How does red shift (Galaxies moving away at various speeds) tell that the universe is expanding?
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Because not only is everything moving away from us, everything is moving away from everyone. And further, the speed that everything is moving away isn’t uniform, but it’s predicable based on the distance between those two objects. So it’s not like an explosion where everything is moving away from us at a set speed. Each and every object that isn’t gravitationally bound with us is moving at speed based precisely on how far away they are from us, and this is true between any two objects (that aren’t gravitationally bound) in the universe. This is consistent with space, everywhere, expanding by very small amounts, but that builds up to large speeds as the distance between objects becomes very large.
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