As the universe expands the scale factor increases, but the density decreases as matter (or energy) becomes spread out. For the standard model of the universe which contains mainly matter and radiation for most of its history, decreases more quickly than increases, and so the factor will decrease. Since the time of the Planck era , shortly after the Big Bang, this term has decreased by a factor of around [ 4 ] and so must have increased by a similar amount to retain the constant value of their product.