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Number Density vs Energy Density of Radiation and Matter 
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#1
May1811, 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 time...is that remotely correct?), but I don't understand why the number densities are the same.
Thanks for the help! 


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