  The dark energy in the Universe is described in the context of modified
Friedmann equations as a fluid parameterized by the density of dark matter and
undergoing an adiabatic expansion. This formulation is applied to the
Cardassian model. Choosing then parameters consistent with the supernova
observations, it gives a background expansion in which the cosmic temperature
fluctuations are calculated. The resulting spectrum is quite similar to what is
obtained in the standard concordance model. If the Cardassian fluid is
interpreted as a new kind of interacting dark matter, its overdensities are
driven into oscillations when the interaction energy is rising in importance.
This does not occur in a description of the Cardassian fluctuations motivated
by theories of modified gravity. There the energy of the underlying matter is
also conserved, which requires appearance of effective shear stress in the late
Universe. In both approaches that allow fluctuations the thermal power spectrum
at large scales is much too strongly enhanced by the late integrated
Sachs-Wolfe effect. With the interacting dark matter assumption, we conclude
that the Cardassian model is ruled out by observations, expect in a small
neighbourhood of the $\Lambda$CDM limit.
