= Cosmological perturbation theory
{wiki}
<Cosmological perturbation theory> studies departures of the <metric tensor> and matter fields from a homogeneous and isotropic <FRW metric>, order by order. <Linear cosmological perturbation theory> keeps only first-order terms. Higher orders describe nonlinear coupling between perturbations. <Nonlinear cosmological perturbation theory> also uses expansions that allow finite amplitudes, such as the <long-wavelength approximation in cosmology>.
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