According to Weyl's postulate timelike geodesics should be hypersurface orthogonal, this in itself seems to clash with the GR principle that there should be no physically preferred frame or slicing of the spacetime manifold (general covariance). Usually there is much insistence in textbooks that the preferred frame choice of standard cosmology, the comoving chart, is not a physical one but a practical one in terms of calculations, which is sometimes difficult to discern (the physical versus practical subtletty). The worldline congruence indeed sometimes seems quite physical, mathematically is the same thing that we apply to stationary models when we want to make them static (by making the timelike killing fields hypesurface orthogonal so there is no time-space mixed components) but in our case there is no killing fields of course. I say that the congruence seems very physical because if we didn't have it I find it hard to understand things like the second law of thermodynamics and the consensus we all have on the direction of time, or the fact that we only observe retarded potentials,or the very fact that the universe is expanding for everyone-if we didn't have the congruence of timelike geodesics, the univers could be expanding for some contracting for others and neither for others depending on the coordinate system they used- all these seem to be "physical" consequences of having worldline congruence. So can someone explain to me why we share all those physical observations if the congruence itself is not physical but a choice to make calculations in GR easier?