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, pₗ=-A/\ ^\ₗ (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 (wₗ = pₗ / \ₗ) and to the parameter \ are -1\ wₗ<-0. 8, 0 \ \ < 0. 2, respectively. Moreover, we show that a Chaplygin gas (\ =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 (\₂₃₌=0) is ruled out at more than the 99. 99 % CL.