Could pressureless dark matter have pressure?

A two-fluid dark matter model, in which dark matter is represented as a two-component fluid thermodynamic system, without interaction between the constituent particles of different species, and with each distinct component having a different four-velocity, was recently proposed in Harko and Lobo [T. Harko, F.S.N. Lobo, Phys. Rev. D 83 (2011) 124051]. In the present paper we further investigate the two-fluid dark matter model, by assuming that the two dark matter components are pressureless, non-comoving fluids. For this particular choice of equations of state the dark matter distribution can be described as a single anisotropic fluid, with vanishing tangential pressure, and non-zero radial pressure. We investigate the properties of this model in the region of constant velocity galactic rotation curves, where the dynamics of the test particles is essentially determined by the dark matter only. By solving the general relativistic equations of mass continuity and hydrostatic equilibrium we obtain the geometric and physical parameters of the dark matter halos in the constant velocity region in an exact analytical form. The general, radial coordinate dependent, functional relationship between the energy density and the radial pressure is also determined, and it differs from a simple barotropic equation of state.

► We model dark matter as a mixture of two fluids.
► Then dark matter can be described as a single anisotropic fluid.
► We analyze dark matter in the constant tangential velocity region.
► The distributions of the density and effective pressure are obtained.
► Dark matter models with pressure give a better description of observations.