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Boarder of universe

  1. Feb 10, 2005 #1
    what makes the boarder of a universe and why and can it change shapes and size
     
  2. jcsd
  3. Feb 10, 2005 #2

    chroot

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    The observable universe has no real "border." The "edge" of the universe is just defined as the furthest distance from which light has had time to reach us. It's about 47 billion light-years away.

    - Warren
     
  4. Feb 10, 2005 #3

    JesseM

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    According to this article, the furthest point in space from which light has had time to reach us would currently be about 78 billion light-years away (although of course it wasn't that far away at the time the light was actually emitted).
     
  5. Feb 10, 2005 #4

    chroot

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    Oops, I guess I remembered the wrong number -- or perhaps the right number, but for the wrong question. :biggrin:

    - Warren
     
  6. Feb 10, 2005 #5

    ohwilleke

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    I am a boarder of the universe and so are you, because we all live here. I change shapes and sized when I eat too much. :surprised :wink:
     
  7. Feb 10, 2005 #6
    ok then why cant we interact with other universe or even see signs that they exisit by like nuclear radiation or radio transmitions?
     
  8. Feb 11, 2005 #7

    hellfire

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    No, you were right. In an universe with OL = 0.73, Om = 0.27 and H0 = 71, the (theoretical) radius of the observable universe is equal to 46 Gly.

    The press article is wrong. It refers to the paper of N. Cornish in which the possibility of a finite universe was analized according to possible matches of circles in the CMB. The paper concludes that we can exclude a universe smaller than 24 Gpc (78 GLy) in diameter. Note that the press acticle claims that "The universe is at least 156 billion light-years wide", which is wrong.
     
    Last edited: Feb 11, 2005
  9. Feb 11, 2005 #8

    Chronos

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    I just want to scrape my fingernails across the chalkboard every time I hear this argument. Eeeeek... the observable universe is the only universe that is scientifically relevant. Until we have a workable explanation of what is observed, the rest is meaningless. I agree all things are possible. I do not agree things that are inherently unobservable are meaningful.
     
  10. Feb 11, 2005 #9

    turbo

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    Dear Chronos: To you, I am inherently unobservable. I promise you that I exist. The fact that you cannot see me does not negate my existence, nor make my existence less meaningful (at least to me!). I make this point for a reason:

    Until the invention of some pretty nice optical instruments, only the brighter planets were known, and until the invention (and insightful use) of some much better instruments (telescopes, spectroscopes, etc), it was thought that Andromeda and other "nebulae" were clouds of gas in our galaxy. The existence and natures of these planets and galaxies were in NO way affected by our ability or inability to see them or to understand their natures. They did not miraculously come into existence or change their attributes when we managed to perceive them more accurately. Our viewpoint and perception do NOT matter to the universe.

    Speaking of Andromeda, that galaxy is about 3 million light-years from us. An observer presently living in that galaxy has a "visible universe" that is just a bit different from ours. It is a spherical volume (just like ours), but it is offset by 3Mly. That observer can see things (opposite the direction of the Milky Way) 3M light years more distant than we can see. Looking back in our direction, that observer's visible universe will be foreshortened by 3M light years, compared to our visible universe. Does that limitation (Andromeda observer cannot see quite as far as we do in the direction of the MW) have any effect on OUR perception of the universe, or on the nature of the universe in total? No. That would confer a special nature to the frame of reference of the Andromeda observer, and that is forbidden by the Standard Model (and by simple logic). Similarly, OUR frame of reference has no effect on the existence or nature of the parts of the universe that may be unobservable to us.

    The "observable universe" is an artifact of the limitations posed by our particular location and by the speed of light. The observable universe is a subset of the Universe. The Universe cannot be constrained by our particular limitations (no special reference frame), so one cannot posit a finite Universe on the basis of our observable universe - it is forbidden by the Standard Model. Any model in which our frame of reference can impose limitations on the frames of reference of distant observers, or on the Universe as a whole, is a non-starter. Talk about fingernails on the chalk-board!
     
  11. Feb 12, 2005 #10

    Chronos

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    Turbo-1. There is a difference between unobserved and unobservable. You may be unobserved, but you are not unobservable. Assuming BB is correct, every observer in the universe will conclude the universe is NOW 13.7 Gy old. No observer, NOW or in the future, will ever conclude it is observationally older than we do. How does that confer a priveleged position to any observer? The universe may be metaphysically infinite, but not observationally infinite.
     
  12. Feb 12, 2005 #11

    turbo

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    Dear Chronos:

    If we decide that the visible universe is the only valid framework for scientific inquiry, and that no inference can be made about the extent or nature of the of the universe as a whole because we cannot see past our surface of last scattering, THAT particular choice confers upon us a "special" frame of reference.

    The idea that only the visible universe is fair game for scientific inquiry is a Special Relativity view of the universe. It will give us problems, though, when we try to address the GR/FRW universe of the standard model. The concepts of simultaniety and "no special reference frame" are central to cosmology. They allow cosmologists to envision overlapping light cones, estimate the present size of the visible universe based on apparent lookback time and expansion rate, etc.

    Our recent exchange on this (see post 41 and follow-ups) probably highlights the differences between the SR view and the GR/FRW view as well as can be expected. :smile:

    https://www.physicsforums.com/showthread.php?t=60934&page=3&pp=15
     
  13. Feb 13, 2005 #12

    Chronos

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    How is that? Every other observer in this universe sees exactly the same thing.
    What other universe is 'fair game'? What does SR have to do with that?
    What problems?
    I don't think it is relevant to this discussion, but, I see no conflict between SR and GR. Please explain what GR predictions conflict with SR.
     
  14. Feb 13, 2005 #13

    turbo

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    If you consider the reference frame of a theoretical observer presently existing in a galaxy that we can see at the edge of our detectable limit (about 13Gly away), you will see that over half of his observable universe lies outside of our observable universe. To say that our observable universe is the only universe that is scientifically relevant...
    is to deny the validity of the reference frame of the distant observer.

    The universe, as envisioned by most cosmologists, includes the condition of simultaneity, which allow us to consider the reference frames of theoretical distant observers as equivalent to our own. In SR, gauging the simultaneity of events is not possible, and it is possible to get trapped into characterizing the distant observer's reference frame based on whether or not he can communicate with us, and how long the communication would take, etc.

    For example, the problem of considering only our visible universe (with its limitations arising from our position and the speed of light) as the only "real" universe worthy of scientific inquiry.

    Here is a fairly illuminating exchange regarding the conflicts that can arise if we insist on applying SR's "no simultaneity" to the GR/FRW universe.

    The above exchange (additional comments in parentheses) arose from a proof I gave that a flat or open FRW universe must be infinite (unless we impose a non-trivial topography on it). If I am not mistaken, each of the objections you raised to the proof was based on the SR notion of "no simultaneity" and the insistence on communication to validate the reference frame of the theoretical presently-existing distant observer.

    I did a little searching to find someone who can explain this SR/cosmology problem better than I can, and I found this archived post by Marcus replying to a question by "confutatis maledictis":
    https://www.physicsforums.com/archive/topic/t-16465_Confutatis_maledictis_(text_of_Latin_requiem).html

     
    Last edited: Feb 13, 2005
  15. Feb 13, 2005 #14
    Isn't it possible that galaxy "B" that we observer to be 13Gly away and in the past is now so far away that it is racing away from us faster than light so that we will never see what it is like at the present moment? Isn't this most likely the case since the expansion is accelerating?

    If distant galaxies are leaving behind a cosmological event horizon, then what does that say about the entropy/information content of the observable universe? Perhaps it is more than a coincidence that the universe started to expand about the same time that life arose. Perhaps the complexity of life is compensation for information disappearing behind the cosmological event horizon. What do you think? Thanks.
     
  16. Feb 13, 2005 #15

    turbo

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    Not only possible, but required, if we live in a BB universe with accelerating expansion. Galaxies will be redshifted into the cosmic microwave background and become undetectable, beginning with the most distant ones.

    The galaxies are not crossing a real boundary. Each observer in an accelerating BB universe will have his/her/its own visible universe, each perceiving (given enough time) that more distant objects are being red-shifted into undetectability. These visible universes may or may not overlap, and have no real boundary. The "boundary" is merely an observational limitation faced by each observer.

    Is anybody giving serious consideration the idea that there is an "information budget" for each observer's visible universe that somehow has to be conserved? I haven't heard that one before, nor the idea that the universe started to expand when life arose on Earth. Do you have links or references? (I'm spoiled by the Internet, obviously. :smile: )
     
  17. Feb 13, 2005 #16
    What difference is there between disappearing behind the event horizon of a black hole and disappearing behind the cosmological event horizon? Both have objects being accelerated so fast that we lose all contact with it. So if we have concerns about where the information of objects go when it falls into a black hole, then I would assume that there must be the same concerns about objects falling behind the cosmological event horizon as well.

    I wonder if there is not a type of Hawking radiation associated with the cosmological event horizon as there is for the event horizon of a BH. Perhaps this is the source of zero point energy of space a.k.a, the cosmological constant. Since everywhere is the cosmological event horizon of somewhere else, the Hawking radiation of the cosmological event horizon would have to exist everywhere as well, right?

    Google... "conservation of information"
     
  18. Feb 13, 2005 #17

    turbo

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    When I do that, I get lots of Dembski links. I encourage everyone to try this once, and get it out your systems. Pure crap. Remember the name.
     
  19. Feb 13, 2005 #18
    I think his method might be flawed. But the question still remains. To repeat myself:

    What difference is there between disappearing behind the event horizon of a black hole and disappearing behind the cosmological event horizon? Both have objects being accelerated so fast that we lose all contact with it. So if we have concerns about where the information of objects go when it falls into a black hole, then I would assume that there must be the same concerns about objects falling behind the cosmological event horizon as well.
     
  20. Feb 13, 2005 #19

    JesseM

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    If black holes didn't evaporate through Hawking radiation, then there would be no information loss paradox, because you could assume the information was still present, but just hidden behind the horizon. There's no Hawking radiation with cosmological event horizons, and we're certainly never going to see the rest of the universe beyond the horizon evaporate away to nothing, so again, no information loss paradox.

    Dembski is a creationist crackpot, by the way. Read this for a nice refutation of his arguments.
     
  21. Feb 14, 2005 #20

    Chronos

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    Let me try a different approach. A universe that is observationally finite to all observers may be huge, but still finite. An endless sum of finite quantities is still finite.
     
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