As the universe expands the scale factor formula_4 increases, but the density formula_3 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, formula_3 decreases more quickly than formula_23 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 formula_24 and so must have increased by a similar amount to retain the constant value of their product.