  We construct a general effective dynamics for diffeomorphisms of spacetime,
in a fixed external metric. Though related to familiar models of scalar fields
as coordinates, our models have subtly different properties, both at
kinematical and dynamical level. The energy-momentum tensor consists of two
independently conserved parts. The background solution is the identity
diffeomorphism and the energy-momentum tensor of this solution gives rise to an
effective cosmological constant.
