Fluid interpretation of Cardassian expansion

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(ρM) returns to the usual 8πM /(3mpl 2 ) 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=(3mpl 2 /8π)g(ρM)=ρM+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~ra-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.