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The deceleration parameter $q$ in cosmology is a dimensionless measure of the cosmic acceleration of the expansion of the universe. It is defined by:

- $q\; stackrel\{mathrm\{def\}\}\{=\}\; -frac\{ddot\{a\}\; a\; \}\{dot\{a\}^2\}$

The Friedmann acceleration equation can be written as

- $3frac\{ddot\{a\}\}\{a\}\; =-4\; pi\; G\; (rho+3p)=-4pi\; G(1+3w)rho,$

This can be rewritten as

- $q=frac\{1\}\{2\}(1+3w)left(1+K/(aH)^2right)$

The derivative of the Hubble parameter can be written in terms of the deceleration parameter:

- $frac\{dot\{H\}\}\{H^2\}=-(1+q).$

Observations of the cosmic microwave background demonstrate that the universe is very nearly flat, so:

- $q=frac\{1\}\{2\}(1+3w)$

Before the first indications of an accelerating universe, in 1998, it was thought that the universe was dominated by dust with negligible equation of state, $w\; approx\; 0$. This had suggested that the deceleration parameter was equal to one half; the experimental effort to confirm this prediction led to the discovery of acceleration.

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Last updated on Friday July 25, 2008 at 12:10:06 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Friday July 25, 2008 at 12:10:06 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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