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The accelerating universe is the observation that the universe appears to be expanding at an accelerated rate. In 1998 observations of Type Ia supernovae suggested that the expansion of the universe is speeding up.[1][2] In the past few years, these observations have been corroborated by several independent sources: the cosmic microwave background[citation needed], gravitational lensing[citation needed], age of the universe[citation needed] and large scale structure[citation needed], as well as improved measurements of the supernovae[3][4].

An expanding universe means that density drops.

(*Not if the accelerating expansion of matter is the cause of the spatial acceleration)

If acceleration continues eventually all galaxies beyond our own local supercluster will redshift so far that it will become hard to detect them, the distant universe will turn dark.
(*Not if the accelerating expansion of matter is the cause of the spatial acceleration)

Models attempting to explain accelerating expansion include the (*recently re-engaged, formerly abandoned) cosmological constant, quintessence, and phantom energy, with the latest WMAP data favoring the cosmological constant. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space.

Phantom energy in a scenario known as the Big Rip causes an exponentially increasing divergent expansion, which overcomes the gravitation of the local group and tears apart our Virgo supercluster, it then tears apart the milky way galaxy, our solar system, and finally even atoms (*Not if the factor of expanding matter is considered in the computation). Measurements of acceleration are thought crucial to determining the ultimate fate of the universe, however we should expect the implications of such a major discovery to develop slowly over many years in the same way the big bang model has continued to develop.

The density of dark matter (*The newly adopted name for the reinstated Cosmological Constant) in an expanding universe disappears more quickly than dark energy (see Equation of State (Cosmology)) and, eventually, the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved but the density of dark energy is nearly unchanged (it is exactly constant for a cosmological constant).

[edit] See also
High-z Supernova Search Team
Supernova Cosmology Project
Brian P. Schmidt
[edit] References
<LI id=_note-0>^ Goldhaber, G. and Perlmutter, S, "A study of 42 type Ia supernovae and a resulting measurement of Omega(M) and Omega(Lambda)", Physics Reports-Review section of Physics Letters 307 (1-4): 325-331 Dec. 1998 <LI id=_note-1>^ Garnavich PM, Kirshner RP, Challis P, et al. "Constraints on cosmological models from Hubble Space Telescope observations of high-z supernovae" Astrophysical Journal 493 (2): L53+ Part 2 Feb. 1 1998 <LI id=_note-Leibundgut>^ B. Leibundgut, J. Sollerman (2001). "A cosmological surprise: the universe accelerates". Europhysics News 32 (4). Retrieved on 2007-02-01.
^ "Confirmation of the accelerated expansion of the Universe", Centre National de la Recherche Scientifique, September 19, 2003. Retrieved on 2006-11-03.
Retrieved from "http://en.wikipedia.org/wiki/Accelerating_universe"

(* Refer, http://forums.delphiforums.com/EinsteinGroupie )