The CMB is expected to appear isotropic i.e. the same in all directions. As we observe it, it isn’t – there’s a dipole contribution such that it’s hotter in one half of the sky and cooler in the other?
In: Physics
The CMB is in fact very even from all directions. Surprisingly so. But on the raw data, it will appear hotter in one direction and cooler in the other, because of red- and blue-shift caused by the motion of earth (and the galaxy) through local space.
This is due to the Doppler effect. The solar system and galaxy is currently moving relative to material in the early universe. Or it was moving relative to the Earth’s current motion. According to the Principle of General Relativity, either viewpoint is equally valid. This motion is, however, much less that the overall speed that the universe is expanding.
This motion was acquired some time in the cosmological past. In the same way the earth moves relative to the sun, the Earth’s motion was acquired as the solar system formed
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