How can astronomers determine that the universe is expanding?
By measuring the Doppler shift.
Both light and sound waves gets elongated if the source is getting away from us.On the other hand if an object is coming towards us the waves gets compressed.The elongation waves in light is called red shift. In 1920 Edwin Hubble studied spectrum of several Galaxies and found the red shift.He found tht the far away galaxies move fastest.
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