The deceleration parameter,*q*, indicates the rate at which the expansion of Universe is_{0}*slowing*due to self-gravitation. It is defined by:where

*R*is the scale factor,*R(t)*, of the Universe by which all lengths scale, is the first time derivative (rate of change) of*R*, and is the second time derivative of*R*. In this notation is equivalent to the Hubble parameter,*H*, and its present value is*H*, the Hubble constant._{0}

Recent observations have suggested that the rate of expansion of the Universe is currently accelerating, perhaps due to the effects of dark energy. This yields*negative*values for the deceleration parameter.