Gravitational Instability for a Multifluid Medium in an Expanding Universe

We consider the gravitational clustering of a multicomponent fluid in an expanding Newtonian universe, taking into account the mutual gravitational interactions of the medium. We obtained a set of exact and approximate solutions for two fluid components and for several fluid components: primordial hydrogen and helium or cosmological massive neutrinos of different masses and/or velocity distribution may be described by these solutions. The ratio between the two fluid perturbations, even if originally equal to unity, changes monotonically with time; this phenomenon leads to different epochs for nonlinear clustering that may be reflected into different epochs of galaxy formation.