A distinguished theorist at Princeton, Steinhardt is perhaps best known for two of his main interests: cosmology and quasicrystals.
His cosmological contributions have shaped our understanding of the early formation of the universe. In 1982, together with his graduate student Andreas Albrecht, Steinhardt proposed the first successful working model of cosmic inflation: the theory that soon after the Big Bang, the universe underwent a short burst of exponential expansion. While that work helped to make inflation the dominant study of early cosmology, Steinhardt was unsatisfied with the model’s flaws. So 20 years later, he and physicist Neil Turok, director of the Perimeter Institute for Theoretical Physics, developed radical alternatives, the ekpyrotic and cyclic models, which proposed that the evolution of the universe is an endlessly repeating cycle and that certain key conditions occurred long before the Big Bang. [related news: First Direct Evidence of Inflation and Primordial Gravitational Waves]
While the perfect model has yet to be found, Steinhardt is hopeful. “The data on both a macro and micro scale points to a universe that is very simple,” Steinhardt says. “I think that’s an enormous hint. If there's any chance of trying to explain the universe in its entirety, what gives me hope is that simplicity.”
Then there’s Steinhardt’s other interest: quasicrystals—a term he coined 30 years ago.
The name refers to a new phase of solid matter with symmetries previously thought to be impossible. The laws governing crystal formations state that they can possess symmetries in only two, three, four, and six folds, and this determines many of their physical properties and applications. Quasiperiodic crystals (or quasicrystals for short) broke those laws entirely.
In the early 1980s, while at the University of Pennsylvania, Steinhardt and his then-graduate student Dov Levine developed the first working theory for how these “impossible crystals” could exist.
At the same time, independent researcher Dan Shechtman spotted quasicrystal formations in an aluminum alloy in his lab and in 1984 published his findings (for which he won the Nobel Prize in Chemistry in 2011). “Shechtman had a material without a theory, and we had a theory without a material.” Steinhardt says. “The discovery that these structures could exist opened the door to a whole new spectrum of solid materials.”