WMAP and the generalized Chaplygin gas

We compare the WMAP temperature power spectrum and SNIa data to models with a generalized Chaplygin gas as dark energy. The generalized Chaplygin gas is a component with an exotic equation of state, p_{X}=-A/\\rho ^{\\alpha}_{X} (a polytropic gas with negative constant and exponent). Our main result is that, restricting to a flat universe and to adiabatic pressure perturbations for the generalized Chaplygin gas, the constraints at 95 % CL to the present equation of state (w_{X} = p_{X} / \\rho_{X}) and to the parameter \\alpha are -1\\leq w_{X}<-0.8, 0 \\leq \\alpha < 0.2, respectively. Moreover, we show that a Chaplygin gas (\\alpha =1) as a candidate for dark energy is ruled out by our analysis at more than the 99.99 % CL, while a generalized Chaplygin gas as a unified dark matter candidate (\\Omega_{CDM}=0) is ruled out at more than the 99.99 % CL.

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