= Tensor cosmological perturbation
{title2=$h_{ij}$}
A tensor cosmological perturbation is the transverse, trace-free part of a metric perturbation about a <Friedmann-Lemaitre-Robertson-Walker metric>. In conformal coordinates, its spatial metric has the form $g_{ij}=a^2(\delta_{ij}+h_{ij})$, with $\partial_i h_{ij}=0$ and $h_{ii}=0$. Its two independent polarizations describe a <gravitational wave>. <Neutrino tensor anisotropic stress> can damp its evolution through <neutrino free streaming>.
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