<p>From our observations there is only a lower bound, but not an upper one. The radius of curvature is <span class="math-container">$\rm r_k=c/(H \sqrt{-\Omega_k})$</span> where the observations tell us that <span class="math-container">$\rm \Omega_k=0\pm \Delta$</span> with <span class="math-container">$\Delta$</span> as the margin of error, so it must be at least a proper distance of <span class="math-container">$\rm d=2 \pi c/(H \sqrt{-\Delta})$</span> from you to yourself if the universe was indeed closed.</p>
<p>If it is not about the observations but about the limit of relativity there is none, the universe could be arbitrarily close to zero, infinity or even truly infinite.</p>

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2024-12-22T19:07:02.377Z
2024-12-22T19:12:07.273Z
2024-12-22T19:12:07.273Z
Yukterez
<p>As far as I know, you cant really tell. Thats because of the expanding &quot;motion&quot; of the universe. At some point the space is expanding so fast that the light isn't fast enough to reach us. So basically you can't see further than this Horizon. =&gt; observable universe</p>
<p>And if you wonder why these masses outside of that horizon can move faster than light, they aren't. The space between us and the masses is growing, so they could theoretically stand still, while for us it is looking like they are going faster than light.</p>
<p>Maybe you could calculate it, if you knew the complete mass(and the distribution) and the expansion/forces of the universe at the big bang.</p>
<p>Hope I could help you.</p>

CC BY-SA 4.0
2024-12-22T17:54:03.940Z
It's true that we can't see beyond the horizon of the observable universe, but the fact that we can't detect any curvature implies a much larger minimum size to the whole universe. If I recall, it's about 40 times the radius of the observable universe. I want to know if there's a corresponding upper limit. Since we can't see anywhere near that far, any such limit would be theoretical.
TESch
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2024-12-22T17:59:34.143Z
Also, the distance at which space is receding faster than light represents the horizon beyond which light will never reach us, even given an infinite amount of time. There's a nearer horizon where space is receding at sub-light speeds but the light just hasn't had time to reach us yet. That light will reach us eventually, but we can't see beyond that horizon now.
TESch
CC BY-SA 4.0
2024-12-21T21:13:18.263Z
2024-12-21T22:44:48.820Z
2024-12-21T22:44:48.820Z
rapzap
<p>If the universe is finite, is there an upper limit to its possible size, such that the universe must either be infinite or smaller than some maximum size? Or any reason why a smaller size would be more likely than a larger one? I'm not asking how big it can eventually get by expanding given enough time, but rather, how big (but finite) it might be right now.</p>
<p>If the range of possible finite sizes is unbounded, and it has no &quot;preference&quot; for smaller sizes, it would seem infinitesimally unlikely to fall into a small enough size range for its curvature to be anywhere close to what we could detect. In other words, even a finite universe would be overwhelmingly likely to be vastly, unimaginably larger than the minimum size implied by the smallest curvature we could (and don't) detect. Right?</p>

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2024-12-21T10:41:58.243Z
Related, *possible* duplicate: https://astronomy.stackexchange.com/q/31793/16685
PM 2Ring
<cosmology><universe>
Is there an upper limit to how big a finite universe might be?
TESch
# Is there an upper limit to how big a finite universe might be?

(Asked by: username_0 on 2024-12-21)

If the universe is finite, is there an upper limit to its possible size, such that the universe must either be infinite or smaller than some maximum size? Or any reason why a smaller size would be more likely than a larger one? I'm not asking how big it can eventually get by expanding given enough time, but rather, how big (but finite) it might be right now.

If the range of possible finite sizes is unbounded, and it has no "preference" for smaller sizes, it would seem infinitesimally unlikely to fall into a small enough size range for its curvature to be anywhere close to what we could detect. In other words, even a finite universe would be overwhelmingly likely to be vastly, unimaginably larger than the minimum size implied by the smallest curvature we could (and don't) detect. Right?

**Tags:** cosmology, universe

> **Comments:**
> **username_1**: Related, *possible* duplicate: https://astronomy.stackexchange.com/q/31793/16685  

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## 2 answers

### Answer by username_2 on 2024-12-22. Score: 1 | Accepted answer

From our observations there is only a lower bound, but not an upper one. The radius of curvature is $\rm r\_k=c/(H \sqrt{-\Omega\_k})$ where the observations tell us that $\rm \Omega\_k=0\pm \Delta$ with $\Delta$ as the margin of error, so it must be at least a proper distance of $\rm d=2 \pi c/(H \sqrt{-\Delta})$ from you to yourself if the universe was indeed closed.

If it is not about the observations but about the limit of relativity there is none, the universe could be arbitrarily close to zero, infinity or even truly infinite.


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### Answer by username_3 on 2024-12-21. Score: 2

As far as I know, you cant really tell. Thats because of the expanding "motion" of the universe. At some point the space is expanding so fast that the light isn't fast enough to reach us. So basically you can't see further than this Horizon. => observable universe

And if you wonder why these masses outside of that horizon can move faster than light, they aren't. The space between us and the masses is growing, so they could theoretically stand still, while for us it is looking like they are going faster than light.

Maybe you could calculate it, if you knew the complete mass(and the distribution) and the expansion/forces of the universe at the big bang.

Hope I could help you.

> **Comments:**
> **username_0**: It's true that we can't see beyond the horizon of the observable universe, but the fact that we can't detect any curvature implies a much larger minimum size to the whole universe. If I recall, it's about 40 times the radius of the observable universe. I want to know if there's a corresponding upper limit. Since we can't see anywhere near that far, any such limit would be theoretical.  
> **username_0**: Also, the distance at which space is receding faster than light represents the horizon beyond which light will never reach us, even given an infinite amount of time. There's a nearer horizon where space is receding at sub-light speeds but the light just hasn't had time to reach us yet. That light will reach us eventually, but we can't see beyond that horizon now.  

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