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In <Cosmology> written by Weinberg, in page 109 
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Jul2611, 12:14 AM

P: 10

In <Cosmology> written by Weinberg, in page 109, it says that
[itex]\epsilon=\alpha_{B}T^4+\frac{3}{2}n_{B}Nk_{B}T[/itex] [itex]p=\frac{1}{3}\alpha_{B}T^4+n_{B}Nk_{B}T[/itex] [itex]\alpha_{B}T^4[/itex]is the average energy density of blackbody radition, then what does [itex]\frac{3}{2}n_{B}Nk_{B}T[/itex]means? the energy density of nonrelativistic particles? If so, then should the press of nonrelativistic particles be zero in the second equation? 


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