  The Cardassian universe is a proposed modification to the Friedmann equation
in which the universe is flat, matter dominated, and accelerating. In the
ordinary Friedmann equation, the right hand side is a linear function of the
energy density, $H^2 \sim \rho$. Here, instead, the right hand side of the
Friedmann equation is a different function of the energy density, $H^2 \sim
g(\rho)$. This function returns to ordinary Friedmann at early times, but
drives acceleration of the universe at the current epoch. The only ingredients
in this universe are matter and radiation: in particular, there is NO vacuum
contribution. The new term required may arise, e.g., as a consequence of our
observable universe living as a 3-dimensional brane in a higher dimensional
universe. A second possible interpretation of Cardassian expansion is
developed, in which we treat the modified Friedman equations as due to a fluid,
in which the energy density has new contributions with negative pressure
(possibly due to dark matter with self-interactions). Predictions are shown for
observational tests of generalized Cardassian models in future supernova
surveys.
