Dark energy: Encyclopedia II - Dark energy - Evidence for dark energy

Dark energy - Evidence for dark energy

During the late 1990s, observations of type Ia supernovae ("one a") suggested that the expansion of the universe is accelerating. These observations have been corroborated by several independent sources since then: the cosmic microwave background, gravitational lensing, large scale structure as well as improved measurements of the supernovae. All these elements are consistent with the concordance Lambda-CDM model.

The type Ia supernovae provide the most direct evidence for dark energy. Measuring the velocity of receding objects is accomplished easily by measuring the redshift of the receding object. Finding the distance to an object is a more difficult problem, however. It is necessary to find standard candles: objects for which the absolute magnitude is known, so that it is possible to relate the apparent magnitude to the distance. Without standard candles, it is impossible to measure the redshift-distance relation of Hubble's law. Type Ia supernovae are the best known standard candles for cosmological observation, because they are very bright and ignite only when the mass of an old white dwarf star reaches the precisely defined Chandrasekhar limit. The distances to the supernovae are plotted against their velocities, and this is used to measure the expansion history of the universe. These observations indicate that the universe is not decelerating, which would be expected for a matter-dominated universe, but rather is mysteriously accelerating. These observations are explained by postulating a kind of energy with negative pressure (see equation of state (cosmology) for a mathematical explanation): dark energy.

The existence of dark energy, in whatever form, also solves the so-called "missing mass" problem. Measurements of the cosmic microwave background (CMB), most recently by the WMAP satellite, indicate that the universe is very close to flat. For the shape of the universe to be flat, the mass/energy density of the Universe must be equal to a certain critical density. The total amount of matter in the Universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%.

The theory of large scale structure, which governs the formation of structure in the universe (stars, quasars, galaxies and galaxy clusters), also suggests that the density of matter in the universe is only 30% of the critical density.

Adapted from the Wikipedia article "Evidence for dark energy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki