  It has been shown that the Gupta-Bleuler method of quantization can be used
to impose the Lorentz gauge condition in static space-times but not in
cosmological space-times. This implies that the Gupta-Bleuler approach fails in
general in non-static space-times. More recently, however, the Dirac method of
quantizing constrained dynamical systems has been successfully employed to
impose the Lorentz gauge in conformally flat space-times. In this paper we
generalize this result by using Dirac's method to impose the Lorentz gauge in a
general space-time region where the metric is expressed in synchronous
coordinates.

General Relativity and Quantum Cosmology