I was reading about Hubbles Law on wikipedia and came across this paragraph: My assumption is this is because we are not directly measuring the velocities, just the redshifts, SR was formed using rigid measuring devices and we have none in this case. But we could measure them could we not? We have methods of measuring distances that far, could we not find the change in distance over time? I dont really understand what about SR makes it confined to "small regions", and what exactly defines a small region. If someone could explain that in as simple a way as possible please. Thank you.