how do we know the universe is expanding and will do this till infinity?
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In short, because everywhere we look, everything is moving away from us.
Thanks to the Doppler effect, objects moving towards us get more blue, and those moving away become more red.
As it turns out, everything outside our local area (that is, our galaxy and neighbouring galaxies) is moving away from us. Not only that but the rate at which they’re moving away is proportional to how far away they are.
Both of these would be explained if its space itself that is expanding. It would mean that you would see this no matter where you are in the universe, so it avoids implying that we’re somehow in a special place in the universe
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