Nonlinear growth of density disturbances in an expanding collisionless medium

The evolution of nonlinear density adiabatic disturbances in collisionless expanding medium possessing pressure caused by chaotic motions of particles is considered. The function describing the evolution of density disturbances in such a medium is found out. Relations for estimation of matter density, pressure etc. corresponding to various initial temperatures are obtained. Results may be used for more detailed analysis of combined development of small-scale isothermal fluctuations and large- scale adiabatic ones. The evolution of isothermal fluctuations leads to the formation of collisionless ''gas''. Nonlinear growth of adiabatic disturbances in this ''gas'' causes the formation of ''pancakes'', thus leading to formation of galaxies and/or clusters of galaxies