# Number Density vs Energy Density of Radiation and Matter

1. ### cp05

11
I was going through my notes preparing for finals (this is my last one hooray!), and I saw that the Number Density of radiation and matter changes at the same rate (N=Na^-3) while the energy density is different (E=Ea^-3 for matter, E=Ea^-4 for radiation). I think I get why the energy densities for matter/radn are different (matter exists in 3 dimensions...while radiation exists in 4 because of time...is that remotely correct?), but I don't understand why the number densities are the same.

Thanks for the help!