red shift
when objects are going away from you, their frequency appears lower (Doppler effect).
lower light frequency is red, higher is violet.
when certain frequencies of light corresponding to known transitions in electron states of elements are shifted down in frequency (toward red) as seen from earth, you know the thing making the light is going away from you.
When far away sources in all directions show a shift toward the red, then everything is moving away from you and the universe must be getting less dense, expanding.
what did edwin hubble knowtice about the direction of travelof distant galaxies that led him to preopose that the universe is expanding?
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