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Hubble's Law

Hubble noticed that there was s simple relationship between the distance that a galaxy was away from the Earth and the speed with which it is receding.

Figure 15.2: Hubble's Law
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The fact that galaxies are receding from us came from the observation that the spectrum of light from the galaxies was shifted towards the red end of the electromagnetic spectrum, compared to the light that would have come from a stationary source. This shift is interpreted as a Doppler shift, with the amount of the shift giving the speed of the moving source.