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Here's a non-expert version of my earlier tweet threads about cosmology. The key question behind those threads was and is: what is a good way to think about cosmic expansion? Should you think about space itself expanding? Or about galaxies moving (through space)? #GenRelEdu 1/
I've been arguing (as have a number of others) that the latter viewpoint has more pedagogical merit. (But a judgement like that has of course a subjective component. Others may disagree.) And I admit that the viewpoint introduces a complication. There are at least two 2/
ways of defining the "speed at which a distant galaxy moves away from us as the universe expands". The first way follows directly from the definition of cosmic expansion, which is that distances between far-away galaxies change in a very specific way: they all change by the 3/
