
#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! 


Register to reply 
Related Discussions  
Effect of ratio of energy density of matter and radiation on redshift  Advanced Physics Homework  1  
Mass density of stars from number density  Astrophysics  0  
At what redshift does energy density in matter equal energy density in radiation?  Cosmology  5  
Calculating Energy density in matter & radiation  Cosmology  1  
calculating the redshift at which radiation energy density equaled mass density  Cosmology  4 