Gravitational instability: An approximate theory for large density perturbations.

An approximate solution is given for the problem of the growth of perturbations during the expansion of matter without pressure. The solution is qualitatively correct even when the perturbations are not small. infinite density is first obtained on disc-like surfaces by unilateral compression. The following layers are compressed first adiabatically and then by a shock wave. Physical conditions in the compressed matter are analysed. Key words: Galaxies formation - Cosmology - Gravitational instability