Feryal Özel PhD ’02, RI ’13 and Paul Steinhardt AM ’75, PhD ’78, RI ’13 gave a tour of the universe’s history. The big bang, marking the beginning of the universe, is thought to have occurred about 14 billion years ago. Özel focused on the period of star and galaxy formation, from six million to nine billion years ago, and Steinhardt discussed the earlier and later periods.
Özel showed simulations—created by her team at the University of Arizona—of how galaxies form, a process about which scientists have a good understanding. Steinhardt discussed why the period of galaxy formation came to an end. “What happened is that a new form of energy overtook the universe,” he said. Dark matter and ordinary matter (which we’re made of) self-attract, but dark energy pushes away from other bits of energy. About 15 years ago, scientists discovered that most energy does the same thing. “We are the minority,” he said. “Seventy percent of the stuff in the universe gravitationally repels.” This helps “promote the expansion of the universe, causing it to speed up in its expansion, whereas if the universe were composed mostly of us and dark matter, you’d have the opposite effect. The universe would have been slowing down in its expansion.” When dark energy took over, about four billion years ago, matter couldn’t cluster fast enough to form any more galaxies.
Özel is an associate professor of astronomy and astrophysics at the University of Arizona. Steinhardt—who held the Lillian Gollay Knafel Fellowship at the Radcliffe Institute—is a theoretical physicist whose research ranges from the origin, evolution, and future of the universe to new states of matter. He is the director of the Princeton Center for Theoretical Science and the Albert Einstein Professor in Science at Princeton University, where he is also on the faculty of the departments of physics and of astrophysical sciences.