The generalized Chaplygin gas is characterized by the equation of state p = - A / \rho ^ { \alpha } , with \alpha > -1 and w > -1 .
We generalize this model to allow for the cases where \alpha < -1 or w < -1 .
This generalization leads to three new versions of the generalized Chaplygin gas : an early phantom model in which w \ll - 1 at early times and asymptotically approaches w = -1 at late times , a late phantom model with w \approx - 1 at early times and w \rightarrow - \infty at late times , and a transient model with w \approx - 1 at early times and w \rightarrow 0 at late times .
We consider these three cases as models for dark energy alone and examine constraints from type Ia supernovae and from the subhorizon growth of density perturbations .
The transient Chaplygin gas model provides a possible mechanism to allow for a currently accelerating universe without a future horizon , while some of the early phantom models produce w < -1 without either past or future singularities .