  Einstein field equations with a cosmological constant are approximated to the
second order in the perturbation to a flat background metric. The final result
is a set of Einstein-Maxwell-Proca equations for gravity in the weak field
regime. This approximation procedure implements the breaking of gauge symmetry
in general relativity. A brief discussion of the physical consequences (Pioneer
anomalous deceleration) is proposed in the framework of the gauge theory of
gravity.
