The 2019 Nobel Prize in Physics was awarded "for contributions to our understanding of the evolution of the universe and Earth's place in the cosmos."

One-half of the prize goes to James Peebles "for theoretical discoveries in physical cosmology." Peebles is the Albert Einstein Professor of Science at Princeton University.

The other half is shared jointly by Michel Mayor and Didier Queloz "for the discovery of an exoplanet orbiting a solar-type star." Mayor is a professor at University of Geneva, in Switzerland. Queloz is a professor at University of Geneva, Switzerland, and University of Cambridge, in the U.K.

COSMOLOGY

“Cosmology studies the universe at the largest scales, applying the laws of physics over billions of light-years and all the back to the universe's infancy,” says John Wise, an associate professor in the School of Physics.

Wise traces the emergence of cosmology as an active research field to 1964, when scientists from Bell Labs discovered the cosmic microwave background (CMB), also known as the afterglow of the Big Bang. James Peebles “was a graduate student at the time, and he made an immediate impact on the young field of cosmology,” Wise says.

Over the next three decades, Wise says, Peebles published a multitude of papers about dozens of discoveries, laying the theoretical foundations of physical cosmology. Among many accomplishments, Peebles explained how atoms form after the Big Bang, the origin of the cosmic microwave background, how and why galaxies grow and spin, the role of dark matter in the universe, and how the largest structures in the universe – the cosmic web – form.

“Peebles’ framework allowed future researchers to develop a rich physical picture of galaxy formation, both analytically and computationally,” Wise says.  His work led to the discovery of dark energy, a mysterious force that is accelerating the universe's expansion. Additionally, Peebles pioneered the paradigm of cold dark matter, whereby small clumps of dark matter, known as halos, merge into larger halos.

“It was later established that dark matter is the dominant ingredient in the mass of the universe, outnumbering the mass of all atoms by nearly six times,” Wise says.  “Normal matter is thus along for the ride as dark matter dictates the dynamics of galaxy formation on the largest scales.”

Numerical simulations are a good tool to explore the subsequent consequences of Peebles’ work in cosmology. Wise’s research uses cosmology simulations of star and galaxy formation.

“Without the pioneering work of Peebles, none of our research would be possible,” Wise says. Researchers including Wise can now investigate how myriad physical processes interact within Peebles’ picture of physical cosmology. For Wise, these investigations have led to understanding and visualization of how the very first stars and galaxies formed in the universe.

“We routinely use his theoretical work when initializing our simulations, which run on the nation's largest supercomputers,” Wise says.  “We now know how the CMB informs us about mass-energy content of the universe, the future evolution of the observed CMB perturbations, and how much hydrogen and helium is produced just after the Big Bang, to name a few.  All of these are essential inputs into our simulations.” 

EXOPLANETS

Michel Mayor and Didier Queloz discovered the first extra-solar-system planet, or exoplanet, orbiting around a Sun-like star. The star is called 51 Pegasi and is about 50 light years away. The exoplanet is 51 Pegasi b, which is about half the mass of Jupiter. It orbits 51 Pegasi once every four days and at a much closer distance than Mercury is to the Sun.

“This discovery marked a breakthrough in the field of astrophysics and led to various fields of interests,” says Gongjie Li, an assistant professor in the School of Physics and the School of Earth and Atmospheric Sciences. Among Li’s research interests is the dynamics of exoplanets.

The discovery not only showed that planets can indeed exist around other Sun-like stars, Li said. It also revolutionized our understanding of planet formation. “Nobody expected such a massive planet – around 150 times that of Earth – could exist so close to its host star with an orbital period of less than 10 days.”

To explain such puzzles, Li has studied various mechanisms that can produce a massive planet close to its host star. They include gravitational interactions between the planet and its companion sibling planets, as well as tidal interactions between the planet and its host star. Both can bring a planet close to its host star after it is formed initially at a farther location.

Li now studies the long-term stability of planetary systems. With graduate student Renyi Chen, undergraduate student Karthik Yadavalli, postdoctoral researcher Billy Quarles, and collaborator Molei Tao, assistant professor of mathematics, she is investigating the stability and the effects of planetary spin-axes oscillations, which govern the seasonal and climate variations of a planet.

The work of Mayor and Queloz “was a truly transformational achievement,” says Chris Reinhard, an assistant professor in the School of Earth and Atmospheric Sciences. Not only did it deeply transform understanding of the formation and evolution of planetary systems. It also sparked exploration of exoplanets.

“It helped catalyze an entirely new field of scientific inquiry that brings together astronomers, planetary scientists, chemists, geologists, and biologists in an effort to characterize planets beyond our solar system and probe them for signs of life,” Reinhard says.

For his part, Reinhard is developing techniques to detect signs of life on exoplanets. “In other words,” he says, “as we continue to characterize planets beyond our solar system, how will we know with confidence if we've found a living world? “This is an incredibly challenging question that pulls from many scientific disciplines, including my own, and this discovery represents one of the major foundational advances underlying all of this work.

To date, more than 4,000 exoplanets have been discovered. More than 20 of them are rocky, like Earth, and could support life – they are habitable.

Says Reinhard: “The effort to discover and characterize these extrasolar worlds will continue to be one of the most vibrant, exciting, and truly interdisciplinary areas in all of science in the coming decades and will undoubtedly reshape our understanding of planetary systems and perhaps the distribution and fate of life in the universe.”