Saul Perlmutter, who led one of two teams that simultaneously discovered the accelerating expansion of the universe, has been awarded the 2011 Nobel Prize in Physics, to be shared with two members of the rival team.
Perlmutter, a professor of physics at the University of California, Berkeley, and a faculty senior scientist at Lawrence Berkeley National Laboratory (LBNL), led the Supernova Cosmology Project that, in 1998, discovered that galaxies are receding from one another faster now than they were billions of years ago.
He will share the prize with Adam G. Reiss of Johns Hopkins University and Brian Schmidt of Australian National University's Mount Stromlo and Siding Spring Observatories, two members of the competing High-z Supernova Search team. When the discovery was made, Reiss was a post-doctoral fellow at UC Berkeley working with astronomer Alex Filippenko, who at different times was a member of both teams.
The accelerating expansion means that the universe could expand forever until, in the distant future, it is cold and dark. The teams' discovery led to speculation that there is a "dark energy" that is pushing the universe apart. Though dark energy theoretically makes up 73 percent of the matter and energy of the universe, astronomers and physicists have so far failed to discover the nature of this strange, repulsive force.
In recent years, Perlmutter has been working with NASA and the U.S. Department of Energy (DOE) to build and launch the first space-based observatory designed specifically to understand the nature of dark energy. A dark-energy mission was named the top telescope-building priority in an August 2010 report from a blue-ribbon committee of the National Academy of Sciences.
Perlmutter was a post-doctoral fellow at LBNL when he decided to focus on Type Ia supernovae as yardsticks to measure the geometry of the universe. Astronomers knew that the universe was expanding, but the main question at the time was whether the universe was open, and thus destined to expand forever, or closed, meaning that the expansion would eventually stop and the universe would collapse back on itself.
He and his LBNL team were puzzled by initial results in 1997 indicating that, not only was the universe's expansion not slowing down, it was speeding up, contrary to all cosmological theories.
"The chain of analysis was so long that at first we were reluctant to believe our result," Perlmutter said. "But the more we analyzed it, the more it wouldn't go away."
The High-z team came to the same conclusion at the same time, based on an independent set of Type Ia supernovae.
"There was no hint of this when we started the project," Riess said in 1998 while still a Miller Postdoctoral Research Fellow at UC Berkeley. "We expected to see the universe slowing down, but instead, all the data fit a universe that is speeding up."
The discovery, reported by both teams in 1998, has since been bolstered by independent measurements. The earliest and most important of these confirmations were by the Millimeter Anisotropy eXperiment IMaging Array (MAXIMA), a balloon-borne experiment led by UC Berkeley physicist Paul Richards, and the Balloon Observations Of Millimetric Extragalactic Radiation and Geophysics (BOOMERanG) experiment, led by the late Andrew Lange, a former UC Berkeley post-doctoral fellow, and Paolo De Bernardis.
"This discovery was very much a team effort," Perlmutter stressed, 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.
Perlmutter graduated magna cum laude in physics from Harvard University in 1981 and began graduate work at the UC Berkeley, where he gravitated toward the study of astrophysics. He completed his Ph.D. with Richard Muller, UC Berkeley professor of physics, in 1986.
While still a post-doctoral fellow, Perlmutter teamed up with fellow post-doc Carl Pennypacker to develop the technology to use Type Ia supernovae -- which are bright enough to be seen across the universe -- to measure cosmological distances. Other astronomers had observational data suggesting that Type Ias were all about the same intrinsic brightness, so that their apparent brightness from Earth could be used to calculate their distance.
With observing time on several telescopes around the world, the Supernova Cosmology Project was able to test and improve its techniques. When the team eventually sat down with new data on Type Ia supernovae to calculate the basic parameters of the universe, however, the results were too bizarre to be believed.
"The most striking part of the project was the huge skepticism," recalled Pennypacker, now with UC Berkeley's Space Sciences Laboratory and a guest in LBNL's Physics Division. The skepticism was not only about proposed techniques, but about the underlying science. "Nobody believed we could do it," he said, "and it was an enormous challenge to get things done."
Perlmutter, a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences, has received numerous honors, including the 2006 Shaw Prize, shared with Schmidt and Reiss; the 2007 Gruber Cosmology Prize, which he shared with his entire Supernova Cosmology Project team and the High-z Supernova Search team; the 2003 California Scientist of the Year award; and the 2002 E. O. Lawrence Award in physics from the Department of Energy.
He lives in Berkeley with his wife and daughter.
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