When two teams of scientists announced in 1998 that the expansion of the Universe was not slowing down due to gravity but was in fact accelerating, the worldwide scientific community was shocked. The discovery turned many of the prevailing assumptions about the universe upside down. Looking back, perhaps the only thing that wasn't a surprise was that the Nobel Prize Committee should take notice.
Last Tuesday the Swedish Academy of Sciences announced that the 2011 Nobel Prize in Physics would go to three American-born scientists from two rival teams: physicist Saul Permutter, head of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory and the University of California, Berkeley, would receive half of the prize, while Brian P. Schmidt, head of the High-z Supernova Search Team and an astronomer at the Research School of Astronomy and Astrophysics at the Australian National University, Weston Creek, would share the other half with a colleague who wrote the original paper announcing the team's findings in 1998, astronomer Adam G. Riess of Johns Hopkins University and the Space Telescope Science Institute.
Despite popular belief, the two teams did not "discover" dark energy. As Perlmutter points out in the short film above, "People are using the term 'dark energy' basically as a place holder to describe any explanation for why it is that we seem to be seeing the universe's expansion getting faster and faster." What is actually known is that the universe has been expanding for as far back as we can observe, and about 7 billion years ago--roughly half the estimated age of the universe--the expansion began to accelerate.
"Why is it speeding up?" Perlmutter asked during a press conference on the morning his Nobel Prize was announced. "It could be that most of the universe is dominated by a dark energy that pervades all of space and is causing this acceleration. It could be, perhaps even more surprising, that Einstein's Theory of General Relativity needs a little bit of a tweak, perhaps acting slightly differently on these very large scales of the universe. But at this moment I would say that the question is wide open."
The 11-minute documentary above, produced in 2008 by KQED in San Francisco, gives a good overview of how Perlmutter and his rivals measured the red-shift and brightness of light from Type 1a supernovae to plot the universe's rate of expansion across billions of years. For an in-depth history of the project, you can read this three-part article from the Berkeley Lab. Or, if you only have a minute (1:39 to be exact) you can watch this "Minute Physics" episode narrated by Caltech physicist Sean Carroll.