In a theory like string theory or supergravity, gravity is described by gravitons on (usually) Minkowski background. But I don't see how this works in terms of the expansion of the Universe. For example, two galaxies far apart can be moving away from each other at more than the speed of light. This is OK in General Relativity due to the expansion of space by the metric. But in quantum gravity where all matter and forces are described by particles or strings this would mean some particles in Minkowski space are travelling faster than the speed of light. Thereby, (presumably), disobeying the rules of quantum field theory? How is this possible? Also, is this anything to do with propagators having non-zero values for space-like separation?