Instability of Chaplygin gas trajectories in unified dark matter models

In the past few years, the Chaplygin gas has been considered as an appealing candidate for unifying dark matter and dark energy into a single substance. This picture had to face several problems when trying to fit its predictions with cosmological observations. We point out another potential problem of this model, showing that, when the Chaplygin gas is described through a classical scalar field, the corresponding trajectories are strongly unstable. This implies that an extreme fine-tuning must apply to the initial values of the field in order to end up today with values allowed by observational data.