How do scientists know that the universe is expanding? A: The Doppler effect shows stars with yellow shifted light.
B: The Doppler effect shows stars with blue shifted light.
C: The Doppler effect shows stars with green shifted light.
D: The Doppler effect shows stars with red shifted light.

B: The Doppler effect shows stars with blue shifted light.

Explanation:

American astronomer Edwin Hubble and others discovered in the 1920s that the Universe is expanding by showing that most galaxies are receding from the Milky Way — and the farther away they are, the faster they are receding. The roughly constant ratio between speed and distance became known as the Hubble constant. Another explanation is that, Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light from distant galaxies was shifted toward lower frequencies, to the red end of the spectrum. If the galaxies were moving toward Hubble, the light would have been blue-shifted