In <Cosmology> written by Weinberg, in page 109

nadia8999

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 black-body radition, then what does [itex]\frac{3}{2}n_{B}Nk_{B}T[/itex]means? the energy density of non-relativistic particles? If so, then should the press of non-relativistic particles be zero in the second equation?