Scalar neutrino anisotropic stress
= Scalar neutrino anisotropic stress
{title2=$\Pi_\nu=-3\Theta_2/5$}
Using the unweighted <neutrino Boltzmann hierarchy> convention and $\Pi_{ij}=-(4\bar\rho_\nu/3)\Pi_\nu(\hat k_i\hat k_j-\delta_{ij}/3)$, the scalar <scalar anisotropic stress> is $\Pi_\nu=-3\Theta_2/5$. Only the quadrupole contributes, since the trace-free tensor $e_ie_j-\delta_{ij}/3$ has angular degree two. Different stress and multipole sign conventions change this displayed coefficient.