Why is the fact that the cosmic microwave background radiation (CMB) is very smooth considered a problem?
A) A universe as smooth as predicted by the CMB should not have formed as many galaxies as have been observed.
B) A universe as smooth as predicted by the CMB should have collapsed by now.
C) A universe as smooth as predicted by the CMB should be expanding much faster than we are now.
D) A universe as smooth as predicted by the CMB should never occur because quantum mechanical fluctuations would have been imprinted on it.
E) A universe as smooth as predicted by the CMB should never have formed any stars or galaxies.

Question

Quizplus · Accepted Answer

According to the principles of quantum mechanics, there should have been fluctuations in the early universe that would have created small variations in temperature and density. However, these small variations are not observed in the CMB, which is very smooth. This is known as the "horizon problem," and it suggests that there may be a gap in our understanding of the early universe.

Why Is the Fact That the Cosmic Microwave Background Radiation