  There is now strong observational evidence that the expansion of the universe
is accelerating. The standard explanation invokes an unknown "dark energy"
component. But such scenarios are faced with serious theoretical problems,
which has led to increased interest in models where instead General Relativity
is modified in a way that leads to the observed accelerated expansion. The
question then arises whether the two scenarios can be distinguished. Here we
show that this may not be so easy, demonstrating explicitely that a generalised
dark energy model can match the growth rate of the DGP model and reproduce the
3+1 dimensional metric perturbations. Cosmological observations are then unable
to distinguish the two cases.
