Models ================ Homogeneous and Isotropic Neutrino Gas ------------------------------------------------------------ :math:`\vec P_\omega` can be time-independent. Regarding the discussion in Pictures, we know immediately that :math:`\vec H` is aligned or anti-aligned with :math:`\vec P_\omega` since .. math:: \vec H \times \vec P_\omega = 0, as :math:`\vec P_\omega` is time-independent. Anyway the equation we need to solve is then .. math:: \vec a What to expect? ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Without solving the equation, we know that Solving Eqns ~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. admonition:: What is special in this model? :class: note To be added.