9

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
  • 201,751
Brondahl
  • 659
  • 1
    Related: https://physics.stackexchange.com/questions/25928/is-the-cmb-rest-frame-special-where-does-it-come-from – Mauricio Aug 25 '21 at 08:39
  • Even if it were possible to specify such a frame, what difference would it make? It would still be just a frame. – Marco Ocram Aug 25 '21 at 08:39
  • @MarcoOcram Agreed, but I'm still curious to know :D – Brondahl Aug 25 '21 at 08:41
  • 2
    @MarcoOcram I guess it's the difference between "There's no sensible way to define a priveledged frame" and "We can define one but it doesn't achieve anything" – Brondahl Aug 25 '21 at 08:43
  • Indeed. And perhaps the final phrase in your second comment makes a point that is worth stressing! – Marco Ocram Aug 25 '21 at 08:46
  • And I guess even more technically "Our best models of the nature of reality tell us that it wouldn't achieve anything". Remember that General Relativity isn't an absolute truth, but rather "our best guess, which appears to explain all current observations" ... as were Special, Newtonian and Gallilean Relativities when they were positted. – Brondahl Aug 25 '21 at 08:49
  • "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?)" What would it mean for the Universe to have a boundary? If you came up next to it, could you stick your arm through it? If so, where would your arm be? The Universe is not some container; it is just the set of all the stuff that's contained in it. – D. Halsey Aug 25 '21 at 17:53
  • 1
    Where is the center of the surface of a sphere? There isn't one. Where is its edge? There isn't one. It's like that, but the sphere is 4 dimensional and its surface has 3 dimensions. – Schwern Aug 25 '21 at 19:38
  • I often posed basically the same question. Because I have the idea that the comoving frame is a kind of more natural "fixed" frame. I am always told that it is about correct but, as above in the comments, it is just another frame. – Alchimista Aug 26 '21 at 05:38
  • Isn't this the whole point of relativity? – Robbie Goodwin Aug 27 '21 at 12:27
  • @RobbieGoodwin. No. – Brondahl Aug 27 '21 at 13:01

3 Answers3

20

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
  • 4,927
  • 5
    And of course, due to the expansion of space, the comoving frame of the CMB isn't quite like a rest frame in Special Relativity. – PM 2Ring Aug 25 '21 at 10:18
  • 3
    Expanding on @PM2Ring 's comment, we can't quite say that the frame of reference so defined here and the one so defined somewhere else are the "same" reference frame, at least not in the sense one would expect the "same" reference frame in SR to act; a star could be at rest WRT to CMB, but we would still see light from it red shifted even if we were at rest WRT to CMB as well. – Acccumulation Aug 26 '21 at 00:09
  • "invariant with respect to rotations and shifts" - if it's simply connected. If it's a Clifford torus (and there's some recent development that suggests that), it has a set of preferred directions in which it's the shortest. Still no preferred point though. – John Dvorak Aug 26 '21 at 05:58
12

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
  • 4,446
  • 1
    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. Yes ... the latter would imply the former, but I agree the reverse is not the case. – Brondahl Aug 25 '21 at 08:54
3

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
  • 29,356