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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, who in 1998 as leaders of the Supernova Cosmology Project (Perlmutter) and the High-Z Supernova Search Team (Schmidt and Riess) discovered the accelerating expansion of the Universe through observations of distant ("High-Z") supernovae.
The simplest evidence for accelerating expansion comes from the brightness/redshift relation for distant Type-Ia supernovae; these are very bright exploding white dwarfs, whose intrinsic luminosity can be determined from the shape of the light-curve. Repeated imaging of selected areas of sky is used to discover the supernovae, and then followup observations give their peak brightness and redshift. The peak brightness is then converted into a quantity known as luminosity distance (see distance measures in cosmology for details).
For supernovae at redshift less than around 0.1, or light travel time less than 10 percent of the age of the universe, this gives a nearly linear redshift/distance relation due to Hubble's law. At larger distances, since the expansion rate of the universe has generally changed over time, the distance/redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time. The full calculation requires integration of the Friedmann equation, but the sign of the deviation can be given as follows: the redshift directly gives the cosmic scale factor at the time the supernova exploded, for example a supernova with a measured redshift implies the Universe was of its present size when the supernova exploded. In an accelerating universe, the universe was expanding more slowly in the past than today, which means it took a longer time to expand from 2/3 to 1.0 times its present size compared to a non-accelerating universe. This results in a larger light-travel time, larger distance and fainter supernovae, which corresponds to the actual observations: when compared to nearby supernovae, supernovae at substantial redshifts 0.2 - 1.0 are observed to be fainter (more distant) than is allowed in any homogeneous non-accelerating model.
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, X-ray properties of galaxy clusters and observational Hubble constant (H(z)) data.
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 has generally been known as the Standard Model of Cosmology from 2003 through the present, since it is the simplest model in good agreement with a variety of recent observations.
Alternatively, modification of the laws of gravity have been proposed instead. Some authors (e.g. Benoit-Lévy & Chardin, Hajdukovic, Villata) have argued that the universe expansion acceleration could be due to a repulsive gravitational interaction of antimatter.
Another type of model, the backreaction conjencture, was proposed by cosmologist Syksy Räsänen: the rate of expansion is not homogenous, but we are coincidentally in a region where expansion is faster than the background. In this model, inhomogeneities in the early universe cause the formation of walls and bubbles, where the inside of a bubble has less matter than on average. According to general relativity, space is less curved than on the walls, and thus appears to have more volume and a higher expansion rate. If we live inside such a bubble, then it would appear that the universe is expanding at an accelerating rate. Räsänen does not consider the model likely, but without any falsification, it must remain a possibility.
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.
- 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 Journal 517 (2): 565–86. arXiv:astro-ph/9812133. Bibcode:1999ApJ...517..565P. doi:10.1086/307221.
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- "Nobel physics prize honours accelerating Universe find". BBC News. October 4, 2011.
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- A. Benoit-Lévy and G. Chardin, Introducing the Dirac-Milne universe, Astronomy and Astrophysics 537, A78 (2012)
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- M. Villata, On the nature of dark energy: the lattice Universe, 2013, Astrophysics and Space Science 345, 1. Also available here