How did dark matter shape the cosmos? Physicist Tracy Slatyer has ideas

At age 12, Tracy Slatyer felt sad at the predicament of a book. She read a newspaper article about how hordes of people were purchasing A Brief History of Time by Stephen Hawking. “But then … nobody was actually reading it,” says she. “People were just leaving it on their coffee tables.” Determined to correct this wrong, Slatyer got a copy and diligently read each page. The famed physicist’s popular text revealed to her “that math was in some sense an expressive language for describing how things really work,” says she. “That, to me, was exciting.”

Nowadays, Slatyer, a theoretical physicist at MIT, puts to use her mathematical aptitude to come up with new ideas about dark matter. The enigmatic substance makes up around 85 percent of the matter in the cosmos. Yet it has consistently eluded researchers’ attempts to pin it down. Slatyer attempts to figure out what dark matter could be made from, how it might in fact interact with itself or anything else and, most central, the consequences of those interactions.

Physicists are well aware that dark matter exists because they can see its gravitational influence on galaxies, galaxy clusters and the overall progression of the cosmos. Beyond that, there are very few clues to work with. Slatyer has helped envisage the myriad ways that dark matter could leave some subtle autograph on the fabric of reality that would show up in observations.

Among researchers doing such work, “I don’t think there’s been anybody who’s been more impactful,” says Dan Hooper (a physicist at the University of Chicago). “She’s as big a deal as I can make her out to be.”

Discovering the Fermi bubbles

Born in the Solomon Islands, Slatyer grew up in capital city of Australia, Canberra. After her rendezvous with Hawking’s book, she was absolutely sure she wanted to study physics. While in graduate school at Harvard University in the 2000s, she met physicist Douglas Finkbeiner, who was investigating enigmatic signals at the Milky Way’s center.

A research satellite had taken note of odd excesses of positrons, the electron’s antiparticle, and high-energy photons called gamma rays that simply couldn’t be explained with traditional theories. Together, Slatyer and Finkbeiner started looking more intensely at a type of self-annihilating dark matter that might address the mystery. In their specific model, this dark matter would leave behind electrons and positrons, which would interact with starlight to produce gamma rays.

In 2008, NASA launched the Fermi Gamma-ray Space Telescope, which provided unprecedented views of high-energy photons emanating from the galactic plane. If dark matter was really self-annihilating, it would definitely show up in Fermi’s observations. The following year, Slatyer and Finkbeiner used Fermi’s public data to search for the stuff. “We analyzed the data and saw this big fuzzy glow north and south of the galactic center,” recalls Slatyer. “So we’re like, ‘Victory!’”

But the more they and another of Douglas Finkbeiner’s students, Meng Su, looked at the signals, the more they realized that this wasn’t in fact dark matter. Fermi’s images bared an enormous hourglass figure that stretched 25,000 light-years above and below the Milky Way’s plane. Dark matter is believed to be present in a diffuse halo all around our galaxy, but this structure had very sharp edges.

Supermassive black holes nourishing on gas and dust in the centers of other galaxies have been known to belch out material into hourglass figures. Ultimately, Slatyer and her colleagues realized that this could in fact be something similar. These Fermi bubbles, as they came to be known, have been the subject of many follow-up studies, resulting in a long-running debate over the mechanisms driving the bubbles’ creation. Slatyer hadn’t found dark matter, but, she says, “I try not to complain when nature gives me exciting new things, whether or not they were what I was looking for in the first place.”

Much of her work since Fermi bubbles discovery has focused on different dark matter scenarios

Much of her work since Fermi bubbles discovery has concentrated on different dark matter scenarios. For example, some of her research has looked at how the enigmatic substance could have annihilated or decayed in the early cosmos, leaving behind fundamental particles that would result in small variations in the expected temperature of the overall universe. Such an effect might show up in CMB (the cosmic microwave background), a remnant light left over from when the cosmos was just 380,000 years old.