Gravity. It's the law.

(CNN) — Two scientists from the United States and one from Australia have won the 2011 Nobel Prize in Physics for their independent discoveries that our universe is apparently expanding at an accelerating rate, the Royal Swedish Academy of Sciences announced Tuesday. The expansion is happening apparently because of a mysterious force known as dark energy, according to the findings. Half of the award went to Saul Perlmutter from Lawrence Berkeley National Laboratory and University of California, Berkeley. The other half went to Brian P. Schmidt of Australian National University and Adam G. Riess of Johns Hopkins University and the Space Telescope Science Institute. The prize in physics is worth 10 million Swedish kronor (about $1.44 million). The two teams, working separately, each measured the light coming from a specific kind of exploding star, or supernova, in what began as an effort to confirm expectations that the expansion of the universe was slowing down 14 billion years after the Big Bang.