Perlmutter’s Quest: In Pursuit of the Fundamentals
"What is true about the world no matter where, no matter when?" is the kind of question that has fascinated Saul Perlmutter since childhood. After graduating magna cum laude in physics from Harvard in 1981, he did his graduate work at the University of California at Berkeley, where he soon realized that to pursue such fundamental questions in high-energy physics "would require vast machines and involve hundreds of people. So I thought it would be fun to try astrophysics instead, where a small group of people could make important advances."
Many of Perlmutter's subsequent accomplishments, notably his leading role in the discovery of the accelerating expansion of the universe, owe much to the practical methods that he and his colleagues devised for finding and using Type Ia supernovae as "standard candles" to measure the cosmic expansion rate.
Astronomical standard candles are objects whose calculable brightness reveals their distance from our solar system, just as the apparent brightness of a candle depends on its distance across a room. Type Ia supernovae are among the brightest objects in the universe, visible at much greater distances than other standard candles like Cepheid variable stars.
Although the idea had been circulating within the astronomical community for years, Perlmutter says, "In the early days, people thought measuring expansion with supernovae would be too hard." Different kinds of supernovae explode in different ways, and it wasn't apparent that any of them were really "standard."
Moreover, in a universe filled with some hundred billion galaxies, with a hundred billion stars in each, finding random exploding stars with a telescope was a chancy business; in the 1980s, one search for the kind of extremely distant supernovae required to measure changes in the universe's expansion rate found just one, after two and a half years of looking – and that one had already faded past its peak brightness.
The group in which Perlmutter did his graduate work, headed by Berkeley Lab and UC Berkeley physicist Richard Muller, was constructing a robotic telescope to look for relatively nearby supernovae – in particular, Type II "core collapse" supernovae whose brightness, it was thought, could be calculated from the velocity of their expanding shells. Although the robotic search was successful in finding some 20 supernovae, distance measurement with Type IIs was "a tough technique, still not perfected to this day," Perlmutter remarks.
"In the meantime, Carl Pennypacker and I, the two postdocs in the group, got interested in looking at Type Ia supernovae at much greater distances," says Perlmutter, "and with Rich Muller’s support we began what was later called the Supernova Cosmology Project." Type Ia supernovae were not only brighter than Type IIs but – if carefully distinguished from superficially comparable types – had proved to be impressively similar in brightness.
Supernova cosmology -- the early days
To find enough Type Ia's for meaningful data about the expansion of the universe, Perlmutter and Pennypacker wanted to use a wide-area telescope to scan thousands of galaxies at once. But competition for telescope time among astronomers was fierce. It was a time when sensitive CCDs (charge-coupled devices) were fast replacing photographic plates in astronomy, and Perlmutter and Pennypacker found an Australian observatory willing to trade observing time for a novel, custom-made, wide-area CCD camera of their own design.
"In exchange for building the camera we got 12 nights, spaced over many months," Perlmutter says. "The weather was good for just two and a half of those nights."
During those two and a half nights they found what Perlmutter calls a "promising" Type Ia supernova candidate, "but we couldn't prove it." It's what he calls "a major chicken-and-egg problem: you couldn't prove you'd found a supernova unless you could get access to a big telescope, but you couldn't get access to a big telescope unless you could prove you'd find a supernova."
In 1992, working at the Isaac Newton Telescope in La Palma in the Canary Islands, they finally found their first convincing Type Ia supernova. And by 1994 the Supernova Cosmology Project had managed to scrounge enough telescope time to prove a new approach they had developed, which yielded large numbers of "supernovae on demand."
"In retrospect it seems obvious, but we realized that the whole process could be systematized. The key was to clump the observations," Perlmutter explains. "By searching the same group of galaxies three weeks apart, we could find supernovae candidates that had appeared in the meantime. We could guarantee four to eight supernovae each time, and all of them would be on the way up" – growing brighter instead of already fading.
"The first time we tried this scheduling scheme, at the Kitt Peak and La Palma observatories in late 1993 and early 1994, we found five supernovae," Perlmutter says. Their success inspired others. "It became a race to build a statistically significant sample."
Facing up to the cosmos
Years of refining theory and observational techniques and painstaking data analysis followed. In 1998 the Supernova Cosmology Project and the competing High-z Supernova Search Team came to a conclusion that both had initially resisted: the expansion of the universe is not slowing, as everyone had assumed. On the contrary.
"The chain of analysis was long, and the universe can be devious, so at first we were reluctant to believe our result," Perlmutter explains. "But the more we analyzed it, the more it wouldn't go away."
The discovery that the universe is expanding at an accelerating pace, soon to be bolstered by independent measurements of other cosmological parameters, instantly revolutionized cosmology.
For if the universe were composed entirely of matter and light, then according to the Theory of General Relativity gravitational forces would slow its expansion. Because the expansion rate is instead increasing, we know that either the universe is pervaded by some entity unlike matter and light, or the Theory of General Relativity fails when applied to the universe as a whole. The hypothetical new entity is termed “dark energy.”
What distinguishes dark energy from matter and light is its negative pressure. Einstein himself once proposed adding a term to the equations of General Relativity, the “cosmological constant,” to make the equations describe a static, nonexpanding universe. When Hubble discovered that the universe was indeed expanding, Einstein discarded the cosmological constant, declaring it to be “the greatest mistake I ever made.”
We now know that Einstein might have been right all along, but for a reason he could not foresee. His cosmological constant acts much like dark energy and could explain the results of the two supernova teams, although we don’t yet know whether the accelerating expansion of the universe is due to a cosmological constant, some other kind of “unconstant” dark energy, or a real failure of General Relativity. The Joint Dark Energy Mission of NASA and the Department of Energy and other satellite-based experiments have been proposed to try to answer this question, which is recognized as one of the most significant scientific problems of 21st-century science.
"Our discovery was very much a team effort," Perlmutter stresses, citing the efforts of the Supernova Cosmology Project's individual members in theoretical studies of supernova dynamics, the detection of supernovae near and far, data analysis and interpretation, and other research components.
Moreover, Perlmutter says, the sustained effort that led to the breakthrough was possible because of the opportunities he had as a young researcher at Lawrence Berkeley National Laboratory. "It was the freedom to look ahead that the Lab offered. No one knew if the effort would work, and it was 10 years before there was a result. Where else could you find the support to do that?"
Perlmutter is a member of the National Academy of Sciences. His other awards include the American Astronomical Society's Henri Chretien Award in 1996; the Department of Energy's E.O. Lawrence Award in Physics for 2002; the California Scientist of the Year Award in 2003; the City of Philadelphia's John Scott Award in 2005; in that same year the Padova Citta Delle Stelle (Padua Prize), shared with Brian Schmidt; in 2006, the Shaw Prize in Astronomy, which he shared with Adam Riess and Brian Schmidt; and later that year the Lincei Academy's Antonio Feltrinelli International Prize in the Physical and Mathematical Sciences. In 2007, the Gruber Cosmology Prize was awarded to Perlmutter, Schmidt, and the members of the Supernova Cosmology Project and High-z Supernova Search Team. Perlmutter was also awarded the Korolev Medal of the Russian Federation of Cosmonautics in 2007. In 2009 he received the Dickson Prize in Science from Carnegie Mellon University. In 2011, Perlmutter and Riess were corecipients of the Albert Einstein Medal.