= Long-wavelength approximation in cosmology
{title2=$k/(aH)\ll1$}
= Cosmological long-wavelength approximation
{synonym}
When physical wavelengths greatly exceed the <Hubble radius>, spatial derivatives are small compared with temporal expansion scales. A cosmological gradient expansion then approximates neighbouring regions as locally homogeneous universes, while retaining finite perturbation amplitudes. This does not alone discard frozen <tensor cosmological perturbations>.
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