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Does the current acceleration of universe imply that our universe is open? If the universe is closed, from the Friedmann's equation, the acceleration of universe wouldn't be possible, would it be? (Of course, except for the very earlier time of inflation era.)

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The answer is yes. – Ron Maimon May 2 '12 at 16:05

No. Universe that is accelerating can be open, closed, or flat in principle. The latter attributes are determined by how much matter/energy is in the universe relative to the critical value. If the energy is greater than critical then the universe is closed, if it's less than critical the universe is open, and energy equal to critical the universe is flat.

On the other hand, the condition for the universe to accelerate is for it to have a component with negative enough pressure (in technical terms, this is entirely obtained from the second Friedmann equation that gives the acceleration of the characteristic scale of the universe, which is not required when talking about flat/open/closed).

[That said, it happens that current cosmological data strongly favor a universe that is flat. But this is mostly independent of the statement that the universe is accelerating. ]

Without dark energy open/flat (closed) universe implies one that is expanding forever (ends in a Big Crunch). But with dark energy that drives the acceleration of the universe, this "geometry is destiny" link is broken. In particular, the future of the expansion can be arbitrary, as it depends on the future behavior of dark energy. However, if the dark energy continues to accelerate the universe as it is doing now, then the universe will expand - and also accelerate - forever.

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The answer is just yes. – Ron Maimon Jun 3 '12 at 18:30
-1: Although you end up saying correct things, you have to stop youself from making something simple complicated. There is acceleration, the universe is open, end of story. – Ron Maimon Jun 3 '12 at 18:49

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