Number Density vs Energy Density of Radiation and Matter

by cp05
Tags: density, energy, matter, number, radiation
cp05 is offline
May18-11, 03:24 PM
P: 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 that remotely correct?), but I don't understand why the number densities are the same.

Thanks for the help!
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