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The Direction of Space

  1. Jun 12, 2003 #1

    Les Sleeth

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    Here is something I wonder about:

    If the big bang has created spherical expansion, with all resulting galaxies residing on the suface of the sphere, does celestial observation ever involve looking "across" the vast (assumed) empty middle to the other side of the sphere? Does anyone think light might travel around the periphery rather than straight through it? Is there a chance all observed galaxies are only those on the surface all around us, and much further away, on the other side of the expanding sphere, are galaxies we can never see?
    Last edited: Jun 13, 2003
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  3. Jun 12, 2003 #2
    i see what you're getting at, but i think your error is it the assumtion that "all galaxies reside on the surface of the sphere". you should think of it in terms that all current galaxies reside on the surface, but there are older galaxies further to the center. and remember, that there is no "center of expansion" so there really is no center of the spherical expansion. one could just as easily say we're the center of expansion as they could another point in space.
  4. Jun 13, 2003 #3

    Les Sleeth

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    My understanding is that part of the inflation model explains the universe's matter as expanding away from the big bang point, and existing in a huge spherical shape something like the surface of a balloon.

    I could see this "surface" being quite thick, but with inflation there should also be an empty core. I suppose I was asking if light could travel across that core, or if it is too vast and expansion too rapid for that light to ever reach us.
  5. Jun 13, 2003 #4


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    It is quite remarkable how many different visions of space there are!

    The "expanding balloon" model of a space with positive curvature was thrown out by cosmologists some time ago

    but they apparently did not bother to tell the rest of us!

    so people still picture the galaxies as dots painted on the surface of a balloon which is gradually expanding so the dots are getting farther apart----a lovely picture but unfortunately confusing because very wrong.

    a better picture, my cosmology professor told me years ago, is of a rising loaf of raisin-bread dough
    the galaxies are raisins
    and as the 3D loaf of space expands the raisins get farther apart.

    as cosmologists talk, for space to be "spatially flat" means that it is ordinary Euclidean 3D space----only expanding in time.

    It doesnt mean that it is flat like a 2D thing like a flat piece of paper but it has an analogous 3D flatness in that the sum of the angles in any triangle is 180 degrees

    maybe in small scale around stars and blackholes etc it is curved by local concentrations of mass-energy but in large scale space has an overall flatness-----or so they tell us the best and most recent observations show.

    so ordinary 3D space, like raisinbread dough extending to infinity in all directions, seems by far the best model

    and there is no HOLLOW that it curves around and encloses!!!!
    that way lies madness. Perish the thought!!!
    Alan Guth, who graciously invented the "Inflation Scenario"
    for our edification, never said anything about expanding balloon shape or hollow. This is not part of any currently credible big bang or inflation picture at least that I have heard of.
    Last edited: Jun 13, 2003
  6. Jun 13, 2003 #5

    Les Sleeth

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    See, I knew this was a dumb question.

    Thanks Marcus/Maximus
  7. Jun 13, 2003 #6
    If the universe is flat, it doesn't mean that it has a center?
  8. Jun 13, 2003 #7
    Not if it's infinite.
  9. Jun 13, 2003 #8


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    LW Sleeth - As already well explained (so why am I repeating it?!?), the Big Bang was not an expanding sphere from a central point with galaxies on the outer edge (that image is the result of an inadequate balloon analogy...the raison bread analogy is slightly better but still incorrectly implies an outer expanding edge). The Big Bang happened everywhere in the universe, not from a central point. The points of space across the universe just used to be a lot closer together than they are now.

    meteor - Like Mentat said, a "flat" universe is believed to be infinite (or at least boundless)....no center & no edge in 3D space.
  10. Jun 13, 2003 #9
    You can't say that. The universe started in a singularity, and has been growing since that, and only can reach an infinite size in an infinite period of time, that is not the case
  11. Jun 13, 2003 #10
    If the entire Universe, all of spacetime, began as a singularity, then you are right, it cannot be infinite now (IMO). However, it didn't necessarily all start out as a singularity, as it could be just "local expansion" (in that there could be many Universes, that "bubbled up" at the beginning of time, and our's just happens to be expanding).
  12. Jun 13, 2003 #11


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    Marcus, the balloon analogy is just that - an ANALOGY. Part of the analogy is that you must translate the 2D balloon surface into 3D space. It requires some mental gymnastics. It was never, AFAIK, meant to say that everything in the universe resides on a thin shell of a sphere - that would not account for Hubble's observations.

    The loaf of rasin bread analogy is also flawed because it includes an edge for the galaxy, something that is not observed.

    The expanding balloon analogy is often simply misstated and thats why the confusion exists. IMO, the balloon anaogy is the better analogy - it is borderless, finite, and uniformly expanding.

    AFAIK, the universe is not theorized to be infinite.
    Last edited: Jun 13, 2003
  13. Jun 13, 2003 #12
    I have read that the aparition of the universe is interpreted as a fluctuation of the vacuum, more or less like the appearance of virtual particles in quantum mechanics. Can a fluctuation have an infinite size?
  14. Jun 13, 2003 #13
    First off, that is just one hypothesis on the origin of the Universe.

    And, secondly, the fluctuation wouldn't really have size, as their would not yet be any space.
  15. Jun 13, 2003 #14
    Definitely, the balloon analogy is a very bad analogy. It would be a good analogy if the curvature of the universe was different from zero, but it has been showed that the curvature of the universe is zero, and the cake analogy is billions times better. The dream of Einstein of look at a telescope and watch his own back neck is not plausible

    Yes, but this hypothesis first introduced by Tryon is the standard hypothesis. You only have to Google and you will see that the majority of the webs says clearly that the Universe started in a very little fluctuation, and after undergo inflation in a fraction of second, reached the size of a tennis ball
  16. Jun 14, 2003 #15


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    Inflation seems to be the standard hypothesis, but is different than the an ex-nihilo varient floating around these days. While inflation doesn't explain the origins of the pre-existing expanding space-time, some like Alexander Vilenkin have proposed that the entire universe was created out of literally nothing at all, as a quantum fluctuation. Inflation occurs after this fluctuation has already brought the universe into existence.

    I'm not sure how popular this idea is among physicists.
  17. Jun 14, 2003 #16

    how can it ever be infinite? it will always be expanding at the max of a very finite speed (the speed of light). maybe you mean it will exist forever.(if it doesn't recollapse) but at no point in time (now or ever) will somebody observe the size of the universe to be infinite.
  18. Jun 14, 2003 #17
    First off, it is (according to modern measurement) expanding faster than the speed of light.

    Secondly, if all of space is infinite, but our local Universe is expanding, then the whole "Universe" (everything) is infinite, but we would still observe expansion.

    Well, no one can ever actually conclude that it is infinite (since it could just be really big), but it (spacetime) could still be infinite.
  19. Jun 15, 2003 #18
    my understanding is that as the universe expands, space expands with it. so what's beyond it? nothingness: no time/matter, ect...
  20. Jun 15, 2003 #19
    Thats true.
    Scientists already know that the Universe expands. They can see galaxy's spreading far apart, they move out into space. So i guess this means that the Universe does expand.

    I doubt whether the Universe if infinite.....When i think of it, i get all weird inside:wink: for some reason.

    I don't think that the Universe if a sphere, if it is, then what is on the other side? Nothing? Some sort of wall? Hmmm!
    Or is the Universe just a black hole going in reverse. Well thats what Stephen Hawking said, a massive black hole thats in reverse....you know, time starts to go...and eventually it dies. Well i don't know if it will result in the supposedly 'big crunch'
  21. Jun 15, 2003 #20

    Les Sleeth

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    I agree with you that time and space are defined by the beginning and, if it happens, the end of the universe. So they are not infinite and can never be.

    The question of "nothing" I think is a really important one which has no satisfactory answers. To say nothing does anything whatsoever, even something as apparently insignificant as a quantum fluctuation, means it wasn't "nothing" after all.

    In the philosophy area I argued that since everthing which exists must be preceded by the potenial to exist, we can look at what exists now and say something about the potentiality it sprang from.

    Even being conservative about it, we can say potentiality can produce a universe, life and consciousness. So is that really "nothing"?
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