Question: How do we know that the universe is expanding?

Duncan McNicholl answered on 7 Nov 2017:

Oh this is one of my favourites.
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You know the doppler effect? When an ambulance is coming towards you, the siren sounds higher pitched, and when it’s going away it sounds lower pitched. That’s because the movement stretches the wave in one direction, and squeezes it in another.
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Well, if you’re moving fast enough you can see the same thing happen with light, only instead of getting higher or lower pitched it gets bluer or redder, because colour is a bit like pitch. When we started making observations of other galaxies, the first one we measured was andromeda, and it was a bit bluler than we expected, because it’s coming towards us. Almost all of the other galaxies we’ve measured have been moving away from us, so they look red, and the further away they are, the redder they look. That’s because the space in between here and there is expanding, and the whole universe just keeps getting bigger.