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/rho(X)(alpha) ( a polytropic gas with negative constant and exponent). Our main result is that, restricting to a. at 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 less than or equal to w(X) < - 0.8, 0 <= alpha < 0.2, respectively. Moreover, we show that a Chaplygin gas ( a = 1) is ruled out as a candidate for dark energy by our analysis at more than the 99.99% CL. A generalized Chaplygin gas appears much less likely as a unified dark matter candidate (Omega(CDM) = 0) than as a dark energy model, although its chi(2) is only two sigma away from the expected value.