Deceleration Parameter
(a) Quantity designating the rate at which the expansion of the universe is slowing down, owing to the braking effect of the galaxies' gravitational tug on one another. It is a function of the cosmic matter density.
deceleration parameter (q0) - (n.)
A particular measure of the rate at which the expansion of the universe is slowing down.
declination - (n.) ...
Given that the deceleration parameter is close to the Einstein-de Sitter value of qo = 0.5, it is not surprising that evolution of the scale factor a(t) from the last minimum until now follows the EdS curve quite closely.
and the density or deceleration parameter
For both of these, a subscript 0 indicates their evaluation at the present epoch. q is related to the open or closed nature of the universe.
In a universe with a deceleration parameter equal to zero, it follows that H = 1/t, where t is the time since the Big Bang.
The research program carried out in the 1920s and 1930s by Edwin Hubble to measure the recessional speeds and distances of a large number of galaxies and to attempt to measure the deceleration parameter.
One can estimate the age of the Universe from the Hubble constant and the "deceleration parameter", which in turn depends on the energy densities in the Universe (usually expressed as Omega and Lambda). The article at ...
 See also: Universe, Energy, Light, Gravitation, Density
  
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