### The Award:

The 2011 Nobel Prize in Physics is awarded "

*for the discovery of the accelerating expansion of the Universe through observations of distant supernovae*" with one half to Saul Perlmutter and the other half jointly to Brian P. Schmidt and Adam G. Riess.### The Science:

In the 1920's, astronomer Edwin Hubble discovered that the universe was expanding. In 1998, two different research teams were trying to measure the expansion rate of the universe. One group was led by Saul Perlmutter, the other group by Adam G. Riess & Brian P. Schmidt. Both teams got a result that they didn't match expectations. They expected the expansion rate to be slowing down, but instead it was speeding up!

### The Big Deal:

Albert Einstein's original work on general relativity had predicted an expanding universe, but he thought this was nonsense. He added a term to his equation, called the cosmological constant to remove this expansion. After Hubble discovered the expansion, Einstein (and many others) believed that the cosmological constant wasn't needed, so scientists spent years researching the expansion of the universe to prove that this term equaled zero. This research, however, proved that there was some sort of energy within the universe that is pushing it apart.

### More on Dark Energy:

This energy, apparently inherent in the very fabric of the universe, is often called dark energy. Research performed over the last 13 years - including the WMAP research into the cosmic microwave background (CMB) radiation, which earned the 2006 Nobel Prize in Physics - has confirmed these results and, in fact, has indicated that dark energy makes up nearly 75% of the energy within our entire universe.

### How the Discovery was Made:

The research teams looked for a certain type of supernova, called a supernova 1a, which happens when a white dwarf star gains enough extra matter to make it unstable. This sort of supernova is so powerful that it can outshine even a galaxy. Since we understand this sort of supernova well, it can be used as a "standard candle" to measure the distance to the supernovas and this measurement can be used to calculate the expansion rate of the universe over time.

### About Saul Perlmutter:

Saul Perlmutter is a UC Berkeley astrophysics professor and does research at Lawrence Berkeley National Laboratory, where he heads the Supernova Cosmology Project. He graduated from Harvard with honors in 1981 and went on to receive a 1986 PhD from UC Berkeley. He has received other prizes, including the Gruber Cosmology Prize (also with Schmidt & Riess) and the Albert Einstein Medal, for his work discovering the universal acceleration due to dark energy.

### About Brian P. Schmidt:

Brian P. Schmidt is an astrophysicist at the Australian National University's Mount Stromlo Observatory. Schmidt grew up in Alaska, but went to the University of Arizona for his bachelor's degree. He earned a PhD in 1993 from Harvard University. He was a lead researcher on the High-z Supernova Search Team, the first team to announce discovery of universal expansion due to dark energy. This discovery has earned Schmidt numerous awards, including the Gruber Cosmology Prize in 2007 (also along with Perlmutter & Riess).

### About Adam G. Riess:

Adam G. Riess is an astrophysicist at Johns Hopkins University. He graduated from the Massachusetts Institute of Technology in 1992. He earned his PhD from Harvard in 1996. Two years later, he was a lead researcher on the High-z Supernova Search Team, along with Brian P. Schmidt. Riess has received the Gruber Cosmology Prize and a MacArthur "Genius" Grant, as well as other awards, for this discovery.