= Gravitational-wave damping by free-streaming neutrinos
For either tensor <helicity>, <neutrino free streaming> produces the memory equation
$$
\ddot h+2\mathcal H\dot h+k^2h
=-24\mathcal H^2f_\nu\int_0^\eta
K(k(\eta-\eta'))\dot h(\eta')\,d\eta',
\qquad
f_\nu=\frac{\bar\rho_\nu}{\bar\rho_{\rm tot}}.
$$
The <neutrino tensor free-streaming kernel> encodes the angular propagation of the perturbation. Energy transferred to neutrino directional anisotropy damps the oscillation amplitude of a <gravitational wave> after horizon entry. This effect was calculated by https://arxiv.org/abs/astro-ph/0306304[Weinberg].
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