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The accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In 1998 observations of Type Ia supernovae suggested that the expansion of the universe is accelerating.

Corroboration

In the past few years, these observations have been corroborated by several independent sources: the cosmic microwave background, gravitational lensing, age of the universe and large scale structure, as well as improved measurements of the supernovae.

Density drops

An expanding universe means that density drops due to continual space being added between all matter. If acceleration continues, eventually all galaxies beyond our own Local supercluster will redshift so far that it will become hard to detect them, and the distant universe will turn dark.

Explanatory models

Models attempting to explain accelerating expansion include some form of dark energy: cosmological constant, quintessence, or phantom energy, with the latest WMAP data favouring the cosmological constant. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space.

Divergent expansion

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. 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.

Dark energy dominates

As the Universe expands, the density of dark matter declines more quickly than the density of dark energy (see equation of state) 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).

See also

• High-z Supernova Search Team
• Supernova Cosmology Project
• Brian P. Schmidt

References

1. 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
2. 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
3. arXiv:astro-ph/0604051v2
4. B. Leibundgut, J. Sollerman (2001). "A cosmological surprise: the universe accelerates". Europhysics News. 32 (4): 121. doi:10.1051/epn:2001401. Retrieved 2007-02-01.
5. "Confirmation of the accelerated expansion of the Universe". Centre National de la Recherche Scientifique. September 19, 2003. Retrieved 2006-11-03.

• Physical cosmology