Dark energy discovery wins physics Nobel

Three astrophysicists honored for grasping universe's accelerating expansion

Dark energy discovery wins physics Nobel

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

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 class of distant supernovas. Because these exploded white dwarf stars tend to have the same brightness, 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 — a consequence of Einstein’s theory of general relativity, in which gravity puts the brakes on the expansion of the universe.

So did Brian Schmidt of the Mount Stromlo Observatory and the Australian National University in Weston Creek and Adam Riess of the Johns Hopkins University and Space Telescope Science Institute in Baltimore, who will split the other half of the prize. Their High-z Supernova Search Team, launched in 1994, also used large telescopes to search big 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 and farther away than expected. Instead of slowing down, the expansion of the universe had been speeding up for billions of years, starting at about the same time that the solar system first 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 of supernovas, also suggest that about three-quarters of the universe’s mass-energy may be invisible dark energy that is driving the accelerating expansion. 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.”

Revealing the nature of this hypothetical dark energy will likely require new instruments — such as a giant camera recently constructed at the Fermi National Accelerator Laboratory in Batavia, Ill. This 570 megapixel camera, destined to be mounted atop a telescope in Chile, will survey a broad area of the night sky, taking snapshots of very distant supernovas and galaxies to pin down exactly how the expansion of the universe has changed over its history.