# Neutrino mass as counted in Dark Matter

If I try to add up neutrino masses (let's assume 1 eV rest mass equivalent each) to count as DM, do I use the rest mass or relativistic mass?

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The total energy density is the $T_{00}$ term, but the neutrino momentum also contributes to other $T_{0i}$ terms. –  John Rennie Jan 3 '13 at 12:44
@JohnRennie True, but if the neutrino momentum distribution is isotropic then the net $T_{0i}=T_{i0}=0$. You will get pressure terms down the diagonal however: $T_{ij} \propto p \delta_{ij}$. For an anisotropic distribution then kinetic theory is probably the way to go. Whether the OP needs this heavy machinery depend on the purpose. To me it sounded like he just wanted to tally up the neutrino contribution to the dark matter. Including $\rho$ and $p$ should suffice for this. –  Michael Brown Jan 3 '13 at 13:38