SAT Subject Physics Practice Question 163: Answer and Explanation
9. Which provides the basis for the observation that the universe is expanding?
A. Newton’s law of universal gravitation
B. Red shift of light from other galaxies
C. The fact that every element of atomic number greater than 83 is radioactive
D. The zeroth law of thermodynamics
E. Mass–energy equivalence
Correct Answer: B
The red shift of light refers to the increase in the wavelength (or, equivalently, the decrease in the frequency) of light from a distant source when it’s measured here. This change in wavelength (and frequency) is the Doppler effect and implies that the source and detector are moving away from each other. This provides evidence for the expansion of the Universe.