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UC Astrophysics, Mathematics Senior Narrows Search for Dark Matter

2017 Goldwater Fellow Madelyn Leembruggen’s summer research examines nature of axion particles.

Date: 8/19/2017 8:00:00 AM
By: Jonathan Goolsby
Photos By: University of Cincinnati

UC ingot   Just what is the nature of the universe? Why does it seem to have more mass than can be explained by objects we can directly observe?

Those are exactly the questions University of Cincinnati Honors Scholar senior and astrophysics/mathematics double major Madelyn Leembruggen — a 2017 Goldwater Fellow — is investigating.

This summer, she’s participating in a National Science Foundation-funded Research Experience for Undergraduates (REU) that is studying axions — a type of “force” particle, like photons and the recently-confirmed Higgs boson, that mediate interactions between regular “matter” particles like quarks and anti-quarks.

We can’t observe axions. Not yet, anyway. Their existence is purely hypothetical.

Should axions exist, they would help to explain the lack of observed violations of CP-symmetry (or, charge conjugation parity symmetry), which are predicted by quantum chromodynamic theory.

CP-symmetry, and predicted violations of it, play important roles in cosmologists’ attempts explain the dominance of matter over anti-matter in the present universe, and in the study of weak interactions in particle physics.

Leembruggen and other researchers are examining mathematical models that could help astronomers and physicists develop methods for detecting axions. And their results could fundamentally change our understanding of the universe.

Looking after a rising star.

Photo of Madelyn Leembruggen
Madelyn Leembruggen

Leembruggen’s interest in space began at an early age. She was fascinated by the stars as a child, and encouraged to learn about them by her father, an aerospace engineer, and mother, a communications instructor at a community college.

“Both my parents are incredibly smart people,” she said. “Every time I’d ask questions, they’d be like, ‘Well, let’s find the answer!’”

In high school, her childhood interests blossomed into a career path.

“Someone has to be an astrophysicist,” Leembruggen thought. “Why shouldn’t I be an astrophysicist?”

“I buckled down. I took extra math classes, I took physics early and decided that I loved it,” she remembered.

Initially, Leembruggen applied to the University of Cincinnati solely because of its affordability. However, on meeting the Department of Physics’ undergraduate advisor, Dr. Richard Gass, she became quickly enthused about life as a Bearcat.

“He sat me down and said, ‘These are all the classes you’re going to take freshman year, here’s what you’ll take sophomore year, here are all the opportunities we can help you achieve,’” she recalled. “So, from the very beginning I felt reassured. I felt cared for.”

She said that she felt additionally cared for after her sophomore year, when her father lost his job, placing her in jeopardy of needing to step away from her education until her family’s finances stabilized.

When members of the Department of Physics learned about Leembruggen’s unexpected financial need, and knowing that they intended to offer her UC’s Violet M. Diller Scholarship for upperclassmen women in Physics to cover her junior and senior year tuition, they offered her a special scholarship to bridge the gap.

“I had no idea, at the time, that they were planning on giving me a full ride for my junior year,” Leembruggen said, smiling shyly.

Knowing that the department is keenly interested in ensuring her success, she emphasized, has been a strong motivator, driving her continued success.

“I’m coming to that place where I can accept that people think I’m good at this, that maybe I am good at this,” she said. “It’s part of why I want to keep doing this and become a role model in my field, because the farther I’ve gotten, the fewer women there were.”

Research through the Women in Science and Engineering program.

The immediate seeds for Leembruggen’s theoretical particle research were sown just after that tumultuous sophomore year.

“I came to college thinking I wanted to do computational astrophysics. Tried it out — was not a fan,” she laughed. “I went to [Dr. Gass] and I said, ‘Hey, I’m thinking about doing research this summer.’ And he said, ‘I know someone who’s looking for a student to mentor this summer.’”

That someone was Dr. Rohana Wijerdhana, a UC Physics professor with particular interests in field theories, black holes and cosmology, who had a research opportunity available under the University’s Women in Science and Engineering (WISE) program.

WISE is a 12-week summer research mentorship that pairs undergraduate women with participating faculty members from a multitude of disciplines. It was during that opportunity that Leembruggen began studying axions.

“I had to catch up a lot on quantum mechanics and particle physics,” she said. “They have all these equations that govern them. We have a set of equations that can explain any classical system, and then we have more particular equations that govern axions themselves.”

Leembruggen synthesized both sets of equations to predict how aggregations of axions would behave. The research poster she produced about her findings took first prize in an undergraduate competition at the American Physical Society’s 2016 National Mentoring Community Conference in Houston.

“At low temperatures, these particles like to be in the same system, and they form little clumps,” she explained. “My project last summer was to look at [their] stability.”

Expanding our knowledge of the universe on her Research Experience for Undergraduates (REU).

Following on last year’s success, Leembruggen was offered positions with National Science Foundation REU sites this summer at Stanford, MIT, Columbia, Chicago, Fermilab, Northwestern, Michigan and Indiana. She chose Stanford.

According to the model Leembruggen and her cohorts are examining, axions should be affected by gravity in much the same way that “normal” particles are, but shouldn’t be affected as much by the three other fundamental forces, except in extreme circumstances.

“It takes a very, very strong normal force to interact with axions,” she explained.

Some of those predicted axion clumps — asteroid-sized collections termed “axion stars” — may be detectable if they were to come into close proximity with magnetars, a special class of neutron stars (themselves small, incredibly dense, rapidly spinning supernova remnants) that generate an intense electromagnetic field.

A magnetar-generated EM field would certainly qualify as extreme circumstance — it’s roughly one quadrillion times more powerful than the Earth’s.

Were an axion star were to pass through a magnetar’s EM field, mathematical models suggest that its constituent axions would destabilize and convert into photons — visible light — which could be measured by astronomical observatories and space-based telescopes like Hubble.

Indeed, some experiments are now testing whether axions can be detected as they pass through simulated magnetar fields.

An alternate detection method?

That’s what the Stanford-based team is hoping to devise.

As axions collect, they should achieve a stable energy state in structures on the magnitude of 200 km in diameter. Anything larger than that, and an axion star would be predicted to collapse in on itself, due to the force of gravity, into another stable energy state structure with a diameter of roughly 7 meters.

This would be roughly analogous to a large, dying star collapsing into a neutron star remnant during a supernova. As in a supernova, a certain amount of energy would be released; a collapsed axion star would likely decay, freeing many of its constituent axions.

Those free axions would move fast enough to escape a galaxy’s gravity, so scientists could attempt to monitor distant galaxies’ masses over time and watch for signs that would indicate they are shrinking due to free axion loss.

“This is a tricky thing,” Leembruggen said. “It doesn’t seem like it would be very much dark matter leaving a galaxy, but it’s still something that we could potentially look at. We could look at similar types of galaxies, of different ages, and compare their dark matter haloes.”

If older galaxies are found to have have less dark matter, on average, than their younger counterparts, that finding would support the Stanford team’s mathematical models and have far-reaching implications for humanity’s understanding of the nature of the universe.

What’s even more interesting, Leembruggen noted, is that the tools she and her fellow student researchers are using are essentially the same tools Einstein, Planck and Fermi were using over a century ago to describe cosmology’s basic principles.

“All the calculations we do, we can do on a chalkboard,” she said, smiling. “It’s a lot of calculus, a lot of physics and a lot of looking up in textbooks how to do certain integrals.”

Making astrophysics accessible to everyone.

Photograph of Leembruggen presenting her work
Leembruggen presents her work.

Leembruggen has already co-authored several papers that have appeared in well-regarded publications, including the Journal of High Energy Physics. She intends to apply to PhD programs next year and, hopefully, land a tenure-track position.

She has a passion for teaching — Leembruggen is a UC Peer Educator and has worked in the University’s Learning Assistance Center since her freshman year. She also has a passion for ensuring that people of all backgrounds and socioeconomic strata feel that they can pursue and achieve careers in the sciences.

“I don’t want to just research physics. I’d like to research how minority groups view and experience STEM fields, because I feel like there’s still not quite enough research about it,” she asserted. “We can still do better at encouraging women and minorities in physics.”

She’s particularly interested in incorporating more humanities studies into STEM education.
“One thing I love about UC is that it truly [offers] a liberal arts degree,” Leembruggen said. “I’ve had a lot of opportunities, because of those extra classes, that I probably wouldn’t if I weren’t in a liberal arts program.”

“There are so many fantastic stories,” Leembruggen noted. For example, “Every time my Quantum Mechanics professor would introduce a new topic, he’d tell the story of how it was discovered, and it was always so interesting.”

“You look at these math systems and medical contributions that ancient Egyptians and ancient Africans invented — so much of that richness has been lost in our storytelling,” she said. “Humanities and STEM can’t be separated if we want to have a truly equitable and a truly representative pool of thinking.”

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