I know this is probably a silly question but i'm not sure I understand why it is special that radiation is cooler on one side of our solar system than the other. It seems like it would indicate that the "temperature" of the observable universe might have a direction to it, but why does it mean our location might be special?

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    $\begingroup$ Exactly what does this have to do with an axis of evil? $\endgroup$ Commented Sep 6, 2018 at 18:22
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    $\begingroup$ @ZeroTheHero news to me: en.wikipedia.org/wiki/Axis_of_evil_(cosmology) $\endgroup$
    – JEB
    Commented Sep 6, 2018 at 18:31
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    $\begingroup$ @JEB the things you learn on PSE! $\endgroup$ Commented Sep 6, 2018 at 19:13
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    $\begingroup$ Interesting. Never heard of the term "Axis of evil". Edited the title to make it clear that "Axis of evil" is a term used in cosmology and not a flippant term introduced by the OP. $\endgroup$
    – user93237
    Commented Sep 6, 2018 at 20:02

1 Answer 1


A blueshift in the radiation on one side of us and a redshift on the opposite side could indicate that we're moving relative to the cosmic microwave background "rest frame" (since redshifts and blueshifts are generated by motion). Though this doesn't make any particular location special, it does identify a particular "special" inertial reference frame: the one in which you are at rest relative to the cosmic microwave background. Given that one of the fundamental precepts of special relativity is that all inertial reference frames are equivalent, this may initially seem surprising. See Is the CMB rest frame special? Where does it come from? for a more involved discussion on this frame.

  • $\begingroup$ I guess that was my confusion, it doesn't seem to indicate anything about our location being "special" $\endgroup$
    – Yogi DMT
    Commented Sep 6, 2018 at 19:04
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    $\begingroup$ well it talks about aligned quadrupole and octopole moments. Would those be translationally invariant? I suppose the dipole alignment isn't, unless you use comoving coordinates. $\endgroup$
    – JEB
    Commented Sep 6, 2018 at 20:51
  • $\begingroup$ Alas for this answer is incomplete. The axis of evil is aligned with the ecliptic, and that's what wants explaining. $\endgroup$
    – Joshua
    Commented Nov 30, 2023 at 4:47

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