We prove that in space-times a velocity field that is shear, vorticity and
acceleration-free, if any, is unique up to reflection, with these exceptions:
generalized Robertson-Walker space-times whose space sub-manifold is warped,
and twisted space-times (the scale function is space-time dependent) whose
space sub-manifold is doubly twisted. In space-time dimension n = 4, the Ricci
and the Weyl tensors are specified, and the Einstein equations yield a mixture
of two perfect fluids.Comment: 11 page
