Degeneracy between the dark components resulting from the fact that gravity only measures the total energy-momentum tensor

Gravity probes only the total energy-momentum tensor, which leads to a perfect degeneracy for generalized dark energy models. Because of this degeneracy, Omega(m) cannot be measured. We demonstrate this explicitly by showing that the combination of cosmic microwave background and supernova data is compatible with very large and very small values of Omega(m) for a specific family of dark energy models. We also show that for the same reason interacting dark energy is always equivalent to a family of noninteracting models. We argue that it is better to face this degeneracy and to parametrize the actual observables.