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, pX=-A/\ρ \αX (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 (wX = pX / \ρX) and to the parameter \α are -1\≤ wX<-0.8, 0 \≤ \α < 0.2, respectively. Moreover, we show that a Chaplygin gas (\α =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 (\ΩCDM=0) is ruled out at more than the 99.99 % CL.