In this article Greek indices take space-time values 0, 1, 2, 3 and Latin indices spatial values 1,
2, 3. Cartesian coordinates are assumed throughout and boldface notation is often used for ordinary Euclidean vectors. In
Section 11 upper Latin letters will refer to tetrad indices 0, 1, 2, 3 with the corresponding spatial values 1, 2,
3. Our signature is +2; hence the Minkowski metric reads . As usual and are
Newton’s constant and the speed of light.