Discovery of dark energy
On 8 January 1998, researchers announced the startling discovery that the universe’s expansion is speeding up . Saul Perlmutter (right), leader of the Supernova Cosmology Project, made the announcement at a press conference during the American Astronomical Society’s annual meeting. Another team, the High-Z Supernova Search Team, independently confirmed the discovery soon after. Until then, most astronomers had thought that cosmic expansion should be slowing due to the gravitational attraction among stars, galaxies, and other matter. The key to proving that assumption wrong was tracking the distances to and velocities of type Ia supernovae, which are considered standard candles because of their predictable intrinsic brightness. Beyond demonstrating that the universe’s expansion is accelerating, the two teams also showed that the most significant component of the cosmos (as measured by mass–energy) is not matter but dark energy. Scientists still aren’t sure what dark energy is, though it may be described by the cosmological constant that Albert Einstein introduced in his equations of general relativity. In recognition of the discovery of an accelerating universe, half the 2011 Nobel Prize in Physics was awarded to Perlmutter and the other half to two of the leaders of the High-Z team, Brian Schmidt and Adam Riess. Perlmutter detailed the discovery and its implications in the April 2003 issue of Physics Today. (Photo credit: American Astronomical Society, courtesy AIP Emilio Segrè Visual Archives)
Date in History: 8 January 1998
