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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?

safesphere
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Paul Young
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  • 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. – Joe Iddon May 04 '19 at 15:43
  • @JoeIddon Search web for CMB Dipole. – safesphere May 04 '19 at 16:08
  • @JoeIddon - question modified, let me know if it is now clear – Paul Young May 04 '19 at 16:12
  • 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. – safesphere May 04 '19 at 16:20
  • @safesphere That was certainly interesting to see that we can determine an "absolute speed", but this is still relative the CMB photons (see here) so I still believe my question is valid, albeit a bit silly as clearly the OP meant at rest relative to them. Thanks though, certainly cool! – Joe Iddon May 04 '19 at 16:24
  • @JoeIddon Which question of yours is valid? You have 3 in your comment. The answers are: (1) zero dipole; (2) no; (3) CMB. – safesphere May 04 '19 at 16:30
  • @safesphere I don't know what you mean, I was admitting my questions were silly as I was not aware of the CMD dipole, however they are still valid in the sense that they able to be answered as you have just demonstrated. :) – Joe Iddon May 04 '19 at 16:34
  • I've edited your title to match the meaning of your question. Otherwise some responders get confused and post irrelevant answers that should have been comments instead. – safesphere May 04 '19 at 16:37
  • @safesphere - thanks, but the title now contains part of the answer - which is "the concept of a CMB rest frame is a valid one" - I had been uncertain of that -tks! – Paul Young May 04 '19 at 16:51
  • I have rolled back my edit and my upvote. I thought your question was if CMB frames could be used for measuring the expansion of the universe, which would be a meaningful question. However, based on your last comment, I am no longer sure what your actual question is and it makes no sense to me. There are no absolute frames, so your title has no meaning. – safesphere May 04 '19 at 17:14
  • I do think you understand me, and Ben has answered my question to my satisfaction ... what I thought of as "an absolute rest frame" is a thing called "the hubble flow frame" and while it is not an absolute rest frame, it will serve for the purposes of measuring the expansion of the universe - which is the main question - I have never seen this addressed elsewhere – Paul Young May 04 '19 at 18:25
  • I think it is important because it adds a certain "reality" to the expansion of the universe – Paul Young May 04 '19 at 18:25
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    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. – safesphere May 04 '19 at 21:25
  • @safesphere - tks, in fact ...this comment comes answers one of those questions that was on my mind but unsaid ... I am thinking i should pose a few of these so that you can answer and other people can google for them later – Paul Young May 04 '19 at 22:55

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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.

  • 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? – Paul Young May 04 '19 at 16:55
  • tiny note - I just went to google "frame of the hubble flow" and your answer is the number three hit :-o – Paul Young May 04 '19 at 17:04
  • @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. – safesphere May 04 '19 at 17:33
  • @PaulYoung "your answer is the number three hit" - This means the concept is not widely used, so not a good sign. – safesphere May 04 '19 at 17:35
  • @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 ... – Paul Young May 04 '19 at 18:22
  • one thing you have added to my understanding is that "if both stations are at equal blue-shift" the experiment works just as well and should work just as well – Paul Young May 04 '19 at 18:26