themselves with “the weak force,” which is the weakest of the four forces of nature. Billions go through your body every second.
We created calculations to better understand how WIMPs would interact with ordinary matter. Then we figured out a detector would need to be built a kilometer below ground. Otherwise, electromagnetic interactions would swamp the detector.
The LUX-ZEPLIN experiment is one of several detectors that got built underground in an abandoned gold mine in South Dakota. UT’s professor Scott Kravitz is part of that experiment, where they recently found — for the first time — an event in the detector that may be due to dark matter. What they found is not explained by any of the expected background signals like junk signals or electron recoils.
With my student, Dionysios Theodosopoulos, for the first time in my life I wrote a paper in 24 hours, proposing this could be a Higgsino. That’s the supersymmetric partner of the Higgs boson, the famous particle that was the last missing piece of the Standard Model of Particle Physics, found in 2012, and it gives mass to the other particles. All the particles that we know about in the Standard Model of Particle Physics could have supersymmetric partners. We don’t know exactly its nature, but one of the possibilities is a Higgsino, which was a perfect fit to the data, so we’re pretty excited.
Q: Why is it important to figure out what dark matter is? What would we understand about our universe that we don’t understand today?
What makes us very special as humans is our curiosity, wanting to figure out what’s out there, understand the unknown and explore. We don’t know what 95% of the universe is made of. Learning about that, through fundamental research, can be a process that also leads to many different practical applications someone never would have predicted.
We got the World Wide Web because of earlier experiments with particle accelerators, as one example.
Q: You’ve spent decades searching for something we can’t see. What keeps you excited about the hunt for dark matter?
What I find most fun is having ideas, figuring things out, and noticing something that nobody else noticed before. That’s pretty exciting.
I’m also always inventing new stuff — that’s what I really love the most. Something new is paleo detectors, a joint effort with others where we realized you could dig up little rocks from deep underground that had been sitting around for a billion years, collecting tracks from dark matter particles hitting them. This idea would replace the large volumes of standard detectors (the LUX-ZEPLIN detector is made of 7 tons of xenon) with long periods of collection time.
Paleo detectors are a worldwide effort happening now, and my students are working on that. Ironically, the paleo detector work was what made us think about the Higgsino. It’s the synergy between all these different ideas that’s really fun.
