Much like conservation of energy is a consequence of time translation symmetry. If you mix spatial and time coordinates through relativity theory you can see th

Much like conservation of energy is a consequence of time translation symmetry. If you mix spatial and time coordinates through relativity theory you can see that these are strongly coupled. Your photon example works because there is no time translational symmetry in the case of metric expansion and it's a general relativity model. In general relativity there is no conservation of momentum either. It would be hard to even define since coordinate systems change from point to point. We can only say that the energy-momentum vector is going through parallel transport, which is like conservation in the local coordinate system.