= Standard perturbation theory in cosmology
{title2=SPT}
\b[Standard perturbation theory in cosmology] expands the matter <density contrast> and peculiar-velocity divergence in powers of a small initial density field. The linear term evolves independent <Fourier modes>; higher terms couple them and predict departures from Gaussian statistics. The usual fluid treatment assumes pressureless, irrotational, single-stream motion. It is useful before strongly nonlinear evolution and <shell crossing> invalidate that approximation.<Large-Scale Structure of the Universe and Cosmological Perturbation Theory by F. Bernardeau and others>[Sections2.2–2.4: fluid equations and Eulerian nonlinear expansion]{ref}
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