What phenomenon observed in the light spectra from distant galaxies indicated they were moving away from Earth?

Answer

Redshift

Hubble and Humason analyzed light spectra and observed redshift, meaning the spectral lines were shifted toward the red end, indicating recession velocity.

What phenomenon observed in the light spectra from distant galaxies indicated they were moving away from Earth?

#Videos

The Man the Hubble Space Telescope was Named After - YouTube

telescopeAstronomerinstrumenteponym