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Let us say that we know the Hubble parameter now. Why cannot Cosmic Background Radiation and/or the Comoving 3D frame be taken as absolute reference frames (at least far enough from massive bodies, if this is relevant)?

In such frames and continuously correcting via the scale factor, objects to which no force is applied are at rest. This seems to me an absolute inertial reference frame big at least as the observable universe.

Please try to explain me what I do wrong with this reasoning taking into account that my technical knowledge of GR is basically zero.

Edit: for instance who is travelling between the twins of the paradox will be the one changing comoving coordinates. He does so by firing rockets but the same could be deduced on purely kinematic bases.

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    $\begingroup$ "Though the rest frame of the universe is the preferred frame for cosmology, it is not a particularly good frame of reference to use, for example, in describing the motion of the planets in our solar system. The laws of physics are the same in all inertial (unaccelerated) reference frames, so none of them can be “special”—or absolute. An absolute frame of reference would be one in which the laws of physics would be different—indeed simpler—but no such reference frame exists", Cosmic Reflections blog . $\endgroup$
    – Conifold
    Commented Dec 4, 2017 at 20:43
  • $\begingroup$ Consider to spell out acronyms. $\endgroup$
    – Qmechanic ♦
    Commented Dec 5, 2017 at 15:49
  • $\begingroup$ That the laws of physics are the same does really implies that an absolute reference frame has others? I don't see how.... moreover I am asking why they are not. Not why they are not on use to describe my walk to the train station. .... $\endgroup$
    – Alchimista
    Commented Dec 15, 2017 at 9:39
  • $\begingroup$ Well, what do you mean by "take X as absolute reference frame"? You can choose to use whatever frame you like, and what may be convenient for a given application, but what makes it more absolute than others? $\endgroup$
    – Toffomat
    Commented Dec 15, 2017 at 11:56
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    $\begingroup$ You can identify proper motion with respect to any reference frame you like, that doesn't make it absolute. I think you might mean that the CBR frame is a "natural" frame for the observable universe as a whole, in the same sense that the centre-of-mass frame is a particlularly simple one in classical mechanics, even though there is no absolute frame there either. $\endgroup$
    – Toffomat
    Commented Dec 20, 2017 at 10:14

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I think I could self answer as far CBR is concerned. We move with respect to CBR by virtue of space expansion. Any other, such having a proper speed, does not modify our distance to CBR which is ubiquitous.

My question is still open concerning comoving coordinate, which seems to me a grid that in principle extends beyond the observable universe.

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You can call that reference frame "absolute" if you want, but it's just a word. Applying a label to the frame doesn't imbue it with extra physical properties. Its physical properties (like isotropy) are due to the matter distribution. If you removed the matter from a region of spacetime, there would no longer be anything special about the frame of isotropy of the (no longer present) background radiation in that region.

for instance who is travelling between the twins of the paradox will be the one changing comoving coordinates.

You seem to be presupposing that the inertially moving twin is stationary in the comoving frame. That needn't be true. Even if it is true, you can still choose some other reference frame to do your calculations, and you'll get the same answer for any physically meaningful quantity that you calculate (such as the age difference of the twins when they reunite).

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  • $\begingroup$ I am not saying that the twin on earth must be stationary. But as I intend if I would see right the same background all around of me I would know to be stationery respect to the comoving frame. Isn't that? While the travelling one would see it split it in a redder and bluer hemisphere. I did upvoted bcz I grasp that my question isn't clear, in the sense that one can still choose whatever frame of convenience. It is just that the traveller one can know s/he is moving without reading an accelerometer or touching any firing button. Thanks. $\endgroup$
    – Alchimista
    Commented Feb 17, 2021 at 11:37
  • $\begingroup$ @Alchimista You can detect your speed relative to the cosmic background photon gas. On Earth we tend to say we're moving when we're moving relative to the ground. If you want to say moving means moving relative to the cosmic microwave background, that's fine too, but it's just relative motion whatever you call it. $\endgroup$
    – benrg
    Commented Feb 17, 2021 at 17:16

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