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Our universe could be accelerating.

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The Accelerating universe is the idea that our universe is undergoing accelerated expansion. Accelerating universe suggests distant objects are receding from our galaxy with speeds that increase over time. The accelerating universe is related to the Hubble Law. While the Hubble parameter may be decreasing with time, it is doing so slowly enough that distant objects continue to accelerate away from us.

Universe Accelerating.
The accelerating universe.

In the late 1990s, observations of Type Ia supernovae suggested that the expansion of the universe is accelerating. 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.

If the acceleration continues indefinitely, the ultimate result of this trend will be that galaxies outside the local supercluster will move beyond the cosmic horizon and will no longer be visible. The unusual energy thought to be responsible for the acceleration is called Dark energy. It is "dark" because it is not known what form the energy takes: it is not directly visible and has not been detected in particle physics experiments. The leading candidates are a cosmological constant and quintessence, with recent WMAP data favouring the cosmological constant. The most important property of dark energy is that it has negative pressure and can be distributed relatively homogeneously in space. An even more hypothetical form of energy, known as phantom energy, causes divergent expansion, which will tear apart the Virgo supercluster and cause a Big Rip. Measurements of acceleration are crucial to determining the Ultimate fate of the universe in Big Bang theory.

Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter and baryons. The density of Dark matter 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).

The observation of an accelerating universe presents problems for Dyson's eternal intelligence. This theory relies on a decelerating universe which, for many years, was the dominant model in cosmology since (in the absence Dark energy) the gravitational attraction of ordinary matter in the universe would act to slow the expansion. By seemingly ruling out a Big Crunch, the accelerating universe also presents great problems for Tipler's Omega point.

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