For almost a hundred years, it has been widely accepted that the Universe is expanding, and that it's been doing so ever since the Big Bang occurred approximately 14 billion years ago. It was initially assumed that the rate of expansion was slowly declining. What came as a surprise to many scientists, however, was the relatively recent announcement that the rate is in fact increasing. That was the remarkable conclusion reached by three physicists located in two countries, and it has just earned them the Nobel Prize in Physics for 2011.
Half of the 10 million Swedish krona (about US$1.5 million) in prize money went to Saul Perlmutter, based out of the University of California, Berkeley's Lawrence Berkeley National Laboratory. The other half went jointly to Brian P. Schmidt of Australian National University, and Adam G. Riess of Johns Hopkins University and Space Telescope Science Institute.
In 1988, Perlmutter set about leading his Supernova Cosmology Project team to visually locate the most distant supernovae. Schmidt launched the competing High-z Supernova Search Team in 1994, in collaboration with Riess.
Both teams were searching for a specific variety of supernova, known as type Ia (with an upper-case "i"). This sort of supernova results from the explosion of a white dwarf, a compact star that is as small as the earth but as heavy as the Sun. Just one type Ia supernova can emit as much light as an entire galaxy, over the course of a few weeks.
The two teams found 50 such supernovae in total, although their light was weaker than anticipated - this was apparently because they were farther from the earth than expected, which indicated that the Universe's rate of expansion is increasing. The two teams were bolstered by the fact that they had each separately reached the same conclusion, based on the same data.
Although the expansion of the Universe is thought to be driven by dark energy, just what dark energy is is still anyone's guess. Perhaps another Nobel prize awaits the people who can provide the answer.