Cosmologist sheds light on very early dark energy
Alex Sobotka’s modeling of the universe’s earliest days may change our understanding of space and time.
To understand what might happen as our universe expands, double Tar Heel Alex Sobotka ’22 (MS), ’25 (PhD) looks for clues from its earliest moments. Alongside Carolina professor and theoretical cosmologist Adrienne Erickcek, he has studied the possibility that “very early dark energy,” or vEDE, accelerated the universe’s growth during its first 380,000 years.
Astronomers think that vEDE might resemble the dark energy driving the 13.8 billion-year-old universe’s expansion today. In the past, it may have behaved differently, subtly altering how matter clumped together without disturbing the cosmic radiation astronomers still measure.
“Think of our universe as a sheet that’s being stretched out,” Sobotka said. “Dark energy is causing space-time to evolve and warp, creating that stretching.”
Sobotka, now an assistant professor at High Point University, and Erickcek, along with Tristan Smith of Swarthmore College, modeled how vEDE would affect the early growth of structures that would later become galaxies.
They built simulations based on observations of the cosmic microwave background — the radiation afterglow of the big bang sometimes called the universe’s “first light.” These microwave signals, invisible to our eyes but measurable with telescopes, reveal patterns in the distribution of galaxies, empty spaces and other primordial features.
“We predicted that if very early dark energy existed then, we should observe certain things today,” Sobotka said.
Among other things, their calculations published in the American Physical Society’s Physical Review Journals show that vEDE could increase the number of dwarf galaxies like those that orbit the Milky Way.
That’s where the Rubin Observatory comes in. Located in Chile, its telescope is equipped with the world’s largest camera. It will soon take images of the entire sky every three nights.
Sobotka is eager to see what those images reveal. “We are on the cusp of potentially being able to see the signatures that our model predicted,” he said. “We stuck something interesting into the early universe to see the implications for the present-day universe. And we can use current observations to learn about what happened when the universe was a fraction of its age.”
He’s always liked to think big. “Growing up, I always loved thinking about the big questions like how the universe began, almost branching on philosophical,” he said. “Cosmology is the closest you can get to that in science.”
Sobotka’s boyhood interest in big questions continued through his undergraduate years at the University of Texas at Austin, extending to his visit to and eventual enrollment in the graduate program in physics and astronomy in the UNC College of Arts and Sciences.
“The faculty were working on so many interesting things, so I thought I’d have a good choice of research,” he said. He sat in on several groups before joining Erickcek’s lab. The fact that his girlfriend, now wife, Hannah, was studying at the UNC Eshelman School of Pharmacy made the decision easier.
As a cosmologist, Sobotka tries to answer questions that remain as vast as the universe. Astronomers estimate that about 2 trillion galaxies exist, each with billions of stars and planets.
“When we say the universe is expanding, that’s based on Einstein’s theory of general relativity,” he said. That theory treats the dimensions of space — up-down, left-right, back-forth — and time as part of the same fabric. Matter, like planets and stars, warps that fabric, determining how it stretches and evolves.
