 Standard perturbation theory in cosmology

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.^[ref]