Is the Universe Both Flat and Accelerating?

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The universe is nearly flat, suggesting it should expand forever, but it is also experiencing acceleration, which complicates the model. An open universe does not accelerate; all geometries—open, flat, closed—are decelerating in the presence of ordinary matter. However, accelerating spacetimes can still be consistent with flatness, as they tend to become dynamically flat. The introduction of dark energy, which is believed to dominate the universe's energy content, alters the traditional understanding of these models. Current theories suggest the universe is flat, with dark energy driving an accelerating expansion, although the exact nature of dark energy remains uncertain.
oXDawidXo
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Hi!

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?
 
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oXDawidXo said:
Hi!

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?
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.
 
oXDawidXo said:
Hi!

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?

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.
 
I think I get it now
Thank you
 

Thread 'Matter density right after the decoupling'

https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
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Thread 'Hubble Constant, Dark Energy and the Expanding Universe'

Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now increasing. Can you clarify this enigma? Also., if the Hubble constant eventually decreases, why is there a lower limit to its value?
View full post »

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