APS News | People and History

Bharat Ratra, Winner of APS Lilienfeld Prize, on What’s Next for Cosmology

Ratra thinks physicists may answer questions about dark energy and the universe’s geometry within a decade.

By
Oct. 16, 2024
Bharat Ratra stands at a whiteboard writing equations.
Bharat Ratra
Kansas State University

Decades ago, when applying to college in India, Bharat Ratra missed a score cutoff for entrance into one of his preferred engineering programs. That pushed him into physics — a lucky accident for the young Ratra, who would go on to become a distinguished professor in theoretical physics at Kansas State University.

Today, Ratra is the recipient of the 2025 Julius Edgar Lilienfeld Prize for his pioneering research in cosmology and particle astrophysics, and for his dedication to students and public engagement outside the classroom. “There are so many people whom I admire who have been previous recipients,” says Ratra, so having his own work recognized is “pretty humbling.”

Although he’s best known for his contributions to the quantum mechanics of cosmic inflation and dynamics of dark energy, Ratra at times faced a less certain path.

Many Indian schools in the 1970s lacked funding for high-grade lab equipment, so Ratra often found college labs “pretty frustrating,” he says. “It was easier to get better at the math and do more theoretical things.” He excelled in all his early theory-intensive physics courses, and when the class “had the chance to do relativity,” he was hooked.

In 1981, Pakistani theoretical physicist Abdus Salam, recipient of the 1979 Nobel Prize in Physics, was invited to India to deliver a series of talks on his work to develop the standard electroweak model, which would later evolve into the Standard Model. When Salam visited Delhi, Ratra — then an undergrad at the Indian Institute of Technology — was so intrigued that he approached Salam after one of his talks. The two spoke for an hour, on topics ranging from graduate school to unified models of particle interactions. The conversation solidified Ratra’s interests in theoretical physics.

The following year, Ratra started his doctorate at Stanford University. “It was a really interesting time,” he says. The concept of inflation was brand new, and one of Ratra’s first tasks was developing a quantum mechanical approach to modeling the energy density fluctuations generated during inflation of the early universe — a treatment that was more consistent than any done before.

Two years later, when physicists were developing superstring theories, Ratra shifted course again. “I really liked the mathematics of superstrings,” says Ratra. “It’s really beautiful, and it’s the only known way of consistently combining gravity with quantum mechanics.” His new trajectory landed him a postdoc at Princeton University in 1986.

But as his research interests evolved, Ratra realized he wanted to rely more on real-world measurements. “It was pretty clear to me by 1987 that there would be a lot of data coming in from telescopes,” he says. That data was likely to open entirely new research in cosmology.

In the four decades since, rapid advancements in physics, aided by increasingly precise measurements of supernovas, cosmic background radiation, and baryon acoustic oscillations, among other phenomena, have been akin to “a revolution,” he says.

More big shifts could be coming. From the Dark Energy Spectroscopic Instrument (DESI) survey, the Rubin Observatory Legacy Survey of Space and Time (LSST), and other near-future experiments, “we should get more and better data in the next decade that will allow us, hopefully, to determine whether dark energy is constant, or weakly varies in time and space,” Ratra says.

He hopes we’ll soon have more precise and accurate constraints on other cosmological parameters, like the Hubble constant, and that we’ll find a more definitive answer on the geometry of space — whether our universe is flat, as current data suggests, or whether new data will point toward a different geometry, like a saddle shape.

But Ratra’s priorities extend far beyond research: The Lilienfeld Prize recognizes his contributions to students and the public, as well. Ratra loves teaching, and his research mentorship has helped more than a dozen undergraduates continue onto graduate school, and some, to faculty careers of their own. “It’s really satisfying,” he says.

Ratra also co-developed a Kansas State general education course “Origins: Humanity, Life, and the Universe,” designed to touch on a range of topics that have faced mounting public skepticism in parts of Kansas in recent decades. “We’re in the center of the country, where people have very strong opinions about what should be taught, and what shouldn’t,” he says.

“I’ve been here 28 years, and twice the state school board has ruled that high schools shouldn’t be teaching evolution, and once, … the Big Bang model,” he says. “People have their beliefs, and a few feel they should be allowed to dictate what’s taught.”

“I think that’s a really dangerous thing,” says Ratra. “It’s useful for people to understand the scientific background of what we deal with in everyday life,” from the depletion of regional aquifers to the likelihood of new pandemics.

Using cosmology as a lens for teaching general education courses or developing educational outreach materials is an “ideal” way to introduce the public to “big questions about life,” he says. After all, “everybody is fascinated by the Big Bang.”

And through the high school outreach program QuarkNet, a National Science Foundation-funded program with hubs across the country, Ratra works with high school teachers and students from Kansas and Arkansas to expand students’ interest in physics. Ratra and other Kansas State physicists bring real data from CERN and other cutting-edge scientific facilities directly into the classroom.

In one project, students and teachers use Fermilab-built detectors to measure the cosmic ray muon flux — particles produced by the interaction of cosmic rays with our atmosphere — and then correlate their measurements with weather data from NOAA. This lets students “track cold fronts as they move two to three hundred miles across the Midwest,” he says.

“It’s pretty impressive,” he says. “Hopefully this award will help publicize the great cosmology research and physics outreach programs that we’ve built here at Kansas State.”

Liz Boatman

Liz Boatman is a science writer based in Minnesota.

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