Regularized expression for the gravitational energy-momentum in   teleparallel gravity and the principle of equivalence
  The expression of the gravitational energy-momentum defined in the context of the teleparallel equivalent of general relativity is extended to an arbitrary set of real-valued tetrad fields, by adding a suitable reference space subtraction term. The characterization of tetrad fields as reference frames is addressed in the context of the Kerr space-time. It is also pointed out that Einstein's version of the principle of equivalence does not preclude the existence of a definition for the gravitational energy-momentum density. 
J. W. Maluf, M. V. O. Veiga and J. F. da Rocha-Neto