Two-fluid dark matter models
  We investigate the possibility that dark matter is a mixture of two
non-interacting perfect fluids, with different four-velocities and
thermodynamic parameters. The two-fluid model can be described as an effective
single anisotropic fluid, with distinct radial and tangential pressures. The
basic equations describing the equilibrium structure of the two-fluid dark
matter model, and of the tangential velocity of test particles in stable
circular orbits, are obtained for the case of a spherically symmetric static
geometry. By assuming a non-relativistic kinetic model for the dark matter
particles, the density profile and the tangential velocity of the dark matter
mixture are obtained by numerically integrating the gravitational field
equations. The cosmological implications of the model are also briefly
considered, and it is shown that the anisotropic two-fluid model isotropizes in
the large time limit.
