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Question about the expansion of the universe/inflation

  1. Jun 19, 2011 #1
    As far as I understand, it's accepted to say that the universe is expanding.
    And I had some thoughts on this recently.
    I think it was Michio Kaku who said that the dispersion pattern of planets, stars etc within the universe does not suggest an actual "explosion" in reference to the bang bang. Then i heard about inflation.

    So here's my question:
    For something to expand, don't the universe need something to "push" it from the inside or "fill" it? I saw a documentary where they used a large balloon to explain this. But it expands or inflated as it was, because air is pushing on it's insides. So what is pushing on the insides of the universe (assuming there is an outside).. Man i hate physics, so complicated :D

    edit/ I believe i put this in the wrong section? If a mod could move it for me that would be appreciated :)
    Last edited: Jun 19, 2011
  2. jcsd
  3. Jun 20, 2011 #2


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    Yes, you've run across one of the difficulties with trying to use every day analogies to describe some of the more nuanced aspects of our universe! The balloon analogy is a good one, but you have to know what about it is true and what is not -- the expanding into something part falls under the category of not true.

    If you like, you can imagine two ways in which an object could expand, either intrinsically or extrinsically. Normally, we think of objects expanding extrinsically, that is, with reference to some other object. We say the balloon has gotten twice as big, because we see that it is now twice as large as the balloon we haven't blown up. However, what if there is nothing else to compare the size of the balloon to? Well, it can still expand! This is rather the case of the universe. Mathematically, we have no trouble just describing it expanding without reference to anything else. Hence, there's no need for it to expand into anything at all!

    Now, the expansion itself is driven by the laws of gravity. You can think of the expanding universe (for the most part) as being due to an initial kick. If you throw a ball up in the air, the ball continues to rise even though it has left your hand -- it has inertia. Similarly with the universe. It was given an initial kick (at the instant of the big bang), and has inertia in its expansion.

    I'm ignoring dark energy and inflation to simplify the discussion, because I don't think they're relevant to what I'm trying to explain. Have I answered your question?
  4. Jun 20, 2011 #3
    So it's the inertia following the big bang? Interesting :) thx
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