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Universe expansion

  1. Sep 14, 2007 #1


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    hey , i was wondering if this would be true :

    at school we often tell us that the universe is in a constant expansion , that gravity only works on small scale distances, that most objects have a powerful cinetic energy that make them expand

    But now, even if the gravitational attraction between all the universes objects is fairly weak (big distances), it should still slowly decrease the universes expansion by decreasing the objects cinetic energy ?
    meaning that there will be a time T where most of the universes object will stop expanding ?
    And then most of this object will gain cinetic energy , but on the other direction (gravity), meaning that all of these objects would move back to reunite. and so it will form a very small , very dense object , and then re-explode like the bing bang , to start another cycle ?

    like if an universes has a cycle of : expanding , reuniting, and exploding

    Because to me that sounds more logic, than saying that it will expand to infinity.
    Because we would have to consider how come so much particules reunited itself in a small volume to then explose (big bang) ?
  2. jcsd
  3. Sep 14, 2007 #2
    There are 3 possibilities depending upon whether the mass in the universe is critical, less than critical or more than critical. But these 3 possibilites are themselves based upon Friedmann models and the Robertson-Walker metric, which may not represent the real universe. In arriving at these models, certain assumptions are made, for example ,that the mass of the universe is fixed at the time of its genesis, and that the velocity of light is constant in all eras - so unless all the assumed factors are actually true, our models are flawed and in the last analysis we cannot predict the ultimate fate of the universe.

    In recent years, new data has cast doubt on all previous models because the universe appears to be accelerating. You are however, safe to bet on any outcome; it is doubtful whether anyone will be around at the end to ask you to pay-up
    Last edited: Sep 14, 2007
  4. Sep 15, 2007 #3


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    the universe accelerating ? with what force ?
    is it that it appears to be that most objects are charged with the same charge, so the electromagnetic force countering the gravitational force ?
    or with some other massic objects around our universe ?
  5. Sep 15, 2007 #4


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    The object attributed to explaining the acceleration of the universe is the mysterious "dark energy." There are various theories as to what it is, but in reality, cosmologists do not really know. Here's a link to a few NASA articles on dark energy:

  6. Sep 15, 2007 #5


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    ok so from what i understood of these articles :

    Dark Energy : energy in space, constantly changes from matter to energy ( a bit like matter and anti-matter clashing to form energy, and latter this energy reforms matter) .
    And that this matter has negative mass, which to me would mean negative energy !! ??
    Which would repell ordinary matter away, making the universe expand. IF this it true, would it mean that this Dark energy is more in the middle than in the surface ? because if it was in the surface , it would make all the universes objects come together ?
  7. Sep 15, 2007 #6
    Same space, different time?

    Knowing matter cannot occupy the same space at the same time, does the Earth (or any amount of matter) ever occupy the same space at different times? Do we ever occupy the same spacial position in the universe?
    Last edited: Sep 15, 2007
  8. Sep 15, 2007 #7


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    The correct spelling is kinetic energy.

    It may seem more logical that the universe should work in the way you describe, but recent measurements have shown that the acceleration of the universe is not 'slowing down', as you argue it should, but actually accelerating.

    This requires something very weird - as you point out, gravity attracts, and we would expect the expansion to deaccelerate. (It's not really quite that simple, but we'll skip over the more complex analysis - the conclusion you come to is basically right)

    This very strange thing that causes the universe's expansion to accelerate has not been directly observed, but has been given the name "dark energy".

    You might want to read some popular articles about it, for instance http://physicsworld.com/cws/article/print/19419

    There are other comments one could make, but "dark energy" is probably the biggest difference between how you think the universe should work and how we've observed that the universe actually appears to work (at least, as nearly as we can tell).
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