Global rotation

Global rotation in cosmological models is defined on an observational basis. A theorem is proved saying that, for rigid motion, the global rotation is equal to the ordinary local vorticity. The global rotation is calculated in the space-time homogeneous class III models, with Gödel's model as a special case. It is shown that, with the exception of Gödel's model, the rotation in these models becomes infinite for finite affine parameter values. In some directions the rotation changes sign and becomes infinite in a direction opposite to the local vorticity. The points of infinite rotation are identified as conjugate points along the null geodesies. The physical interpretation of the infinite rotation is discussed, and a comparison with the behavior of the area distance at conjugate points is given.