The invisible glue of the universe

For years, astrophysicist Stacy Kim wanted to be a writer. That changed during her senior year of high school in Phoenix, Arizona, when she took her first physics course. She soon discovered that the advanced mathematics she loved could be used to do far more than solve abstract math problems — it could help explain how the physical world works.

As a self-described “painfully shy” teenager, Kim found that astronomy held a personal appeal. She was drawn to a night sky that initially revealed very little but that held an intricate and layered whole when examined up close. “It’s dark. There are some pinpricks of light,” she explains. “But if you look really carefully with powerful telescopes, there’s so much complexity and beauty in the universe.”

Now the Clare Boothe Luce Assistant Professor of Physics and Astronomy at Mount Holyoke College, Kim uses computer simulations of the tiniest galaxies in the universe — called “dwarf galaxies” — to investigate one of the universe’s largest unanswered questions: What is dark matter?

We know dark matter exists because of gravity. When we measure how fast stars move within galaxies, we can calculate how much mass they are orbiting. However, when we add up all the visible stars, gas and dust particles, the numbers don’t match the motion we observe. Something unseen, the theory states, must be providing that additional mass.

The ordinary matter that builds our planets, stars and other visible celestial bodies makes up a mere 5% of the universe. Dark matter accounts for approximately 25%, while dark energy claims the remaining 70%. As Kim explains, dark matter “literally holds us all together.” Without its gravity pulling together the gas and dust needed to form stars and planets, life as we know it could not have developed.

But what exactly is dark matter and what physical properties does it hold? While particle physicists have theorized various ideas for what dark matter might be, each model predicts a different blueprint for how a galaxy should form and evolve. To test these theories, Kim places these models into computer simulations. By recreating actual dwarf galaxies under different dark matter models, she identifies unique cosmic signatures. These simulations give astronomers using telescopes the ability to determine which dark matter models are most consistent with the observed universe.

At Mount Holyoke, Kim wants her students to experience how astronomical knowledge is produced. In ASTR-226: Cosmology, students recently joined a live, remote observing run using telescopes in Chile, led by Laura Hunter ’17. Kim’s students also get to run their own simulations of some of the tiniest dwarf galaxies. Because these galaxies are exceptionally small, students can complete their simulations in just a few days. By comparing their results with real-world observations, they can potentially contribute to current scientific research.

Beyond the stars, dark matter and galaxies, Kim wants her students to recognize the incremental process that expands human understanding. As an example, she points to our growing knowledge of the existence of dark matter itself. Answering seemingly simple questions about how celestial bodies move revealed the universe to be much larger, more mysterious and more intricate than anyone imagined.

Kim was especially attracted to Mount Holyoke’s culture of mutual support. During her campus visit, she witnessed faculty mentoring students, students collaborating with each other and colleagues at all levels looking out for one another. She was especially struck by the genuine sense of community and support within the Physics and Astronomy Department, noting the exceptional care shown by faculty and staff, particularly to those earlier in their careers. “That is the sort of environment I really, really love,” she enthuses. “That is something special.”

In her free time, Kim enjoys the outdoors and is an avid gardener and hiker.