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Message: <blockquote data-quote="Umbran" data-source="post: 9810584" data-attributes="member: 177">That sounds likely...So, the problem is that this... isn't a thing.  That's not a "principle" of cosmology.  What I really think we have here is a misunderstanding about what is important in Einstein's Special Relativity.  That there is an inertial frame in which we might observe the Cosmic Microwave Background as isotropic isn't actually a big deal. It isn't meaningful as far as Special Relativity is concerned. To a human, "isotropic" would seem to have some special significance.  But, indeed, the entire point of Special Relativity is that differences in view of the universe due to constant linear motion don't really matter.  That these relative motions are inconsequential is the point!  Thus the name!If you had to be accelerating in order to see the CMB as isotropic would probably tell us that the curvature or expansion of the (observable) universe was asymmetric in some manner, but to do so in an inertial rest frame doesn't actually say much of anything.  By the transitive property, if you are instantaneously at rest with respect to the CMB, and they are instantaneously at rest with respect to the CMB, then they are instantaneously at rest with respect to you.  The word "instantaneous" here is important, because apparent movement due to cosmological expansion isn't actually movement.</blockquote>