recession of the galaxies

recession of the galaxies

An important discovery made by Edwin Hubble in 1929 and based on the fact that the spectral lines of all galaxies beyond the Local Group are systematically shifted toward longer wavelengths. These observed redshifts are interpreted as being due to the Doppler shift and imply that all distant galaxies are moving rapidly away from our own. The recession velocities of the galaxies can be determined from their redshifts and are found to be roughly proportional to their distances from us. Thus the Universe as a whole appears to be expanding, and it follows that the Universe was denser in the past. See Big Bang theory; expanding Universe; Hubble's law.
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The question remained: Was the recession of the galaxies a result of their motion through a medium or the expansion of the medium itself?