September 26, 2002
Berkeley Lab Research News
Saul Perlmutter Wins E. O. Lawrence Award in Physics
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BERKELEY, CA   Saul Perlmutter, a member of Lawrence Berkeley National Laboratory's Physics Division and leader of the international Supernova Cosmology Project based here, has won the Department of Energy's 2002 E. O. Lawrence Award in the physics category. Perlmutter is Berkeley Lab's 25th recipient of the prestigious award, which includes a gold medal and $25,000.

Saul Perlmutter

Perlmutter will be cited at the awards ceremony in Washington D.C. on October 28 "for his leading contributions to an unexpected discovery of extraordinary importance: the determination, through the careful study of distant supernovae, that the expansion of the universe is speeding up rather than slowing down." The announcement of the "accelerating universe" in 1998 was named scientific "breakthrough of the year" by the journal Science.

"We are all enriched by the contributions these researchers have made, ranging from understanding the genetic code to measuring the expansion of the universe itself," said Secretary of Energy Spencer Abraham.

Said Berkeley Lab Director Charles V. Shank, "We are proud that the techniques for measuring cosmic expansion were developed and proven at Berkeley Lab under Saul's leadership of the Supernova Cosmology Project. His Lawrence Award recognizes the kind of imaginative basic research done here to address the most fundamental questions about nature, yielding knowledge whose benefits we may only begin to imagine."

The Lawrence Awards, established by Dwight D. Eisenhower in 1959 as a memorial to Ernest Lawrence, are chosen by independent panels from thousands of nominations by scientists and research organizations. The awards recognize achievements in atomic research, broadly defined, and are intended to encourage the careers of scientists who show exceptional promise.

In addition to Perlmutter's award in physics, this year's winners are, in chemistry, Keith O. Hodgson of the Stanford Linear Accelerator Center; in environmental science and technology, Benjamin D. Santer of Lawrence Livermore National Laboratory; in life sciences, Claire M. Fraser of the Institute for Genomic Research; in materials research, C. Jeffrey Brinker of Sandia National Laboratory and the University of New Mexico; in national security, Bruce T. Goodwin of Lawrence Livermore National Laboratory; and in nuclear technology, Paul J. Turinsky of North Carolina State University.

The fundamental questions
"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 headed for 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."

Many of Perlmutter's subsequent accomplishments, notably his leading role in the discovery of the universe's accelerating expansion, owe much to the practical methods he and his colleagues devised for using 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. 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 hard." Different kinds of supernovae explode in different ways, and it wasn't apparent that any were really "standard."

Moreover, in a universe filled with some hundred billion galaxies of a hundred billion stars each, finding random exploding stars with a telescope was a chancy business; in the 1980s, one search for the extremely distant supernovae required to measure changes in the universe's expansion rate found only a single supernova after two and a half years of looking -- and that one was already faded past its peak brightness.

The group in which Perlmutter did his graduate work, headed by Berkeley Lab and UCB physicist Richard Muller, was constructing a robotic telescope to look for relatively nearby 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, finding some 20 supernovae, distance measurement with Type IIs was "a tough technique, still not perfected," 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 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 impressively similar in brightness.

Supernova cosmology -- the early days
To find enough Type Ia's for meaningful data about expansion, 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 they found an Australian observatory willing to trade observing time for a custom-made CCD camera with a novel wide-area 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, "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 found a supernova."

In 1992, working at the Isaac Newton Telescope in La Palma, the Canary Islands, they finally found their first convincing Type Ia supernova. By 1994 the Supernova Cosmology Project had managed to scrounge enough telescope time to prove it could produce 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 who had initially been skeptical. "Since then everybody has done it. 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 so long that 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 bolstered by independent measurements of other cosmological parameters, instantly revolutionized cosmology. Apparently some mysterious "dark energy" drives cosmic acceleration and constitutes two-thirds of the density of the universe; the nature of dark energy is one of the most significant questions facing high-energy physics in the 21st century.

"This 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 Berkeley Lab's unique status as a national laboratory. "It was the freedom to look ahead that the Lab offered. No one knew if the effort would work, and it was ten years before there was a result. Where else could you find the support to do that?"

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Additional information
More about the Supernova Cosmology Project
More about the SuperNova/Acceleration Probe (SNAP)
More about Saul Perlmutter
The Department of Energy's announcement of the 2002 E. O. Lawrence Awards