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Visibility of Matter in the Universe

  1. Oct 27, 2015 #1
    When we view objects in the universe our eye sees the light that traveled from said object to our eye. When we see the Andromeda galaxy we are actually seeing the Andromeda galaxy as it was 2.5 million years ago.

    This raises a question about matter within the visible universe:

    Is there a possibility that we are seeing the same atoms in multiple places within the visible universe?

    Could some of the hydrogen from one of the first galaxies to form now be part of the Andromeda galaxy but because of the distances we are seeing the same atoms 13.8 billion years ago and at Andromeda 2.5 million years ago?

    Could some of the atoms in me also be seen when I look at the Hubble Deep Field?
     
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  3. Oct 27, 2015 #2

    phinds

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    No, this is impossible. It would require faster than light travel of any particles that we "saw twice".
     
  4. Oct 27, 2015 #3

    Chronos

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    No. Matter cannot travel at the speed of light, so, any matter that emitted photons we observe cannot be in more than one place along the path traveled by those photons. In fact, matter almost always travels much slower than the speed of light. So, for example, let's assume 14 billion years ago a clump of cmb matter was traveling at an amazing .5c [a highly improbable velocity]. 14 billion years later that clump could only have traveled 7 billion light years. If it instantly started emitting light at that time it would still take another 7 billion years to reach us [discounting expansion]. The universe could be no less than 21 billion years old by the time those photons could reach us.
     
    Last edited: Oct 27, 2015
  5. Oct 28, 2015 #4

    Chalnoth

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    Yes, but not in the sense you're proposing. No matter can outrun a light beam, so what you describe isn't possible. However, gravity bends light, which makes it so that massive objects act as lenses. Sometimes, when there is one object directly behind another, we can see multiple images of the background object as the light is bent around the foreground object. For example:
    https://en.wikipedia.org/wiki/Einstein_Cross
     
  6. Nov 6, 2015 #5
    What if the universe is closed and finite?

    Is it possible that we then could be seeing some of the same matter in different locations? Could we be seeing early formations in the universe at different times?
     
  7. Nov 6, 2015 #6

    Jorrie

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    Present evidence is that the universe may perhaps be spatially closed and finite, but the cosmological constant is causing a cosmological horizon around us at about 16 billion light years. Photons from beyond that distance can never reach us, so no photon can ever 'circumnavigate' our universe.
     
  8. Nov 6, 2015 #7

    phinds

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    Perhaps, yes, but I thought the consensus is now that it is looking more and more like it's flat and infinite. At least, I think that's what I see here more than any other point of view.
     
  9. Nov 6, 2015 #8

    Jorrie

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    I think the error bars still include the Omega > 1 case, so we cannot rule out closed and finite yet. My point is that even if this is the case, the old idea of circumnavigation does not hold for a positive cosmological constant. The 'balloon is just being blown up too fast' for anything to circumnavigate it.
     
  10. Nov 6, 2015 #9

    Chronos

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    This gets us back to an Olbers paradox scenario, in an infinitely old universe there would be plenty of time for circumnavigation, but, once again - why isn't the sky ablaze with circumnavigating photons?
     
  11. Nov 6, 2015 #10

    phinds

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    Ah. I over-interpreted your statement. I was not suggesting that it be ruled out, I was making the point that you said it seemed most likely to be finite/closed and that seems to be NOT the most likely scenario but I see you didn't really say "most likely" or anything like it. My bad.

    Yes, I agree.
     
  12. Nov 6, 2015 #11

    marcus

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    That's a good way of putting it!

    To address the other point, I personally have not detected any signs of a consensus that the U is spatially flat and infinite!

    I suppose people generally use the flat case in computation because it is simpler to compute with and gives a good approximation. You could say that the U is "for all practical purposes" flat because it is nearly flat or IOW flat enough so that we can treat it as such.
     
  13. Nov 6, 2015 #12

    phinds

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    Thanks Marcus. I may be overemphasizing (to myself) one side of the finite/infinite discussions that I have read here.
     
  14. Nov 6, 2015 #13

    marcus

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    I suppose you could be, Phinds. I didn't see the discussions you read---I'd probably tend to overlook threads focused on a U finite vs U infinite debate.
    Also the people participating in those threads wouldn't necessarily be representative of cosmologists or the scientific community as a whole. It's risky to draw conclusions about consensus of professional researchers, of course, from what you see in one locale.

    Maybe you would agree that for now we can't say anything about the comparative likelihood of flat infinite, or the alternatives. The Ωk confidence interval contains zero, yes, but it also contains an uncountable infinity of non-flat, non-infinite possibilities. It doesn't seem like a question that is ready to be addressed. Pursuing it might be an unwise use of one's time. I'm just speculating.

    I can guess but it is only a guess that the question could eventually be resolved when conditions before and around the start of expansion are better understood. There is a significant effort now to TEST different models for what replaces the Initial Fail (the big glitch, or "singularity")
    Some authors to look up (Ashtekar, Agullo, Wilson-Ewing...).
    There is a lot of observational data, and more will be coming in---so this is the kind of thing that can reach consensus. Models are testable. So I can imagine one or another model of the start of expansion eventually winning out and being generally accepted.
    Then it might be possible to say something about the flat infinite idea. For example the consensus model might be a bounce, with a particular mechanism that would not support the spatially infinite case. I suppose that is one possible outcome. Anyway I am just speculating what ifs here and should go think about something that for the time being is more accessible
     
  15. Nov 6, 2015 #14

    Chalnoth

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    If we saw the universe wrapped back on itself, this would cause strong correlations between different places on the cosmic microwave background. There are no such strong correlations. So if the universe is closed and finite (which is a definite possibility), then we are only seeing a small fraction of the whole, and due to the expansion won't ever see anything but a small fraction of the whole.
     
  16. Nov 6, 2015 #15

    Grinkle

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    Can one contrive a pre-expansion / post-expansion scenario where we might see a pre-expansion echo of a particle that looks closer to us than the post-expansion particle ended up? I think pre-expansion no photons had formed, so perhaps not.
     
  17. Nov 6, 2015 #16

    marcus

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    If you want a comparatively straightforward example of a bounce scenario, you can google
    "LambdaCDM bounce" which will get the Dec 2014 article by Cai and Wilson-Ewing titled "A LambdaCDM Bounce Scenario"
    they take a copy of our standard model universe that is contracting and follow it through bounce to an expanding version of the standard model.
    The link to the article is here http://arxiv.org/abs/1412.2914
    It goes through a radiation-dominated phase at the bounce with near Planckian energy density.
    The premise is that at extreme density quantum corrections make gravity repel which is what prevents a singularity and causes a rebound.
    I think visual information would be hopelessly scattered.
     
    Last edited: Nov 6, 2015
  18. Nov 13, 2015 #17
    I thought the consensus is that it is as flat as we can measure, but we will never be able to tell whether it is finite or infinite, because the accuracy of our measurements will never reach 100%.

    However, the more accurate our measurements get, the larger the universe would have to be (in the finite case) in order for the spatial curvature to be lower than the measuring error.
     
  19. Nov 13, 2015 #18

    phinds

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    Agree.
     
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