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Work on the Accelerating Expansion of the Universe Wins 2011 Nobel Prize in Physics

10/06/2011 07:02PM

Work on the Accelerating Expansion of the Universe Wins 2011 Nobel Prize in Physics
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, Adam G. Riess and Brian Schmidt.

Perlmutter, 52, 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. Riess, 41, of The Johns Hopkins University and Brian Schmidt, 44, 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, Riess was a postdoctoral fellow at UC Berkeley working with astronomer Alex Filippenko, who at different times was a member of both teams.

Perlmutter is the fifth Nobel winner for UC Berkeley in the past 11 years, and the 22nd Nobelist overall. This is the ninth Nobel in Physics awarded to a UC Berkeley faculty member, the most recent winner being George Smoot in 2006.

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 postdoctoral 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 postdoctoral 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."

Adam Riess is an astronomer at the Space Telescope Science Institute and Krieger-Eisenhower professor in physics and astronomy at The Johns Hopkins University in Baltimore. The academy recognized him for leadership in the High-z Team's 1998 discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained "dark energy" filling the universe. Critical parts of the work were done with NASA's Hubble Space Telescope.

"The work of Riess and others has completely transformed our understanding of the universe," said Waleed Abdalati, NASA chief scientist. "This award also recognizes the tremendous contributions of the technological community that engineered, deployed, and serviced the Hubble Space Telescope, which continues to open new doors to discovery after more than 20 years of peering deep into the universe. With the future launch of the even more powerful James Webb Space Telescope, NASA is ensuring more revolutionary science discoveries like these."

Space Telescope Science Institute director Matt Mountain added, "The power of this discovery is that NASA has kept Hubble going for 20 years. This meant that Adam was able to track the history of the universe using science instruments that were upgraded from one servicing mission to the next. That is why this work has been recognized with the Nobel Prize."

Riess led the study for the High-z Supernova Search Team of highly difficult and precise measurements of objects that spanned 7 billion light years that resulted in the 1998 discovery that many believe has changed astrophysics forever: an accelerated expansion of the universe propelled by dark energy.

"We originally set out to use a special kind of exploding star called supernovae to measure how fast the universe was expanding in the past and to compare it to how fast it is expanding now," Riess recalled. "We anticipated finding that gravity had slowed the rate of expansion over time. But that's not what we found." Instead, Riess' team was startled to discern that the rate of expansion was actually speeding up.

Richard Griffiths, Hubble program scientist in the Astrophysics Division at NASA Headquarters, Washington, said, "The role of the Hubble Space Telescope in this work was to measure how the brightness of some of the most distant supernovae changed over time. This established the acceleration of the universe and by inference that the agent of acceleration is dark energy."

The importance of Hubble was to obtain images of the high-redshift supernovae of type Ia, exploding white dwarfs that have accreted gas from their companion stars in a binary system and reached a mass limit beyond which they can no longer support themselves against gravity. Since the brightness of these supernovae change with time in a way that correlates with their intrinsic peak brightness, observations of their light can point to how bright, and therefore how distant, their host galaxies are.

The precision of Hubble measurements of the high redshift supernovae, which had been discovered from the ground, was crucial in the demonstration that distant supernovae were fainter than expected, and that the initial deceleration of the universe has astoundingly transformed into an accelerating expansion due to the effects of dark energy.

Although Hubble played a critical role in the discovery of dark energy, nearly every major observatory on Earth also contributed to the study of this mysterious energy. Ground-based telescopes run by the National Optical Astronomy Observatories, especially the 4-meter Blanco telescope at the Cerro Tololo International Observatory in Chile, and at the Kitt Peak National Observatory in Arizona, as well as European telescopes on the Canary Islands, are credited with discovering of the majority of the supernovae ultimately used to track the expansion rate of the universe. The astronomers also used the W. M. Keck Observatory in Hawaii, the MMT Observatory in Arizona, and European Southern Observatory's 3.6-meter telescope in Chile to measure the spectra of the discovered supernovae and the distances of their host galaxies.

"One of the most exciting things about dark energy is that it seems to live at the very nexus of two of our most successful theories of physics: quantum mechanics, which explains the physics of the small, and Einstein's Theory of General Relativity, which explains the physics of the large, including gravity," Riess said.

"Currently, physicists have to choose between those two theories when they calculate something. Dark energy is giving us a peek into how to make those two theories operate together. Nature somehow must know how to bring these both together, and it is giving us some important clues. It's up to us to figure out what [those clues] are saying."

Riess is continuing his Hubble Space Telescope observations of distant supernovae to characterize dark energy. He also is involved in searching for the exploding stars with the Panoramic Survey Telescope and Rapid Response System, a series of ground-based telescopes at the University of Hawaii's Institute for Astronomy. The sky survey is expected to find thousands of new supernovae.

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