Are distant galaxies moving faster than the speed of light? Wouldn that violate relativity?
A profound feature of relativity is that two objects passing by each other cannot have a relative velocity greater than the speed of light. An even more profound feature, one which has received much less publicity, is that the concept of “relative velocity” does not even make sense unless the objects are very close to each other. In Einstein’s general theory of relativity (which describes gravity as the curvature of spacetime), there is no way to define the velocity between two widely-separated objects in any strictly correct sense. The “velocity” that cosmologists speak of between distant galaxies is really just a shorthand for the expansion of the universe; it’s not that the galaxies are moving, it’s that the space between them is expanding. If the distance isn’t too great, this expansion looks and feels just like a recession velocity, but when the distance becomes very large that resemblance breaks down. In particular, it’s perfectly plausible to have distant galaxies whose “recessi
