Could pressureless dark matter have pressure?

TL;DR

This paper explores a two-fluid dark matter model where both components are pressureless and non-comoving, resulting in an anisotropic fluid description that can explain galactic rotation curves through exact analytical solutions.

Contribution

It introduces a novel two-fluid dark matter model with pressureless, non-comoving components leading to anisotropic fluid behavior, and derives exact solutions for galactic rotation regions.

Findings

Dark matter distribution can be modeled as a single anisotropic fluid.

Exact analytical solutions for dark matter halos in constant velocity regions.

The energy density and radial pressure relationship is non-barotropic.

Abstract

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 & Lobo, [Phys. Rev. D83, 124051 (2011)]. 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 the 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.