Why does light from distant galaxies get redshifted?

As photons travel through expanding space, their wavelengths get stretched, which shifts their light toward the red end of the spectrum. This cosmological redshift is one of the main ways astronomers estimate distance in the universe. Edwin Hubble used this relationship in the 1920s to show that distant galaxies recede faster than nearby ones. Redshift is not the same as a Doppler shift from ordinary motion, although the two ideas are related. The greater the redshift, the earlier the light was emitted in cosmic history.