Covariant Galileon
  We consider the recently introduced "galileon" field in a dynamical
spacetime. When the galileon is assumed to be minimally coupled to the metric,
we underline that both field equations of the galileon and the metric involve
up to third-order derivatives. We show that a unique nonminimal coupling of the
galileon to curvature eliminates all higher derivatives in all field equations,
hence yielding second-order equations, without any extra propagating degree of
freedom. The resulting theory breaks the generalized "Galilean" invariance of
the original model.
