system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know space itself is expanding, and that stars aren't just moving away from us in a static vacuum?


Is the universe the actual vacuum of space itself, or is it the visible matter inside it, like stars and galaxies etc, or is it both?

And if we know that "the universe is expanding" by observing the light from distant stars, then how do we know that the actual vacuum of space is expanding, and that it's not just those stars or galaxies moving away from us?

Edit: TIL!
gpt
Because metric expansion and motion aren't equivalent. If they were actually in motion, then some far galaxies would be moving at relativistic speeds, and that means that we should observe relativistic effects. We do not.