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 obtain 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 mass 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.   Additional note: see also D. Fargion (Nuovo Cimento 65,1981,316), L.V. Solov'eva & I.S. Nurgaliev (Sov. Astron. 29(3),1985,267-272), L.V. Solov'eva & A.A. Starobinskiy (Sov. Astron. 29(4),1985,367-371), I.S. Nurgaliev (Sov. Astron. Lett. 12(2),1986,73-76) and compare with astro-ph/9605133. 
D. Fargion (Phys. Dept. University of Rome "La Sapienza"-INFN)