The deceleration parameter, *q _{0}*, indicates the rate at which the expansion of Universe is

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 _{0}*, the Hubble constant.

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

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