A figure that describes the slowing-down of the expansion of the Universe. In a Friedmann universe, the deceleration parameter is simply half the density parameter, Ω‎. A value of q greater than 0.5 indicates that the expansion is decelerating quickly enough for the Universe eventually to collapse. A value less than 0.5 indicates that the expansion will continue forever. In models with a cosmological constant, q can even be negative, indicating an accelerated expansion, as in the inflationary universe. Current observations suggest we live in a Universe with a negative deceleration parameter, i.e. an accelerating Universe.