An Etymological Dictionary of Astronomy and Astrophysics

The deduction based on the observation that the most distant → Type Ia supernovae
are fainter than that expected from their → redshifts in a matter-only dominated expanding Universe. The faintness is attributed to larger distances resulting from an accelerating Universe driven by presence of a new component with strongly negative pressure. This
component that makes the Universe accelerate is named → dark energy. The deceleration or acceleration of an expanding Universe, given by the general relativistic equation, is:
R^../R = -(4/3)πGρ(1 + 3w),

where R is the linear → cosmic scale factor of the expanding Universe, G the → gravitational constant, ρ the mean density of the Universe, and w the → equation of state parameter representing dark energy. The expansion accelerates whenever w is more negative than -1/3. The Nobel Prize in Physics 2011 was awarded to the initiators of this concept,
Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess, for their discovery of the accelerating expansion of the Universe through observations of distant supernovae. See also the original paper: Perlmutter et al. 1999, ApJ 517, 565.

See also: → accelerating; → universe.