In <Cosmology> written by Weinberg, in page 109

In <Cosmology> written by Weinberg, in page 109, it says that

$\epsilon=\alpha_{B}T^4+\frac{3}{2}n_{B}Nk_{B}T$
$p=\frac{1}{3}\alpha_{B}T^4+n_{B}Nk_{B}T$

$\alpha_{B}T^4$is the average energy density of black-body radition, then what does $\frac{3}{2}n_{B}Nk_{B}T$means? the energy density of non-relativistic particles? If so, then should the press of non-relativistic particles be zero in the second equation?

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 Tags cosmology, weinberg