Question: What is the proof for an expanding universe?

David Wilson answered on 11 Nov 2014:

it comes from a thing called the Doppler effect. If you’ve every been passed by an ambulance with a siren you would have heard this- the pitch of the siren changes as it goes past. This is because the sound waves get bunched up as the siren goes towards you, and spread out as it gets further away. It’s the same with light waves- if an object emitting light is going towards you the light waves get shorter and more blue, but if its going away from you the light looks more red. All of the galaxies we can see, except for the really close ones, look redder than we expect- and the further away they are, the more red they become. This means that they must all be speeding away from us, so the Universe is expanding.