On the onset of the Jeans instability in a two-component fluid
  Conditions for the establishment of small density perturbations in a self-gravitating two component fluid mixture are studied using a dynamical system approach. It is shown that besides the existence of exponentially growing and decaying modes, which are present for values of the perturbation wave-number $k$ smaller than a critical value $k_{_M}$, two other, pure oscillatory, modes exist at all scales. For $k < k_{_M}$, the growing mode always affects both components of the fluid and not only one of them. Due to the existence of a resonance between the baryonic and the dark perturbations, it is shown that the onset of structure formation in the post recombination epoch is substantially enhanced in a narrow scale band around another critical value $k_{c}$. For dark matter particles having a mass $\sim 30$ eV, the corresponding critical mass scale for the establishment of density perturbations at the time of recombination is of the same order of magnitude as the galactic one. 
J. P. M. de Carvalho and P. G. Macedo (Grupo de Cosmologia do   C.A.U.P.)