Fluid interpretation of Cardassian expansion

A fluid interpretation of Cardassian expansion is developed. Here, the Friedmann equation takes the form H²=g(ρ_M) where ρ_M contains only matter and radiation (no vacuum). The function g(ρ_M) returns to the usual 8πρ_M/(3m_pl²) during the early history of the Universe, but takes a different form that drives an accelerated expansion after a redshift z∼1. One possible interpretation of this function (and of the right-hand side of Einstein’s equations) is that it describes a fluid with total energy density ρ_tot=(3m_pl²/8π)g(ρ_M)=ρ_M+ρ_K containing not only matter density (mass times number density) but also interaction terms ρ_K. These interaction terms give rise to an effective negative pressure which drives cosmological acceleration. These interactions may be due to interacting dark matter, e.g. with a fifth force between particles F∼r^α-1. Such interactions may be intrinsically four dimensional or may result from higher dimensional physics. A fully relativistic fluid model is developed here, with conservation of energy, momentum, and particle number. A modified Poisson’s equation is derived. A study of fluctuations in the early Universe is presented, although a fully relativistic treatment of the perturbations including gauge choice is as yet incomplete.