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Questions about the balloon analogy

  1. Jan 13, 2016 #1
    I've been thinking about the balloon surface analogy and I want to ask some questions. Perhaps I am pushing it too far. But in that case it seems important to find its limits. My understanding is that the this analogy is used to illustrate the expansion of the universe and in particular how it is that even though (with the exception of a few close galaxies) everything in the universe is moving away from us. This does not mean that we are the center or source of the universe. It actually means that every point in the universe is moving away from every other point. Just like when you inflate a balloon. All points on the surface of the balloon move away from each other as it inflates.

    To the questions:

    1. The balloon surface analogy is a two dimensional illustration of something that is happening in three dimensions. If so, can we talk about what that thing is? Would this image work: Say you have a highly compressed rubber ball and that there are tinny grains of sand randomly embedded in it. Then imagine that there is a mechanism by which we can uniformly increase the volume of all the rubber, gradually. In this scenario all the grains of sand would also be moving away from each other, but in three dimensions. Is that an accurate way to bring the analogy in to three dimensions?

    2. We know that "all" galaxies are moving away from us because of red shift. We also know that the speed of recession is relative to distance from us. The farthest galaxies recede at a faster pace than the nearest ones. Taking both speed and distance into account we can "turn back" the clock and verify that all those galaxies meet back "here" at some point in the past, showing that the universe was once extremely dense. Now to my question: presumably we are also moving. If so, how come extra-galactic objects seem to be moving away from us? Shouldn't they be moving away from a different point since we are presumably not in a "direct line" with that denser state/place (we have since moved)? It doesn't seem possible for us to both be moving away from that core and still be in direct line with all the other objects that are moving away from it. This probably just shows that I con't get what is happening at all! One explanation might be that extra galactic objects have lateral speed which we cannot detect. But if this were the case we would not be able to measure a consistent speed to distance ratio in those objects. But we do. Therefore there is little lateral velocity in red shifted extra galactic objects (right?).

    3. The other notion the balloon image is meant to express is that the universe itself (not simply the objects in it) is expanding. Are we to think, then, that objects are being "carried along" by "the universe" and that is why they are receding, not because of the "explosive" effect of the Big Bang. In any case, what exactly is "the universe" if it is not matter? What is mean by saying that "the universe" is expanding if that means something more than or other than the objects in the universe? Are we saying that there is no such thing as empty space or dimensions outside the universe?

    Thanks for reading. I'm looking forward to responses.
  2. jcsd
  3. Jan 13, 2016 #2
    Firstly, can I point you to to this excellent Physics Forums article where you should find all you need to know about the balloon analogy and its limitations. To pick up on a couple of things you should check up on:

    I prefer to say that everything is "getting further away", rather than "moving away" - I will come back to this later.

    Correct - in fact there is NO single point that is the center or source of the universe.

    No that doesn't work - the rubber ball has a centre and grains of sand near the centre get further away from each other more slowly than those near the surface. There isn't really any way to bring this analogy into three dimensions other than considering the 3D Universe itself.

    Delete the words 'back "here"' from that and this is correct.

    No, nobody is moving - everything is just getting further apart (or receding). This is where the balloon analogy really works for me - a point marked on the balloon does not move across the surface of the balloon as it inflates, but it does get further away from other points.

    If you take in the points I have made above and/or in the Insights article I linked to, you should now be able to see why all of this is wrong.

    Yes - and there is no explosive effect of the Big Bang (do not confuse this with the concept of "inflation" - you don't need to know about inflation until you have thoroughly grasped the main concept of the expansion of space).

    In addition to all matter, the Universe also includes all energy and the space in which the matter and energy exists.

    It means that the objects are getting further apart.

    The Universe includes by definition all matter, energy and space so there is no matter, energy or space outside the Universe. I don't know what you mean by "dimensions outside the Universe".
  4. Jan 13, 2016 #3


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    Although MrAnchovy's answer is pretty much complete I would like to stress out some points: There is no "inside" and no "outside" the balloon, in the balloon analogy. So, saying
    is not the proper way to think about it.

    What does it mean here "different point" and "direct line"? According to relativity these things cannot be defined in the obvious classical way.

    The Universe has definitely matter but as mentioned has energy and space too, as it has dark matter and dark energy as is explained by today's acceptable cosmological models.
  5. Jan 13, 2016 #4
    Thanks for the answers and the article on the balloon analogy is great. I guess one of the things that is so compelling about the balloon analogy is that the popular mind wants a way to visualize what the universe is, but apparently physics cannot provide the requisite image. And so the balloon seems very attractive. I think that behind all my questions, lies this one: what IS the shape of the universe? Is there no answer to this? Is it not an object that we can imagine? Or do we just not know?
  6. Jan 13, 2016 #5


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  7. Jan 13, 2016 #6
    I noticed a mention in the article about the "raisin bread analogy". I guess that is the correct 3D version of the analogy. There is no center in that: http://map.gsfc.nasa.gov/universe/bb_tests_exp.html
  8. Jan 13, 2016 #7
    No, the "raisin bread" analogy is wrong. NASA is good, but nobody is perfect.

    As to the shape of the Universe, we know that it is very near to being flat, however the margin of error is such that it might be either infinitely expanding or ultimately limited, we don't currently know.
  9. Jan 13, 2016 #8
    When you say "flat", are we to imagine that space is longer and wider than it is high? Flat but with height extension? Like a pancake (to use a wildly incorrect image, no doubt! But just for the point of height). Then this feeds into the question of whether space is flat, positively or negatively curved: http://www2.connectseward.org/shs/students/students15/adamsiebrandt/Space%20Website/shape-of-universe.jpg [Broken]. Am I getting on track here?
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  10. Jan 13, 2016 #9


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    Hi Rob.
    No, that's not what 'flat' means in the context of cosmology. 'Flat' describes geometry of space - i.e., what happens to parallel lines (do they stay parallel, diverge or converge?) or what is the sum of angles in a triangle (180 deg? more, less?).

    Why do you dislike it? It's not that much different from the balloon. As long as we remember that both are just analogies with limited applicability...
  11. Jan 13, 2016 #10
    Not really. Whilst these questions seem easy, they are not and the answers to them are not relevant to the rest of physics. Accept that the Universe is really big and move on.
  12. Jan 13, 2016 #11
    The 3D raisin bread has a centre of expansion, the 3D Universe does not.
  13. Jan 13, 2016 #12


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    I'm o.k. with that limitation. All you need to imagine is an infinite raisin bread :) .
    The balloon has its own share of issues: an extra dimension, and positive curvature. Judging by the questions asked here on PF, the balloon analogy breeds many more misconceptions than the bread ever did.
    All I'm saying is that as long as we know how to use the analogies, and remember that they're all 'wrong' it's all fine.
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