Conserved quantities in the perturbed Friedmann world model

The evolutions of linear structures in a spatially homogeneous and isotropic world model are characterized by some conserved quantities: the amplitude of gravitational wave is conserved in the super-horizon scale, the perturbed three-space curvature in the comoving gauge is conserved in the super-sound-horizon scale, and the angular momentum of rotational perturbation is generally conserved.