The Copernican Principle states that Earth is not at a special place in the Universe, and by extension, that there are no "special places" in the Universe (per homogeneity of the universe, aka the cosmological principle). However, the frame in which the CMB is isotropic appears pretty special:
- There is exactly one reference frame in which the CMB is isotropic
- It's independent of the observer's motion
- Every observer agrees which reference frame that is
- It's not trivial, since it makes the CMB simpler (and the CMB underpins much of modern cosmology)
Does the frame in which the CMB is isotropic violate the Copernican Principle? If so, why do we still believe in the Copernican Principle?