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 α 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 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. A generalized Chaplygin gas as a unified dark matter candidate (ΩCDM = 0) appears much less likely than as a dark energy model, although its χ 2 is only two sigma away from the expected value.