# deceleration parameter

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## deceleration parameter

(dee-sel-ĕ-ray -shŏn) Symbol: q 0. The rate at which the expansion of the Universe changes with time as a result of self-gravitation. It is dimensionless and can be given in terms of the cosmic scale factor, R:
q 0 = –R (d2R /dt 2)/(dR /dt )2

Values of q 0 less than 0.5 are associated with an ever-expanding open Universe; values exceeding 0.5 imply that the Universe is closed and will collapse at a finite time in the future. If q 0 is equal to 0.5 then the Universe is infinite and space is Euclidean. See also mean density of matter; Universe.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
Several cosmological models were obtained with constant deceleration parameter, where the matter is in the form of perfect fluid or ordinary matter.
obtained the deceleration parameter of the present universe to be [q.sub.0] ~ -0.48 for the flat spacetime (for a closed spacetime, [q.sub.0] is smaller, e.g.
For best performance, the assumed deceleration parameter of the algorithm must be less than or equal to that of the driver and the assumed reaction time must be greater than or equal to that of the driver.
Allan Sandage has described cosmology as "the search for two numbers." He meant the Hubble constant, [H.sub.0] (the cosmic expansion rate), and the deceleration parameter, [q.sub.0] (the slowing down or speeding up of that expansion).
One of these numbers is the deceleration parameter, [q.sub.0].
The deceleration parameter q can be obtained by taking the time derivative of (2), which lead to
The classical deceleration parameter q(t) is found to have the value q([T.sub.0]) = -0.570 for the current Universe age and the current radius of curvature should be R([T.sub.0]) > 100 Gly.
Ever since, cosmologists have tried to gauge the expansion: its present-day rate (mathematically described by the Hubble constant, or [H.sub.0]), and the change in that rate with time (described by the deceleration parameter, or [q.sub.0]).
The deceleration parameter depicts that the rate of expansion is growing for closed universe, while it is slowing down for open universe.
One approach involves studying the slowing down of galaxies' outrushing motions, a measure of which is the so-called deceleration parameter, |q.sub.0~.

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