As the universe expands the scale factor a increases, but the density ρ ρ 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, ρ ρ decreases more quickly than a 2 increases, and so the factor ρ ρ a 2 will decrease. Since the time of the Planck era, shortly after the Big Bang, this term has decreased by a factor of around 10 60 , and so ( Ω Ω − − 1 − − 1 ) must have increased by a similar amount to retain the constant value of their product.