Three members of the American Astronomical Society (AAS) have been named recipients of the 2011 Nobel Prize in Physics. The Royal Swedish Academy of Sciences has announced that half of the SEK 10 million ($1.44 million) award will go to Saul Perlmutter (Lawrence Berkeley National Laboratory and University of California, Berkeley) and half will be shared by Brian P. Schmidt (Australian National University) and Adam G. Riess (Johns Hopkins University and Space Telescope Science Institute).

The 2011 Nobel Prize in Physics is being given "for the discovery of the accelerating expansion of the universe through observations of distant supernovae."

Perlmutter led the Supernova Cosmology Project, and Schmidt and Riess led the rival High-z Supernova Search Team. In the 1990s these large international collaborations sought to determine how fast the expansion of the universe was slowing down due to the gravitational pull of its galaxies. They measured the expansion rate over the history of the universe using Type Ia supernovae, stellar explosions so luminous that they can be seen across vast cosmic distances.

To their astonishment, both teams found that the expansion isn't slowing down at all - it's speeding up! Announced in 1998, this discovery was dubbed "the accelerating universe" and led to the concept of dark energy, a mysterious cosmic force that acts like antigravity or negative pressure. Figuring out what dark energy is and how it works is one of the biggest challenges facing astrophysicists in the 21st century.

"The work being celebrated was published in our journals and presented at our meetings, with a key news briefing taking place at our January 1998 conference," says Kevin Marvel, AAS Executive Officer. In 2002 the AAS honored Riess with the Helen B. Warner Prize, which recognizes a significant contribution to astronomy by a recent Ph.D. recipient younger than age 36 (Reiss, born in 1969, earned his doctorate at Harvard University in 1996).

"We've congratulated everyone involved," says Marvel. "This is a great day for the AAS, for our nation, and for astrophysicists worldwide."

"This is the third time in 10 years that the Nobel Prize in Physics has gone to astronomers," notes AAS President Debra Elmegreen (Vassar College).

"Astronomy is a relatively small field, but it produces some of the most exciting and astounding results in all of science. The discovery of the accelerating universe and dark energy by Saul, Brian, Adam, and their colleagues is a particularly noteworthy example, and I'm tremendously gratified to see it recognized as a scientific landmark by the Nobel Prize committee."