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Flat and accelerating Universe

  1. Mar 11, 2010 #1

    I'm doing physics project and I'm stuck with a really awful paradox.:confused:

    Universe appear to be flat (nearly), so it should expand forever (the critical universe). On the other hand we've got acceleration of the universe so open model. Which model is the right one?
  2. jcsd
  3. Mar 11, 2010 #2


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    An open universe is not accelerating. In the presence of ordinary matter and radiation, all three geometries-- open, flat, closed -- are decelerating. Closed universes eventually begin to contract again. Flat and open universes expand forever at a decelerating rates.

    An accelerating spacetime is not inconsistent with flatness. In fact, accelerating spacetimes are driven towards flatness dynamically.
  4. Mar 11, 2010 #3


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    It sounds like you are reading some slightly older material. At one point we classified the Universe under the simple 'open/flat/closed' system because we thought the only thing that mattered was the density of matter. If there is not enough, we have an open universe, too much we collapse and just right leads to asymptotic decceleration.

    It turns out though that there is more to the Universe. In fact we think most of the energy today is not in the form of matter but 'dark energy' which rather than acting to slow the expansion speeds it up.

    Once you include dark energy, the geometry (open/flat/closed) and the fate of the universe (expand forever/collapse) become different questions, it depends on the properties of dark energy which are not yet fully understood.

    Our present best guess is that the universe is flat, in that the sum of matter + dark energy density equals the critical density, but that the dark energy dominates and the expansion is accelerating. Out best guess says that this will continue in the future and the expansion will grow ever more rapid, but that question is far from settled.
  5. Mar 13, 2010 #4
    I think I get it now
    Thank you
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