Photon quadrupole in tight coupling with polarization
= Photon quadrupole in tight coupling with polarization
For the expansion $\Theta(\mu)=\sum_\ell(-i)^\ell\Theta_\ell P_\ell(\mu)$ without a $(2\ell+1)$ factor, the leading polarization relation is $E_2=-\sqrt6\,\Theta_2/4$. The <photon Boltzmann hierarchy> then gives $\Theta_2=-8k\Theta_1/(9\dot\tau)$. This small <photon quadrupole> supplies the shear-viscosity part of <Silk damping>.