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Cosmic acceleration discovery wins physics Nobel

Three astrophysicists honored for grasping universe's accelerating expansion

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7:39am, October 4, 2011
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The shocking discovery that the universe is expanding at a faster and faster rate has taken the 2011 Nobel Prize in physics. Three American-born astrophysicists will divide the $1.5 million prize, announced October 4 in Stockholm.

"This is tremendous news, in recognition of a fundamental discovery that has changed our picture of the universe,” says Robert Caldwell, a theoretical physicist who specializes in cosmology at Dartmouth College.

Half the prize goes to Saul Perlmutter of Lawrence Berkeley National Laboratory in California. In 1988, he started the Supernova Cosmology Project to measure the brightness of a certain type of distant supernovas. Because these exploded white dwarf stars tend to put out the same amount of light, they offer a cosmic yardstick for measuring how fast distant objects are moving. Perlmutter expected to find evidence that the fabric of spacetime has been expanding at an ever slower rate as a consequence of gravity putting the brakes on the universe's growth.

So did Brian Schmidt of the Australian National University in Weston Creek and Adam Riess of Johns Hopkins University and the Space Telescope Science Institute in Baltimore. The two will split the other half of the prize. Their High-z Supernova Search Team, launched in 1994, used ground-based telescopes to search large swaths of the night sky for supernovas.

In 1998 the rival teams both announced something utterly unexpected: Compared with closer supernovas, distant supernovas were dimmer than existing theories would predict. Instead of slowing, the expansion of the universe was found to be accelerating. It was later shown that the expansion started about 4.5 billion years ago, around the same time that the solar system formed.

“It was with a fair bit of trepidation that we ended up telling our group and eventually telling the world that we had this crazy result,” Schmidt told the Nobel committee during the announcement of the prize.

These observations, which have since been verified by more precise measurements, suggest the existence of a kind of repulsive gravity in addition to the familiar attractive force. Dark energy, an invisible form of energy though to make up about three-quarters of the universe’s mass-energy, is widely believed to be the entity behind this push. A similar idea was first proposed by Einstein himself, then rejected. Observations of radiation left over from the Big Bang support the existence of this dark energy.

“This is a hot research topic,” says Olga Botner, a physicist at Sweden’s Uppsala University and a member of the Royal Swedish Academy of Sciences. “The hottest candidate is what is called dark energy. We don’t know what it is.”

In some theories, dark energy is constant and spread uniformly throughout the universe. In others, its density in different places changes over time. Some theorists discard the notion altogether, preferring to explain the mysterious acceleration by tinkering with how gravity itself works.

"This puzzle is the most profound mystery in all of science," says Michael Turner, a theoretical astrophysicist at the University of Chicago. "Maybe not the most important, but certainly the most profound."

Revealing the nature of this hypothetical dark energy will likely require new instruments. Using the South Pole Telescope, astronomers will examine the cosmic microwave background radiation — a faint remnant of the Big Bang — to understand how dark energy fights gravity in shaping groups of galaxies. And a giant camera recently built at the Fermi National Accelerator Laboratory in Batavia, Ill., is destined to be mounted atop a telescope in Chile, where it will take snapshots of very distant supernovas and galaxies to pin down exactly how the expansion of the universe has changed.

NASA's proposed Wide-Field Infrared Survey Telescope was given top priority by an astrophysics panel last year in part because it would provide especially detailed information about dark energy. Already delayed, the project's future remains uncertain due to NASA's tight science budget.

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