In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. This matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a, while radiation is anything whose energy density scales to the inverse fourth power of the scale factor (ρ ∝ a). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge of the box decreases the density (and hence energy density) by a factor of eight (2). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.