Why you should care

Dark matter is one of physics’ greatest mysteries.

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Tracy Slatyer picked up A Brief History of Time, Stephen Hawking’s seminal book on the origins of the universe, after reading a review that called it fascinating yet dense and cited a survey reporting that most people couldn’t get past page 30 or so. Slatyer finished the book in two weeks. She was 12 years old.

Today Slatyer is a theoretical physicist at the Massachusetts Institute of Technology, bringing the same drive and thoroughness to her research on dark matter, a mysterious substance thought to make up approximately 80 percent of the matter in the universe — even if scientists have not been able to directly observe it. That’s why Slatyer hunts for signals of dark matter, meticulously searching often messy telescope data for the signature glow the particles that compose dark matter emit when they annihilate — that is, collide with each other and decay. Her work could bring physicists closer to detecting dark matter signals by better enabling them to tease these signals apart from similar ones produced by stars and other objects in the cosmos.

I want to know what 80 percent of the matter in the universe is.

Tracy Slatyer

Detecting dark matter could transform our understanding of the universe. For decades, physicists have relied on the standard model, a theory describing the fundamental particles of matter. But the theory has “a glaring hole,” says Ryan Foley, an astronomer at the University of California, Santa Cruz: It doesn’t account for dark matter. “Imagine if we had that additional knowledge, how different the laws of nature might be.”

To be sure, efforts to detect dark matter have failed thus far, and “there’s no guarantee” they’ll succeed, says James Buckley, a physicist at Washington University in St. Louis. “They require very hard experimental work.” But if physicists do detect dark matter through their collisions in space, Slatyer “is likely to play an important role,” predicts Glenn Starkman, a theoretical physicist at Case Western Reserve University. “Stars don’t have to be flashy. Sometimes they can just be very reliable … Everyone trusts the solidity of Tracy’s work.” On top of that, she “has a lot of big ideas,” Foley says. “She’s really pushed this field forward.”

Slatyer, 33, is soft-spoken but talks quickly and animatedly. Beneath her quiet demeanor, “there’s a fierceness,” says MIT colleague Jesse Thaler. Growing up mostly in Canberra, Australia, Slatyer read voraciously. As she pored over A Brief History of Time, she realized physics sought to answer hefty questions about the universe largely through math, for which she had a natural knack. After majoring in theoretical physics, she earned a Ph.D. in physics from Harvard, did a postdoc at the Institute for Advanced Study and joined MIT in 2013.

Dark matter sleuths like Slatyer rely on various methods. They may use accelerators to hurtle charged particles at each other, hoping their collisions generate dark matter particles, or try to catch dark matter particles bumping into other particles in underground detectors. But typically only the physicists running these massive, multiyear experiments can access the raw data they yield.

Instead, Slatyer mines the wealth of publicly available data collected by space telescopes. She hones in on gamma rays, the most energetic form of light, which could be produced by dark matter annihilation. Since the central regions of galaxies likely contain a high density of dark matter and throw off tons of gamma rays, they’re a promising place to look. The problem is, they house a dense thicket of black holes, stars and other objects that also emit gamma rays. Slatyer works to parse these out so physicists don’t mistake them for dark matter.

As a doctoral student, Slatyer, together with her adviser, Douglas Finkbeiner, and another grad student, Meng Su, used images from the Fermi Gamma-Ray Space Telescope to discover the source of a cloud of gamma rays around the center of the Milky Way galaxy — a pair of spherical structures they dubbed “Fermi bubbles.” She then shifted her focus to a glut of gamma rays smack-dab in the galactic center — far in excess of what physicists would predict from a black hole or other known sources of gamma rays. In 2016, Slatyer’s team and their collaborators theorized that the rays were being emitted by dark matter annihilation.

On closer examination, however, Slatyer and her colleagues found that a statistical model showing multiple objects emitting gamma rays better explained the excess than dark matter annihilation. (The gamma-ray excess looked speckled, whereas dark matter annihilation would emit a smooth haze of gamma rays.) The conclusion? The excess seemed to come from pulsars, or collapsed cores of stars, which spin as they spew gamma rays, appearing as pulses of light.

Still, Slatyer says, “this is an ongoing controversy,” and physicists can’t completely rule out dark matter. Her team is now exploring other statistical techniques to verify whether dark matter or pulsars are responsible. In a few years, radio telescopes, currently under construction, could directly detect pulsars. “That could be the smoking gun for the pulsar hypothesis,” she says.

Will Slatyer be disappointed if the gamma-ray excess is proven to come from pulsars, not dark matter? Absolutely not, since physicists need to know what their gamma-ray emissions look like to avoid getting duped. Plus, a new population of pulsars could shed light on the early history of our galaxy. “It’s like Christmas,” Slatyer says. “Even if it’s not what you wrote down on your wish list, you get an awesome present anyway.” As eager as Slatyer is to detect dark matter, Foley doubts she has an agenda: “She’s trying to figure out what’s right.”

“I want to know what 80 percent of the matter in the universe is,” Slatyer says. “It’s a big puzzle, but it seems like a manageable puzzle.” Even if the final picture remains a mystery, she won’t rest until she pieces it together.

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