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. Which of the following is most likely an observation of the Doppler effect that led to the discovery that the Universe is expanding?