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The n-th expansion question

  1. Jan 18, 2006 #1
    while i have no problem with the whole universe expanding thing, i have qualms with the balloon idea.

    you take an uninflated balloon, draw a dot on it, and blow it up. as well as the balloon increasing in size, the dot does also.

    if (ignoring the X^2/3 law :P) the human body was expanding gradually so proportions stayed the same, you wouldn't perceive your feet getting further from you, because they're getting larger as well. assuming no external reference, you'd never realise the expansion was occuring.

    i guess what i'm tryna ask is: if space is expanding surely everything, objects etc, must be expanding as a consequence, so how can it be stated that "galaxies are moving away as a result of *the expansion*," because we'd never perceive it.

    hope this makes sense
  2. jcsd
  3. Jan 18, 2006 #2


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    This question has been asked many times before in these Forums it is; "If space is expanding then what expands with it?"

    As I said before:

    It is generally accepted that gravitational attraction of the local gravitational fields, of the Earth, Sun, Milky Way galaxy, and possibly the Local Group, overwhelm the cosmological expansion and these bodies do not expand with the universe.

    Einstein himself wrote a paper in the 1940's to prove that the solar system was not co-expanding with the universe. He did so by cutting out a spherical volume from the cosmological model and replacing it with a void with a spherical mass inserted in the middle; thus embedding a Schwarzschild solution inside a cosmological one. The question is how do you take the limit of the Schwarzschild metric as [itex]r \rightarrow \infty[/itex]

  4. Jan 19, 2006 #3


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    Hi Saoist! It would be better to picture the dots as ink droplets resting on the balloon surface, not embedded. Surface tension will prevent the droplets from being pulled apart as the balloon inflates.
  5. Jan 20, 2006 #4
    Plus, there are no "dots" in our 3-d spatial Universe, except for maybe particles by their lonesome. So, what you envision as a "dot" would really be an accumulation of particles. It then should be a question of whether the other forces overwhelm/keep those particles together over what is causing the Universe to expand.
  6. Jan 20, 2006 #5
    If you have heard of Theory of Relativity, it points out a star as a massive object deforming the "fabric of space-time".
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