Montana State physicist wins National Science Foundation CAREER award

BOZEMAN - A Montana State University astrophysicist and strong gravity expert has been awarded the National Science Foundation's most prestigious award for young researchers.

Hang Yu, an assistant professor in the Department of Physics in MSU's College of Letters and Science, will use the $400,000 CAREER award for a five-year study of binary neutron stars, which are pairs of extremely dense stars gravitationally bound to one another. Neutron star binaries generate gravitational waves as they orbit one another or when they collide, and Yu aims to create new, more accurate theoretical models that will enable scientists to better interpret those wave signatures.

Gravitational waves are ripples in space-time created by neutron star interactions and other dynamic events in the universe, such as the mergers of black holes. Their existence was first predicted by Albert Einstein in 1916 as part of his general theory of relativity and confirmed in 2015, when the first gravitational wave was detected on Earth. Neil Cornish, MSU Regents Professor of Physics and internationally recognized expert in strong gravity, along with a team of his graduate students, contributed to that discovery by developing a novel method for extracting gravitational wave signals directly from the detector.

"We will provide to the community, I would say, the most accurate gravitational wave waveform model for neutron stars that can really be used to test neutron star physics to accuracy," Yu said. "None of the previous models can achieve that."

Originally from China, Yu earned his bachelor's degree in physics from Johns Hopkins University and his Ph.D. from the Massachusetts Institute of Technology. He conducted postdoctoral research for three years at the California Institute of Technology and a year at the Kavli Institute for Theoretical Physics before joining the MSU faculty in 2023. He said he was attracted by the reputation of MSU's physics department and its eXtreme Gravity Institute, which is led by Cornish.

John Neumeier, physics department head, said the award announcement affirmed the department's expectation that Yu would win a CAREER award.

"The CAREER program is NSF's most prestigious. It funds the early-career development of the nation's best young researchers and those who show the most promise to serve as academic role models in research and education," Neumeier said.

"We see this already in Hang Yu, after only three years at MSU," he added. "His research culminates in long, detailed research publications that will educate scientists well into the future. He attracts excellent students into our Ph.D. program and has contributed greatly to our teaching mission, even creating a new, interdisciplinary and introductory course in quantum computing."

Yu explained that neutron stars can be viewed as balls of fluid and that tidal interactions between them work exactly as they do between the moon and Earth. When the stars are close to one another, gravity raises tides inside them, and existing waveform models account for that tidal deformability under oversimplified assumptions. The problem, he said, is that those models don't account for many hydrodynamical effects at work within the binary. Yu's study will incorporate those additional effects into existing waveform models, and that may reveal new signatures of the gravitational wave signals.

"This study is mainly to incorporate all aspects of hydrodynamics, including the actual dynamical features and also nonlinear aspects of the fluid," he said, adding that failing to account for those additional effects could show a false inconsistency with Einstein's theory of relativity.

Yu accomplishes his work through pencil and paper calculations compared against numerical relativity simulations, not through direct astronomical observation. That's because current gravitational wave detectors aren't yet sensitive enough to collect precise data about the hydrodynamical aspects of neutron star binaries. Yu said the next generation of gravitational wave detectors, including the Cosmic Explorer currently under active development with support from the NSF, will have sufficient sensitivity to test all the predictions.

He said his study incorporates strong gravity science, theoretical astrophysics and nuclear physics and will be significant for all three fields. The study is relevant in nuclear physics because the density of a neutron star is higher than the density of any atoms that can be studied on Earth, Yu said.

The CAREER award also will support the work of two graduate students and an outreach project in Montana's GEAR UP schools for teacher training and the development of science, technology, engineering and mathematics lesson plans. GEAR UP, which stands for Gaining Early Awareness and Readiness for Undergraduate Programs, is sponsored by the Montana University System to help middle school and high school students in rural areas prepare for postsecondary education after high school.