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.