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I've yet to understand the finite universe

  1. Sep 4, 2004 #1
    They say it "curves in" on itself, and that this somehow resolves the paradox of finity (a time before the Big Bang and a space/non-space/nothing beyond the expanding edge of the expanding universe). Now I have an important question. If I were God, and the universe were floating in a fish bowl, then based on our current understanding would it even have a geometry that could be perceived and even illustrated? Or would that contravene its spacetime laws? Not long ago some scientists claimed it resembles a soccer ball, then they said no,no- it is more like a fountain. But how can they say these things when an absolute medium by which to define its edges necessarily eludes any rationalization? How can space itself have an end?
  2. jcsd
  3. Sep 4, 2004 #2

    IF space is in fact just an empty vacuum, then, I can't imagine a closed system. IF vacuum space is just another form of energy, then, I can imagine a closed system.

    IF GOD exist, I think he/she/it would have to exist outside of a closed system. IF we exist within a closed system, I don't think anything would exist outside of this closed system.

    IF the universe is comprised exclusively of energy in one form or another and the universe is "not" a perpetual motion machine, then, the fundamental force responsible for all motion within a closed system would have to be external to the system it acts upon.

    IF variations in space density are responsible for curved space/time, then, I think a finite universe is a reasonable possibility.

    IF energy can neither be created nor destroyed, then, a closed system is possible.

    IF any of this makes any since, then, maybe i'm not a complete crackpot.

  4. Sep 4, 2004 #3


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    here's a short animation
    if you havent watched it before
    you could watch it, if you want,
    and then come back and ask more question

    Ned Wright's balloon animation

    in this animation the universe is spatially only 2D
    so it is easier to picture it as edgeless

    the little wriggly things are photons
    they get stretched out so their wavelength gets longer
    you can see it happen

    the other things are galaxies
    they stay the same size but get farther apart

    i am not saying our universe is exactly like the one in the animation
    but it is a good picture of an edgeless or boundaryless universe to have,
    and it is finite in the sense that at any time you pick there is a largest distance between any two objects
  5. Sep 5, 2004 #4


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    By definition, a universe includes all that is possible to observe within it.
  6. Sep 5, 2004 #5


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    This is the definition of the Observable Universe, not the Universe. If the Big Bang model is correct, there is a LOT of Universe that we cannot observe, but that is posited to exist. In fact, there may exist an infinite Universe, of which our observable universe is a subset. Bear in mind, that if the Universe is indeed infinite, it is pointless to use relative adjectives like "small subset", "tiny subset" etc. Our (purportedly 13.7 Gy) observable universe collapses to a mere point relative to an infinite U.
  7. Sep 5, 2004 #6
    I'm confussed?

    Using Ned wrights analogy;

    Since light speed is C and radiates in all directions from its source, would this imply that in Neds animation, a volume of light would extend much farther out perpendicular from the sphere. Or would the radiation fold back within the confined volume containing all of the energy producing systems?
  8. Sep 5, 2004 #7


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    I'm not a huge fan of the standard big bang theory, but here's my take on cosmology developed under that model. The universe is 13.7 billion years old, and there is a hard limit of 13.7Gy on the chance of us seeing ANY electromagnetic radiation. The universe is infinite in spacial extent, but we cannot see any light (or radio or X-ray radiation) from further away than 13.7Gy (actually even a much shorter period, since the universe was not transparent to EM until much later in this model).
  9. Sep 5, 2004 #8
    i think that our universe or our POST is finite but it exist with an infinite amount of other pocket of space/time
  10. Sep 5, 2004 #9


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    That is certainly a possibility, just that whatever part of it we are precluded from viewing has no consequences we can observe. That makes it a candidate for a shave using Occam's razor. As Turbo noted, we can only observe things within a ~13.4Gy radius [depending upon when recombination occured]. Beyond that, there is nothing left to see because the universe becomes opaque at that distance. The 'eternal now' universe, of course, would look much different than the one we see [an 'eternal now' universe is what it would look like if tachyons exist and we had a tachyon telescope]. Now that would be quite a sight. A fella could make some $$ selling those, especially if you could dial in what speed tachyons you wished to observe.
  11. Sep 5, 2004 #10
    This is a very common misconception, but actually the radio of the observable universe is around 46 billion ly. THe more distant object observed is the protogalaxy with z=10 discovered this year, it is at a distance of 31'5 billion ly
  12. Sep 5, 2004 #11
    so after 46 billion light years our current devices cant see after that?

    and could it be possible to "peek" into other pockets of space time
  13. Sep 5, 2004 #12


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    Misconceptions about red shift are not uncommon, so I will clarify. At z=10 redshift you are looking at an object as it appeared when the universe was .48 billion years old. At present, it is at a distance of 31.5 billion light years. At around z=1000, you would be looking at an object as it appeared 300,000 years after the big bang, which is right at the cutoff point where the universe becomes opaque. At present, it is at a distance of 45.4 billion light years. But, without a tachyon telescope, we cannot see what either of these objects look like at present.

    These values, of course, depend on the values you assign to the Hubble constant and the matter-energy density of the universe. Try out this calculator and see if it helps clear things up.
    http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]
    Last edited by a moderator: May 1, 2017
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