How do we know the universe is ever expanding?
In: Physics
Doppler effect. When the source of a wave is travelling away from you, the waves stretch out causing a shift down. That’s why the sound of a car horn changes as it drives past – the sound waves are getting longer and therefore the pitch is changing.
The same thing happens with light waves – if the source is travelling away from you, the wavelength of light stretches until it drops below the visible spectrum. If this didn’t happen, then you wouldn’t see individual stars at night – the night sky would be full of stars from all over the universe.
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