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.
When observed from Earth, light spectra from stars in other galaxies have redshifts.
B.
The spectrum of light given off by the Sun has a redshift when it is observed from Earth.
C.
When observed from a black hole, all of the stars in the Universe have blueshifts.
D.
The spectra of light given off by stars in the Milky Way Galaxy have blueshifts.

Answer 1

The correct answer is:

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

This observation indicates that those galaxies are moving away from us, which is a key piece of evidence for the expansion of the Universe. The redshift seen from distant galaxies shows that their light is stretched to longer wavelengths, consistent with the Doppler effect for objects moving away from an observer.