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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 Dec 4 '17 at 20:43
  • $\begingroup$ Consider to spell out acronyms. $\endgroup$ – Qmechanic Dec 5 '17 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 Dec 15 '17 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 Dec 15 '17 at 11:56
  • $\begingroup$ @Toffomat. That is right at the core of my question. It seems to me that comoving coordinates are absolute in the sense that I can identify proper (peculiar) motion with respect to it. I.e. any motion not dictated by the expansion of space. $\endgroup$ – Alchimista Dec 15 '17 at 16:13
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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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