Mathematically, the expansion of the universe is quantified by the scale factor, a , which is proportional to the average separation between objects, such as galaxies. The scale factor is a function of time and is conventionally set to be a = 1 at the present time. Because the universe is expanding, a is smaller in the past and larger in the future. Extrapolating back in time with certain cosmological models will yield a moment when the scale factor was zero; our current understanding of cosmology sets this time at 13.787 ± 0.020 billion years ago. If the universe continues to expand forever, the scale factor will approach infinity in the future. It is also possible in principle for the universe to stop expanding and begin to contract, which corresponds to the scale factor decreasing in time.