Challenges to the Generalized Chaplygin Gas Cosmology
  The generalized Chaplygin gas (GCG) model allows for an unified description of the cosmologically recent accelerated expansion of the Universe and of the evolution of energy density perturbations. This dark energy - dark matter unification is achieved through a rather exotic background fluid whose equation of state is given by $p = - A/\rho^{\alpha}$, where $A$ is a positive constant. Observational constraints arising from bounds on the locations of the first few peaks and troughs of the Cosmic Microwave Background Radiation (CMBR) power spectrum from recent WMAP and BOOMERanG experiments are consistent with the model for $\alpha \lsim 0.6$ assuming that $0 < \alpha \le 1$. Most recent Type-Ia Supernova data indicates however, that the range $\alpha > 1 $ must be considered. 
O. Bertolami