In 1929, the American astronomer Edwin Powell Hubble linked the red shift visible in the spectra of distant galaxies, to the constant expansion of the universe.

Hubble suggested that this cosmological red shift is caused by the Doppler Effect.

More than just a statement that the Universe is expanding, Hubble's Law is a quantitative, linear relationship between the distance to a galaxy, and the galaxy's redshift.

This is an important distinction, because with Hubble's law, you can determine the distance to a galaxy, just by measuring its redshift.

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