An Accelerating Universe from Dark Matter Interactions with Negative   Pressure
  As an explanation for the acceleration of the universe, we propose dark matter with self-interactions characterized by a negative pressure; there is no vacuum energy whatsoever in this Cardassian model. These self-interactions may arise due to a long-range "fifth force" which grows with the distance between particles. We use the ordinary Friedmann equation, $H^2 = 8 \pi G \rho/3$, and take the energy density to be the sum of two terms: $\rho = \rho_M + \rho_K$. Here $\rho_M \equiv m n_M$ is the ordinary mass density and $\rho_K \equiv \rho_K(\rho_M)$ is a new interaction term which depends only on the matter density. For example, in the original version of the Cardassian model, $\rho_K = b \rho_M^n$ with $n<2/3$; other examples are studied as well. We use the ordinary four-dimensional Einstein's equations $G_{\mu\nu} = 8 \pi G T_{\mu\nu} $ and assume a perfect fluid form for the energy momentum tensor. Given this ansatz, we can compute the accompanying pressure, and use the conservation of the energy-momentum tensor to obtain modified forms of the Euler's equation, Poisson equation, and continuity equation. With this fully relativistic description, one will then be able to compute growth of density perturbations, effects on the Cosmic Background Radiation, and other effects with an eye to observational tests of the model. 
Paolo Gondolo, Katherine Freese