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I Expansion of the universe, a poor choice of words?

  1. Apr 24, 2016 #1
    I think cosmologists use a poor choice of words when they say the universe is expanding. I don't dispute any of the science behind the "expanding" universe but I do want to dispute the appropriateness of the phrase to the science.
    I think people typically understand expanding to mean: getting bigger.
    But the size of the universe is unknown. It may be finite or infinite and if its the latter the "expansion" of space does not mean the universe is getting bigger. It may have been infinite in spatial extent even at the big bang.
    Many people cannot understand how this could be so, because they think space is expanding in the sense of "getting bigger" hence it could not have been infinite in size at the big bang. Therefore they rule out the infinite universe on the grounds of the poor understanding that was given to them by a bad phrase.
    I think the public are literally being misled by the cosmological community. i don't think this is done intentionally by the community. Cosmologists known that what they mean by expanding is the scale factor is increasing over time. But what the public understand by it is not the same.

    Cosmologists are familiar with bad phrases being forced upon them by history. For example big bang implies a conventional explosion in space, but I have often heard cosmologists frequently saying big bang is a bad phrase and trying to correct the misunderstanding. I don't think I have ever heard cosmologists trying to say that the universe is not expanding, in the sense of "getting bigger" which is incorrect rather than the scale factor is increasing which is correct.
    Consider three phrases to describe the situation in layman terms:
    1 the universe is expanding
    2 the observable universe is expanding
    3 the universe is stretching
    To me , 1 is what is used all the time and it is wrong or perhaps not wrong but misleading (after all the universe could be finite we just don't know. But 2 and 3 could easily be used instead without referring to technical terms such as the scale factor and would help understanding.
    Interested to see what people think.
     
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  3. Apr 24, 2016 #2

    Orodruin

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    I do not agree, "expansion" is a perfectly fine term in the sense that the distance between two comoving objects is getting bigger with time. Your proposition 2 is strongly misleading as it would be true also in a static universe which had existed for a finite time (and even in a contracting universe as long as it does not contract too fast). I think "stretching" is just as prone to misunderstandings as "expanding" is.
     
  4. Apr 24, 2016 #3
    The problem is cosmologists don't say the distance between two coming objects is getting bigger (when they communicate to the public) they say the universe is getting bigger and that implies it could not have been infinite in the past , a statement that is not justified. Also ,are not the Milky way and Andromeda co moving objects? The distance between them is not increasing, it is decreasing. So it is only distant co moving objects where the distant is increasing over time. I will have a think about your statement about stretching perhaps you are right. But why not then just use statement 3 and avoid statement 1?
     
  5. Apr 24, 2016 #4

    Orodruin

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    I think it is perfectly justified, an infinite universe can grow. The standard example of this would be Hilbert's hotel. Your conclusion is therefore wrong.

    No, as evidenced by the motion of the Milky way relative to the CMB rest frame.
     
  6. Apr 24, 2016 #5
    As I understand Hilberts hotel accommodates more guest by moving the guests not by growing in size. On the second point I stand corrected.
     
  7. Apr 24, 2016 #6

    Orodruin

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    The same principle applies. There is nothing strange if you are careful with what "size" means. In cosmology, it is the size of a comoving volume which is expanding.
     
  8. Apr 24, 2016 #7
    I agree but when cosmologists say to the public the universe is expanding that carefulness of "size" is not communicated with it. It gives the definite impression the universe has a definite finite size that grows with time. This is of course compounded by animations on tv that shows the big bang as a little ball that gets bigger. One can blame popular tv but official sources like WMAP and pPlanck images also show a finite little ball getting bigger . I don't blame these visual representation people need visual representations and there is no easy way to draw an infinitely big universe that has a scale factor increasing (iM not saying the universe was infinitely big but it might have been). Cosmologists need to appreciate these images are out there and will always be out there, and hence thats more reason for a better phrase. The reality is the universe's spatial extent is unknown to cosmologists; it may be finite or infinite. Hence by changing the phrase universe is expanding to observable universe is expanding we aid understanding, whereas simply saying the universe is expanding I maintain gives a false impression.
     
  9. Apr 24, 2016 #8
    I am new here.
    hi, I'm Charlie.
    what I don't get is this.
    logically, anyone is going to think this:
    the Universe is expanding.
    ok, so far.
    13.7 billion years ago, there was the Big Bang.
    all of the matter and energy in the observable Universe, was compressed into a small area that was pretty darn dense and hot.
    ok, gotcha.
    now then, if all of that is true, then, lets rewind from right now present day, and watch the observable Universe contract (the opposite of expand).
    how is there somehow, not a center?
    I just don't get this.
    can anyone please tell me what I am missing?
     
  10. Apr 24, 2016 #9
    I think the correct answer is that all of the matter was not compressed into a small area. Rather the density was super high. This is kind of what I ma getting at. when cosmologists say the universe is expanding it creates that impression.
     
  11. Apr 24, 2016 #10
    thank you so much for your reply.
    the thought that immediately comes to mind, though is:
    if the density was high, then doesn't that mean that all the matter and energy was in a small area?
    I must be missing some critical concept.
    if you rewind a movie of an expanding Universe back to the Big Bang (or 13 billion years, lets say), how is there possibly not a center?
     
  12. Apr 24, 2016 #11

    PeterDonis

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    Because rewinding the movie doesn't do what you are imagining. You are imagining the universe as a spherical region of space, and you are imagining that the spherical region gets smaller as we go back in time. Since that spherical region of space has a center, you think the universe must have a center.

    But in fact that is not what our cosmological models say. What our best current cosmological models say is that our universe is an infinite spatial expanse. An infinite expanse obviously has no center. But even if we adopt an alternate model (which is highly unlikely on our current data but not absolutely ruled out) in which the universe is spatially finite (but very large), it still has no center. That is because, in this model, the universe is spatially the "hypersurface" of a 3-sphere. Such a hypersurface has no center, just as the surface of an ordinary 2-sphere has no center. (The interior of an ordinary 2-sphere has a center, but its surface does not; and the spatial geometry of the universe in the model I'm talking about is analogous to the surface only.)

    This idea is not easy to grasp because we have difficulty imagining how a 3-dimensional space could have the geometry of a 3-sphere. The best way I know of to imagine it is to imagine setting out in your rocket from Earth and heading out in the same direction indefinitely. In the 3-sphere geometry, you will eventually return to Earth, even though you never change direction at all. (This is analogous to circumnavigating the surface of a 2-sphere like the Earth.) Such a geometry has no center.
     
  13. Apr 24, 2016 #12

    PeterDonis

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    All of the matter in our observable universe was compressed into a very small volume (not area). And its density was very high. But in our best current model, the universe as a whole is spatially infinite, so yes, all of the matter in the entire universe in this model did not occupy a very small volume.

    (In the alternate model I mentioned in my previous post, where the universe is spatially a 3-sphere, the entire universe did occupy a much smaller volume in the far past--though still much larger than the volume occupied at that time by the matter in our observable universe.)
     
  14. Apr 24, 2016 #13
    thank you so much for your excellent reply.
    will read carefully and reply back.
     
  15. Apr 24, 2016 #14

    QuantumQuest

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    I think that this misconception stems from the fact that you imagine rewinding the expanding universe as you observe it outside this. But the case according to the balloon analogy - or any relevant analogy for that matter, is that we are on the surface of this, while there is no "inside'' and no "outside". This is expanding to all directions, so there can't be a center.
     
  16. Apr 24, 2016 #15

    phinds

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    We have such discussion here from time to time about how one term/phrase or another is not a good choice. At the end of the day, all such discussions are totally pointless since no one is going to change the accepted terminology no matter how cogent an argument (not that you have presented such) can be made against the accepted term.
     
  17. Apr 24, 2016 #16
    That's interesting, you're right we have no idea how big the universe is. We could just be in a bubble in the universe that appears to be expanding while the rest of the universe is collapsing. Besides if the universe is expanding as most people would conceive, what is it expanding into?
     
  18. Apr 24, 2016 #17
    thank you to everyone who replied.

    Peter, thank you for both of your replies.
    I have carefully read them both.
    they are very good.
    they are a great help for me, in this question that I have had for years.
    I follow what you are saying.
    one obvious logical thought that springs to my mind is:
    if the density of all the matter in the observable universe was very high 13 billion years ago, then that means that all the matter in the early observable universe was very close together, and that implies to me that there was a center.
    but, I think that your answer to me would be to learn about the hypersurface of a 3 sphere.
    I just read a little.
    it appears to be far beyond my grasp.
    really unintelligible.
    I get so far, and then, forget it.
    it is a geometry thing, about which I am thinking it would take me years of study to grasp.
    how very non intuitive.
    I am so into learning about Relativity (Special and General).
    I am used to having to re frame things, to go from Newtonian gravitational force to Einsteinian General Relativity geometrical description of gravity.
    but the hypersurface of a 3 sphere..........well, I'm lost.
    thank you very much again for your excellent replies.
    thank you all of you.
     
  19. Apr 24, 2016 #18

    phinds

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    You would be well advised to read some basic cosmology.
     
  20. Apr 24, 2016 #19
    There is a center of the observable universe and that's wherever you are. Everything would get closer and closer to you if you rewound. You'd see the same thing if you were anywhere else. Then that the universe (not the observable universe) is finite or infinite is a different question.

    Now to everybody else, did I get this right already?
     
  21. Apr 24, 2016 #20

    PeterDonis

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    Yes. It might take some time for you to understand, but once you understand it, you will see that it falsifies your intuition that there must be a center because things were closer together in the past.
     
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