Photon temperature multipole
= Photon temperature multipole
{title2=$\Theta_\ell$}
For Fourier wavevector $\mathbf k$ and photon direction $\widehat{\mathbf p}$, the temperature perturbation is expanded as
$$
\Theta(\mu)=\sum_{\ell=0}^{\infty}(-i)^\ell(2\ell+1)
\Theta_\ell P_\ell(\mu),
\qquad
\mu=\widehat{\mathbf k}\mathbin\cdot\widehat{\mathbf p}.
$$