What phenomenon, observed by Hubble and Humason, indicated that distant galaxies were moving away from us?

Answer

Redshift

The observation that light from nearly all distant galaxies was redshifted indicated that these objects were moving away from the observer, with the degree of shift corresponding to their speed of recession.

What phenomenon, observed by Hubble and Humason, indicated that distant galaxies were moving away from us?
HistoryscientistAstronomertheoryPhysicist