The Doppler effect explains that the observed spectrum of light that is given off by a moving light source is different than that observed when the light source is stationary. The observed spectrum of light has higher frequencies when a light source is moving toward an observer. This is called blueshift because blue is the visible light color with the highest frequency. The observed spectrum of light has lower frequencies when a light source is moving away from an observer. This is called redshift because red is the visible light color with the lowest frequency.

Question

Which of the following is most likely an observation of the Doppler effect that led to the discovery that the Universe is expanding?
A.
The spectra of light given off by stars in the Milky Way Galaxy have blueshifts.
B.
The spectrum of light given off by the Sun has a redshift when it is observed from Earth.
C.
When observed from Earth, light spectra from stars in other galaxies have redshifts.
D.
When observed from a black hole, all of the stars in the Universe have blueshifts.

Answer 1

C. When observed from Earth, light spectra from stars in other galaxies have redshifts.

This observation is most closely tied to the discovery that the universe is expanding. The redshift of light from distant galaxies indicates they are moving away from us, which supports the theory of an expanding universe as formulated by Edwin Hubble in the 1920s.