Fluid Interpretation of Cardassian Expansion

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A fluid interpretation of Cardassian expansion is developed. Here, the Friedmann equation takes the form H² = g(ρₘ) where ρₘ contains only matter and radiation (no vacuum). The function g() returns to the usual 8π/(3 mpl²) 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 ρtot = 3 mpl² 8 π g() = + ρₖ containing not only matter density (mass times number density) but also interaction terms ρₖ. 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 rα⁻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.