The temperature of the CMB is hotter on one side of the sky than on the other by approximately 3 mK. What does this tell us?
A) The Earth has a peculiar velocity of about 8,000 km/s with respect to the CMB.
B) The Earth has a peculiar velocity of about 400 km/s with respect to the CMB.
C) The Earth is near the center of the universe's expansion, but not exactly at the middle.
D) The universe is expanding faster on one side of us than the other.
E) The universe is homogeneous but not isotropic.

Question

Quizplus · Accepted Answer

The temperature difference, known as the dipole anisotropy, is caused by the motion of the Earth relative to the CMB. This motion, known as our peculiar velocity, is approximately 400 km/s towards the direction of the constellation Leo. The fact that the temperature difference is not larger suggests that the universe is nearly isotropic on large scales.

The Temperature of the CMB Is Hotter on One Side