Astrophysics (Index) | About |
In cosmology, the deceleration parameter (often represented by q) is a quantification of the deceleration (slowing) of the expansion of the universe:
a d²a/dt² q ≝ - ————————— (da/dt)²
Determinations (specifically, comparing counts of Type Ia supernovae at various redshifts) show the universe's expansion rate is increasing, and the deceleration parameter is in the general range of -0.55 or -0.60, its negative value indicating an acceleration in the universe's expansion. Thus there must be a force counteracting the inward pull of the universe's gravity (which would tend to slow expansion), and the term dark energy was coined for whatever is providing it. A parameter for deceleration rather than acceleration was presumably adopted by cosmologists before such a determination was made and it was presumed that the universe's expansion was slowing.