Open vs. Flat Universe

  • #1

Main Question or Discussion Point

So CMBR points to a flat universe, and this seems to be the generally accepted model. But in a flat universe is expansion not supposed to slow exponentially, stopping after an infinite time? How does this fit with the observation that distant type Ia supernovae show the universe's expansion to be accelerating, which would point to an open universe.
 
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  • #3
PeterDonis
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But in a flat universe is expansion not supposed to slow exponentially, stopping after an infinite time?
Only if there is no cosmological constant/dark energy.

the universe's expansion to be accelerating, which would point to an open universe.
No, it points to there being a nonzero cosmological constant/dark energy. An open universe with zero cosmological constant would still have slowing expansion; but it would never slow to a complete stop, even after an infinite time. The only way to get accelerating expansion is to have a nonzero cosmological constant/dark energy; and such models are possible with closed, flat, or open spatial geometry.
 
  • #4
Only if there is no cosmological constant/dark energy.



No, it points to there being a nonzero cosmological constant/dark energy. An open universe with zero cosmological constant would still have slowing expansion; but it would never slow to a complete stop, even after an infinite time. The only way to get accelerating expansion is to have a nonzero cosmological constant/dark energy; and such models are possible with closed, flat, or open spatial geometry.
Thanks!
 

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