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Let's say I go out into deep space and set up two observation stations a long ways apart. I want to find an absolute rest frame for each one.

I decide that if the microwave background radiation is at the exact same redshift in all directions then I must be standing absolutely still. If it seems a bit blue-er in any direction, I presume I am moving in that direction and I use my retro-rockets to slow myself down. So, I visit each station and survey the surrounding microwave background radiation and bring each station "to rest" by making sure the background is the exact same color in all directions.

I then pick one station and bounce beams of light off the other station and measure how long the round trips take. I suspect they will take longer as the universe expands and my two "at rest" stations become more distant.

In theory, is this a valid way to measure the expansion of the universe?

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  • $\begingroup$ May you explain what you mean by: "bring my station "to rest" by making sure the background is the exact same color in all directions."? Do you mean at rest with respect to the other station? If not, what are you at rest relative to? My understanding is that there is no absolute velocity in the universe, so you can never know at what speed you are going at in some fundamental system, you must always be talking about relative to something else. $\endgroup$
    – Joe Iddon
    Commented May 4, 2019 at 15:43
  • $\begingroup$ @JoeIddon Search web for CMB Dipole. $\endgroup$
    – safesphere
    Commented May 4, 2019 at 16:08
  • $\begingroup$ @JoeIddon - question modified, let me know if it is now clear $\endgroup$
    – Paul Young
    Commented May 4, 2019 at 16:12
  • $\begingroup$ Your idea is valid conceptually, but likely is not very practical. The stations should be enough apart for precision, yet close enough to bounce laser beams. Also, you don't have to measure a roundtrip time, just measure the redshift. Hopefully you get a full answer with calculations. $\endgroup$
    – safesphere
    Commented May 4, 2019 at 16:20
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    $\begingroup$ Please note that the expansion of the universe and the expansion of space are related, but very different concepts. The former is an experimental fact. The latter is a theoretical explanation of this fact based on the specific cosmological model. There is no direct experimental proof that space expands. Such a proof is conceptually possible, but none has been observed. In other words, the universe conceptually may expand just because it was initially pushed by the Big Bang and is still going on inertia with no space expansion. The Milne model of cosmology is one great example of this scenario. $\endgroup$
    – safesphere
    Commented May 4, 2019 at 21:25

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Your procedure works, but what you are determining is not an absolute rest frame, it's just the frame that's at rest with respect to the matter and radiation. People sometimes refer to this as the frame of the Hubble flow.

When we talk about an absolute rest frame, we mean something different. It would be a frame in which the laws of physics have some special, simple, or preferred form, as with aether theories.

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  • $\begingroup$ tks ... is there a reason to believe that the frame of the Hubble flow isn't special? I can almost put some question together in my mind involving frame dragging, but I cannot quite pose it ...but usually physicists are intuitive and you might be able to see what I am (almost) thinking? $\endgroup$
    – Paul Young
    Commented May 4, 2019 at 16:55
  • $\begingroup$ tiny note - I just went to google "frame of the hubble flow" and your answer is the number three hit :-o $\endgroup$
    – Paul Young
    Commented May 4, 2019 at 17:04
  • $\begingroup$ @PaulYoung By "special" BC means related to the underlying spacetime geometry itself rather than to how matter is (randomly) distributed. (Let him correct me if this is not the case.) For example, in a closed universe, a frame, in which the roundtrip takes the longest time would be special regardless of how matter or radiation is distributed. In other words, it is hypothetically possible for one large region of the universe to move differently from the rest, e.g. due to some enormous gravitational anomaly. In this case, the CMB frame there would be way off and not "special" at all. $\endgroup$
    – safesphere
    Commented May 4, 2019 at 17:33
  • $\begingroup$ @PaulYoung "your answer is the number three hit" - This means the concept is not widely used, so not a good sign. $\endgroup$
    – safesphere
    Commented May 4, 2019 at 17:35
  • $\begingroup$ @safesphere - I think you get me. And I think the #3 hit thing might be a bit "anthropic" (haha) because I wouldn't be asking here is a simple google search revealed the answer ... $\endgroup$
    – Paul Young
    Commented May 4, 2019 at 18:22

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