deceleration parameter

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