Displacement of the spectrum of an astronomical object toward longer wavelengths (visible light shifts toward the red end of the spectrum). In 1929 Edwin Hubble reported that distant galaxies had redshifts proportionate to their distances (see Hubble's constant). Since redshifts can be caused by motion of an object away from the observer (the Doppler effect), Hubble concluded that all galaxies are receding from each other. This became the cornerstone of theories of an expanding universe.
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