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The accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor has a positive second derivative, so that the velocity at which a distant galaxy is receding from us should be continuously increasing with time. In 1998, observations of type Ia supernovae also suggested that the expansion of the universe has been accelerating since around redshift of z~0.5. The 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics were both awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for the 1998 discovery of the accelerating expansion of the Universe through observations of distant supernovae.
After the initial discovery in 1998, these observations were corroborated by several independent sources: the cosmic microwave background radiation and large scale structure, apparent size of baryon acoustic oscillations, age of the universe, as well as improved measurements of supernovae and X-ray properties of galaxy clusters.
Explanatory models 
Models attempting to explain accelerating expansion include some form of dark energy, dark fluid or phantom energy. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space. The simplest explanation for dark energy is that it is a cosmological constant or vacuum energy; this leads to the Lambda-CDM model, which is generally known as the Standard Model of Cosmology as of 2003-2013, since it is the simplest model in good agreement with a variety of recent observations.
Theories for the consequences to the universe 
As the Universe expands, the density of radiation and ordinary and dark matter declines more quickly than the density of dark energy (see equation of state) and, eventually, dark energy dominates. Specifically, when the scale of the universe doubles, the density of matter is reduced by a factor of 8, but the density of dark energy is nearly unchanged (it is exactly constant if the dark energy is a cosmological constant).
Current observations indicate that the dark energy density is already greater than the mass-energy density of radiation and matter (including dark matter). In models where dark energy is a cosmological constant, the universe will expand exponentially with time from now on, coming closer and closer to a de Sitter spacetime. In this scenario the time it takes for the linear size scale of the universe to expand to double its size is approximately 11.4 billion years. 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.
In other models, the density of dark energy changes with time. In quintessence models it decreases, but more slowly than the energy density in ordinary matter and radiation. In phantom energy models it increases with time, leading to a big rip.
See also 
- High-z Supernova Search Team
- Supernova Cosmology Project
- Metric expansion of space
- List of multiple discoveries
- Cosmological constant
- Scale factor (cosmology)
- Friedmann–Lemaître–Robertson–Walker metric
- Jones, Mark H.; Robert J. Lambourne (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 244. ISBN 978-0-521-83738-5.
- Is the universe expanding faster than the speed of light? (see final paragraph)
- Adam G. Riess et al. (Supernova Search Team) (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant". Astronomical J. 116 (3): 1009–38. arXiv:astro-ph/9805201. Bibcode:1998AJ....116.1009R. doi:10.1086/300499.
- S. Perlmutter et al. (The Supernova Cosmology Project) (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae". Astrophysical J. 517 (2): 565–86. arXiv:astro-ph/9812133. Bibcode:1999ApJ...517..565P. doi:10.1086/307221.
- Riess, A. G., et al. 2004, Astrophysical Journal, 607, 665
- "Nobel physics prize honours accelerating Universe find". BBC News. October 4, 2011.
- "The Nobel Prize in Physics 2011". Nobelprize.org. Retrieved 2011-10-06.
- Spergel, D. N., et al. 2003, Astrophysical Journal Supplement, 148, 175
- Dark energy is real, Swinburne University of Technology, 19 May 2011
- Chaboyer, B., & Krauss, L. M. 2002, Astrophysical Journal Letters, 567, L4
- Wood-Vasey, W. M., et al. 2007, Astrophysical Journal, 666, 694
- Astier, P., et al. 2006, Astronomy and Astrophysics, 447, 31