CMB spectrum in Cardassian models

Abstract

Department of Physics, University of Oslo, N-0316 Oslo, Norway(Dated: February 2, 2008)The dark energy in the Universe is described in the context of modified Friedmann equations asa fluid parameterized by the density of dark matter and undergoing an adiabatic expansion. Thisformulation is applied to the Cardassian model. Choosing then parameters consistent with the su-pernova observations, it gives a background expansion in which the cosmic temperature fluctuationsare calculated. The resulting spectrum is quite similar to what is obtained in the standard concor-dance model. If the Cardassian fluid is interpreted as a new kind of interacting dark matter, itsoverdensities are driven into oscillations when the interaction energy is rising in importance. Thisdoes not occur in a description of the Cardassian fluctuations motivated by theories of modifiedgravity. There the energy of the underlying matter is also conserved, which requires appearance ofeffective shear stress in the late Universe. In both approaches that allow fluctuations the thermalpower spectrum at large scales is much too strongly enhanced by the late integrated Sachs-Wolfeeffect. With the interacting dark matter assumption, we conclude that the Cardassian model isruled out by observations, expect in a small neighbourhood of the ΛCDM limit.Introduction