At the tender age of 12, Tracy Slatyer was moved by a book. She came across a newspaper article about the widespread purchase of Stephen Hawking's 'A Brief History of Time.' 'But then... nobody was actually reading it,' she recalls. 'People were just leaving it on their coffee tables.' Resolving to change this, Slatyer obtained a copy and read it thoroughly. The renowned physicist's work revealed to her 'that math was, in some sense, an expressive language for describing how things really work,' she says. 'That, to me, was exciting.'
Today, Slatyer, a theoretical physicist at MIT, employs her mathematical prowess to explore new ideas about dark matter. This enigmatic substance constitutes about 85 percent of the matter in the universe, yet it has persistently defied scientists' efforts to understand it. Slatyer seeks to determine what dark matter might be composed of, how it could interact with itself or other matter, and, most critically, the implications of these interactions.
Physicists are aware of dark matter's existence due to its gravitational effects on galaxies, galaxy clusters, and the universe's overall evolution. Beyond this, there are scant clues to guide research. Slatyer has contributed to envisioning the myriad ways dark matter could leave subtle traces on reality, detectable through observations.
Among her peers, '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 significant as I can make her out to be.' Born in the Solomon Islands and raised in Canberra, Australia, Slatyer's encounter with Hawking's book solidified her desire to study physics. During her graduate studies at Harvard University in the 2000s, she collaborated with physicist Douglas Finkbeiner, who was investigating anomalous signals at the Milky Way's center.
A research satellite had detected unusual excesses of positrons and high-energy gamma rays, unexplained by conventional theories. Together, Slatyer and Finkbeiner delved into a type of self-annihilating dark matter that could potentially explain the mystery. In their model, this dark matter would produce electrons and positrons, which would interact with starlight to generate gamma rays.
In 2008, NASA launched the Fermi Gamma-ray Space Telescope, providing unprecedented views of high-energy photons from the galactic plane. If dark matter was self-annihilating, it would manifest in Fermi's observations. The following year, Slatyer and Finkbeiner analyzed Fermi's public data in search of dark matter. 'We analyzed the data and saw this big fuzzy glow north and south of the galactic center,' Slatyer remembers. 'So we're like, 'Victory!''
However, further examination with Meng Su, another of Finkbeiner's students, revealed that the signals were not from dark matter. Fermi's images displayed an enormous hourglass figure extending 25,000 light-years above and below the Milky Way's plane. Dark matter is believed to form a diffuse halo around our galaxy, but this structure had very distinct edges. Supermassive black holes in other galaxies, known to expel material in hourglass shapes, were eventually identified as the source. These Fermi bubbles have since been the focus of numerous studies, sparking a long-running debate about their formation mechanisms.
Although Slatyer didn't find dark matter, 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 subsequent work has explored different dark matter scenarios. For example, she has investigated how dark matter might have annihilated or decayed in the early universe, potentially causing subtle variations in the cosmic microwave background (CMB), the remnant light from the universe's first 380,000 years.
Satellites measuring the CMB have found that it indicates the universe had nearly uniform temperature, with deviations of only one part in 100,000. Slatyer and her team calculated that dark matter annihilation could have produced an even more subtle temperature signature, down to one part in a million. In 2023, they reported how self-annihilating dark matter would distort the CMB—a signal for future instruments to detect.
In a 2024 study, Slatyer and colleagues examined other potential effects of excessive heat from dark matter in the early universe. Under certain scenarios, this heat could have generated surplus free electrons, catalyzing chemical reactions that favored early star formation. Conversely, excess heat might have dispersed gas and dust, potentially reducing star formation and leading to the formation of massive black holes, which could have seeded the first galaxies.
These ideas could help explain the unexpected findings of the James Webb Space Telescope, which has detected unusually large black holes and galaxies in the early universe. Slatyer and her team suggest that dark matter might be responsible for these massive cosmic objects.
By rigorously pursuing her theories, Slatyer has become indispensable to the community of theoretical and observational physicists seeking dark matter. 'She's one of these people who's kind of ubiquitous,' Finkbeiner says. 'She shows up at every meeting. She has her finger in every pie. She's on every panel to figure out what the field should do for the next 10 years.'
Given the limited knowledge about dark matter, Slatyer emphasizes the importance of considering a wide range of possibilities and devising experiments to test them. 'We try to... make sure that we don't miss anything blindingly obvious,' she says.
