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Relationship between expanding universe and time

  1. Apr 21, 2012 #1
    Some background: I am a senior in high school taking astronomy, so admittedly i do not know much about the structure and physics of the universe. However this is something that ive come up with on my own. I spend a lot of time just thinking about the universe, and this is one thing that just came to me a few days ago. No other explanation for it really. Is there any sense in my path of thinking and the connections i make? Are there some things that i am misunderstanding? Any types of opinions are appreciated.

    To begin, something i learned in class and from reading;
    The most popular model of the expansion of the universe is a balloon being blown up. The surface of the balloon is our 3 dimensional world being projected onto the 2 dimensional surface. We cannot leave the surface of this balloon. As the balloon expands, our world expands in all directions at the same time. This is why there is said to be no center of the universe, since the surface of the balloon is round there is no middle point, no point of origin.

    This i believe i understand, but im not really 100% on it. The big bang happened and our universe is inexplicably expanding/accelerating in all directions. Great, but theres something missing about the balloon model. I begin to wonder, what does the interior of the balloon represent? I will return to answer this question.

    I’ve read before somewhere that time is considered a dimension in itself. However time as a dimension behaves differently from the other three dimensions that we live in. Time, for some reason, is that only dimension that we can only travel forwards through, and cannot change our position or go back on. All other dimensions can be altered, but time seems to be this overseeing unchanging rule. It is relative to each point in the universe, and seems to be unstoppably ever-changing.

    So the conclusion i came upon is what if there is a connection between time and that internal dimension of the balloon? What if the expansion of the universe is what is causing us to perceive time itself? What if the same reason we cant go back in time is why the universe continues to accelerate? The interior of the balloon is the dimension of time, the same way that time is relative to every point in the universe, the universe is expanding at every point simultaneously, the expansion of the universe and the passage of time itself are one and the same thing.

    Or maybe im just thinking too much. Where can i find more information about the topic? What do you geniuses think? Where did i go wrong? :P

    Thanks for your time and for reading.
  2. jcsd
  3. Apr 21, 2012 #2
    That's where your error lies. It's not a model. It's an analogy. In this analogy, only the surface of the balloon is considered. It is using a two dimensional ANALOGY of the three dimensional universe. So for the purpose of the analogy, there is no inside of the balloon.
  4. Apr 21, 2012 #3
    Svo, as alexg said, the balloon analogy is just an analogy. General relativity allows for a space to be similar to the surface of a 3-sphere, but not be embedded in 4-dimensional space.

    Think of a one dimensional universe, the line making a circle. This circle has a circumference of 10 meters. Consider a single point x. If a 1-dimensional being on this circle traveled 10 meters from point x, it would find itself back at the position x. This essentially represents the same thing as a universe that repeats after 10 meters, but without being embedded in any extra dimension.
  5. Apr 21, 2012 #4
    I see. I had a feeling that may be where i was going wrong. Where can i read more about the expanding of the universe? I feel as though im not grasping it properly.

    @Mark that doesnt really help me because there is a center to the circle the same way there is a core to the balloon. What exactly do you mean?

    Im not understanding how the universe could repeat itself without being folded in on itself. What would be a proper model of the universe if not a balloon? I dont see any other way to make sense of it.
  6. Apr 21, 2012 #5
    I would suggest The Inflationary Universe by Alan Guth.

    Again, he's just using an analogy to show how something (the universe) can be finite and unbounded.
  7. Apr 21, 2012 #6
    i understand the word analogy but i dont understand how it can be possible.
  8. Apr 22, 2012 #7


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    Your posts are thoughtful but you are overlooking some things.

    First, you are mistaken about there being a center to the circle because you insist on continuing to regard the balloon as a 3D object when the analogy ONLY allows you to consider the 2D surface. To get to the "center" that you insist on for a great circle (or any other circle) on that 2D surface, you HAVE to leave the 2D surface and that is not allowed in the analogy. This is where everyone who has trouble with the balloon analogy has their problem. We live in 3D and find it difficult to truly conceptualize a spherical flatland. You would do well to check out the concept of a geodesic. A great circle on the balloon is a geodesic, and in some very real senses, a geodesic is a "straight line" even though our senses tell us it is curved. That is, on the 2D surface, the "circle" is NOT a circle, it is a straight line.

    Another thing is that the concept of tying time to spacial expansion just doesn't work. Time is a linear flow (forgetting for the moment about relativity and time dilation, which don't apply here) but the expansion of the universe is not at all linear. In the first tiny fraction there was amazing inflation, then there were some 8 or so billion years of slowing expansion, and then for the last 5 billion years or so there has been an accelerating expansion. If the dimension of time varied widely as have the dimensions of space, there are observations that would not have their known values. It just doesn't work.
  9. Apr 24, 2012 #8
    One way of looking at it:
    The inside of the balloon is the past, with the Big Bang in the center. The surface of the balloon is the present, while outside the balloon is the future.
  10. Apr 24, 2012 #9


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    I think you're taking the analogy WAY beyond where it can comfortably go. Just stick to the surface of the baloon. There is nothing else in the analogy.
  11. Apr 24, 2012 #10
    The balloon analogy can be easily represented on a 2d surface that's right in front of you... Time for me to show my age (or not) but the analogy I think can still work if you think of a (computer) game like asteroids, if you go off the left side, you "wrap" around to the right side, if you wish to expand space (though not exactly right, its close enough) simply increase the resolution of the screen. This represents a finite universe that doesn't have an edge (so to speak).
  12. Apr 24, 2012 #11


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    Actually, Octavianus, that is a very good portrayal. The current surface of the balloon represents the present. The balloon was smaller in the past and will be bigger in the future. Any observer at any particular time can only see the surface of the balloon. Now here's the weird part. When you look deep into space you are seeing the past surface of the balloon so it does not even look like a balloon, more like the graph of some exponential function that approaches zero as t approaches zero.
  13. Apr 24, 2012 #12


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    NO, that is not correct. The balloon analogy is NOT intended to represent a finite universe. It is ONLY for the purpose of showing how things all move away from each other. It is a very unfortunate analogy because it causes all kinds of misunderstandings, such as yours.
  14. Apr 24, 2012 #13
    I really don't like this analogy for another reason than what was pointed out by phinds. Your example suggests that the universe wraps around on itself, hence implying *positive curvature. But we know to high precision that our universe is flat and so this 'wrapping around' effect that you're describing is impossible.

    *edit - said negative initially when I meant positive.
    Last edited: Apr 25, 2012
  15. Apr 24, 2012 #14


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    Actually, that's not true. As you say, we know to high precision that our universe is flat but that does NOT say that it IS flat, so on a huge scale, it's possible that it does wrap.
  16. Apr 24, 2012 #15
    I suppose, but by that logic it's equally possible the universe is negatively curved and so it still wouldn't wrap around on itself. If the 3 options are equally likely, even though we currently measure flat, then there's 66% chance it wont wrap around on itself. I guess we'll never know though.

    edit - I said positive when I meant negative. Got the terms mixed up. Negative = hyperbolic, Positive = spherical.
    Last edited: Apr 25, 2012
  17. Apr 24, 2012 #16

    George Jones

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    A closed spatially homogeneous and isotropic universe has positive spatial curvature.
  18. Apr 24, 2012 #17
    Okay, I get it, I am certainly not of the mind that the universe is finite. I am of the mind that the universe has always been infinite.

    But you are right I should have ignored the wrap, the analogy would still work then right? It seems like a simpler explanation than the balloon one, where the same wrapping occurs and has a 3rd dimension that can lead to confusion.

    (and as far as the wrap is concerned if it happened sufficiently far enough away isn't it possible we could still see the observable universe as flat even though in the far far distance (far beyond our observability) that it does indeed wrap?)
  19. Apr 24, 2012 #18


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    That would be a function of our ability to measure it. It is certainly conceiveable to me that it could occur that the universe was so close to flat that it was beyond our ability to measure it within the observable universe.

    I don't believe that is likely to be the case and, further, I am not a big believer in coincidence so I am of the opinion that the fact that the universe is flat within out ability to measure it means that it IS flat. This IS just an opinion, thought, not substantiated by any facts.

    EDIT: It alread IS beyond our ability at the moment, but what I meant was that it is conceivable that it will ALWAYS be beyone our ability.
  20. Apr 25, 2012 #19
    Really? I've been under the impression that under those assumptions we use the FLRW model of the universe, which permits positive, negative, and 0 curvature in the universe.
  21. Apr 25, 2012 #20


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    00Svo you are not the first to come up with that suggested model:

    Self Creation Cosmology - An Alternative Gravitational Theory , published in 'Horizons in World Physics, Volume 247: New Developments in Quantum Cosmology Research', Nova Science Publishers, Inc. New York, 2004.
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