Solution to 2008 Problem 20

From the Wien displacement law, the temperature of the cosmic microwave background (CMB) is inversely proportionaly to the most intense wavelength. The wavelength of a CMB photon is proportional to Freidmann-Robertson-Walker scale factor a. When the temperature of the CMB was 4 times larger, a must have been 4 times smaller. This means that all lengths were smaller by a factor of 4. Therefore, answer (A) is correct.

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