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Prompted by commenting on this question.

I offered the standard "Which frame of reference are you using? Yours? A satellite's? The sun's? The Milky Way's?" observation.

Which prompted me to think ... Is there any sort of Absolute Universal (as in of-the-universe) Frame of Reference?

I suspect not, but I don't know enough Astrophysics/Cosmology to be confident.

  • If the Universe had an observable boundary, then it would necessarily have a Centre, which we could measure w.r.t., but my impression is that it doesn't? (Or at least that if it does it's outside the Observable Universe?)

  • And my broad understanding is that the expansion of the Universe doesn't have a central point ... it's not expanding "away from a point" ... everything is just expanding away from everything else uniformly?

Essentially is there any way to define a fixed "central" point of the universe that isn't either entirely arbitrary, or based solely on the Observable Universe (which I assume is centered on us?)

Qmechanic
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Brondahl
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3 Answers3

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In standard cosmology, the answer to the bolded question is "yes", while the answer to the second one is "no".

There is a way to define an absolute reference frame, in that any observer can measure the dipole moment of CMB radiation and determine the velocity with which they are moving through it (see section 2.1 in this Planck paper, for example).

What this means is that the universe is not invariant with respect to Lorentz boosts, but it is (at large enough scales) invariant with respect to rotations and shifts, so there is no center or preferential direction to speak of.

The existence of this "preferential" frame does not invalidate the general principle of relativity --- physics can be described in other frames just as well if we do a coordinate transformation. However, this particular one is interesting in that we can unambiguously refer to it from anywhere in the universe.

Jacopo Tissino
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Is there a well-defined “speed relative to the universe" and is there a well-defined "central point" of the universe are 2 different questions.

As far as central point is concerned, there is indeed no such point.

Speed relative to the universe is also strictly not well-defined, but the frame of the cosmic background radiation is sometimes considered as some kind of stationary rest frame of the universe, relative to which other events can be described. But this is not a "privileged" frame of reference, just a convenient frame of reference to describe any kind of events anywhere in the universe.

silverrahul
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Is there any sort of Absolute Universal (as in of-the-universe) Frame of Reference?

Sometimes the CMB is considered to be a stationary frame of reference for the universe. But as per this link, one should not consider the CMB as the "single absolute frame of reference" for the universe. We can do things like measure the speed of astronomical objects, like galaxies, relative to the CMB.

the expansion of the Universe doesn't have a central point ... it's not expanding "away from a point" ... everything is just expanding away from everything else uniformly?

This is true. The expansion of the universe is not happening from a point, but instead it is happening everywhere.

The idea of "a fixed "central" point of the universe" lacks a coherent definition according to standard cosmological models.

joseph h
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