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To first order, the Cosmic Background Radiation (CMB) looks the same in every direction. Apparently, however, at the level of parts per million, there are small distinctive variations in the properties of the radiation depending on which direction you look.
Based only on these variations, can we say anything about our absolute position, velocity, acceleration, attitude, or angular velocity with respect to the early universe? Can we discern anything 'special' about our role as an observer?

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    $\begingroup$ The dipole asymmetry, which indicates our velocity with respect to the CMB, is much larger than that. $\endgroup$
    – ProfRob
    Commented Nov 25, 2020 at 19:22
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    $\begingroup$ Actually the cosmic microwave background does not look the same in each direction to us. There is a dipole shift: astronomy.swin.edu.au/cosmos/C/…. This is indeed due to the frame of reference of Earth, and it is only when this effect is removed that one sees the uniformity that you mention. (Edit: Rob beat me to it) $\endgroup$
    – Rococo
    Commented Nov 25, 2020 at 19:22
  • $\begingroup$ @ Rob Jeffries thanks & Rococo! How large is the dipole effect in ppm? $\endgroup$
    – Roger Wood
    Commented Nov 26, 2020 at 2:36

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Special relativity, flat spacetime, has no way to define a rest frame. On cosmological scales, spacetime isn't flat, and we have the CMB. The Milky Way is moving with respect to it at around 600 km/s (0.2% c), so yes, you can define an preferred frame with respect to it.

But there is a problem with calling it "The Rest Frame". It is position dependent. Let's say we put up a space telescope at rest with respect to the CMB, if it sees a galaxy 8 Billion light years away with a zero redshift, that galaxy will not be at rest with respect to the CMB where it is, even though it is "at rest" in our frame...which really isn't that special as far as the rest of the universe is considered.

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  • $\begingroup$ @ JEB is this velocity only with respect to the primordial CMB? Or is it also evident in very ancient/distant galaxies? Or is this velocity just related to our galaxy or our local cluster? $\endgroup$
    – Roger Wood
    Commented Nov 26, 2020 at 2:42
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Note the CMB does not look the same in every direction. There's a very noticeable dipole.

enter image description here

Adjusting to remove this dipole does let us say something about our relative velocity to the frame in which the CMB is isotropic, which is as "special" a frame as any. Our position, velocity, etc will all vary constantly though, since we are rotating around the Sun which is rotating around the Milky Way which is moving in a certain direction relative to the Local Group, etc.

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  • $\begingroup$ @ Allure thanks - is this a high velocity compared with our local motions in space? BTW which direction are we going? $\endgroup$
    – Roger Wood
    Commented Nov 26, 2020 at 2:47

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