Which statement accurately describes how the understanding of the Doppler effect is applied to the study of galaxies?

The Doppler effect describes how light’s observed wavelength changes when a source moves relative to an observer. Hubble used the shifts in light from galaxies to estimate their velocities; a red shift indicates a galaxy is moving away, while a blue shift indicates it is moving toward Earth. The statement that Hubble used the effect to measure galaxy velocities is accurate.

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