<p>Depending on the mass of the universe, the universe has three different possible shapes.
<a href="http://en.wikipedia.org/wiki/Shape_of_the_universe" rel="nofollow">http://en.wikipedia.org/wiki/Shape_of_the_universe</a></p>

<p>It can be finite or infinite depending on the shape it has, and there are some ways we can discard possible shapes (if it started to shrink instead of expand).</p>

<p>Hope this helps</p>

CC BY-SA 3.0
2014-08-14T20:34:51.357Z
Well, it technically has three different curvatures, no three different shapes.
HDE 226868
CC BY-SA 3.0
2014-08-15T20:13:56.237Z
Universe topologies, if you like :)
saken
CC BY-SA 3.0
2014-08-14T18:49:38.513Z
2014-08-14T18:49:38.513Z
saken
<p>Space and time are total different things.</p>

<p>These citations are about the observable universe, and not from the whole.</p>

<p>We can see only a 13.7billion light year sphere from our spacetime. It is not very big.</p>

<p><a href="https://physics.stackexchange.com/questions/88786/lower-limit-of-the-size-of-the-universe-wmap">The current measurement of the WMAP satellties</a> says, the universe is probably infinite big.</p>

<p>Your questions can be explained with the Friedman equations. These equations are the solutions of the Einstein Field Equations for the infinite vacuum of our spacetime.</p>

<p><em>And its spacelike sections of our Friedman-spacetime is planar</em>, at least in the limits of the precision of the measurement. What currently can be said, is that "if the whole universe is finite, and has a (hyper-)spherical geometry, then its radius must be at least 300billion light years".</p>

CC BY-SA 3.0
2014-08-14T19:06:42.537Z
I will look into the Friedman equations.
Paul
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2014-08-15T07:10:24.913Z
The Friedmann equations are much more general than either the vacuum condition or being spatially infinite. The whole section is more than a bit confused. @HDE226868 The implication, which this answer alludes to but fails to communicate clearly, is that the WMAP observations indicate spatially flatness, which under the modeling assumptions of the FRW family of solutions (global isotropy and homogeneity) would imply an infinite universe.
Stan Liou
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2014-08-15T08:40:04.063Z
`We can see only a 13.7billion light year(radius or diameter?) sphere`, nah the universe, as many observations seem to show is about 13.5 billion years old, but the sphere of our visible universe is much bigger, [almost 50 billion lightyears in radius](http://en.wikipedia.org/wiki/Observable_universe)
DrCopyPaste
CC BY-SA 3.0
2014-09-06T21:58:31.180Z
@StanLiou Thanks for the information about WMAP.
HDE 226868
CC BY-SA 3.0
2014-08-14T16:21:43.190Z
2014-08-14T16:21:43.190Z
peterh
<p>So it seems the overall thought is that the universe is infinite. I don't get how.</p>

<p>Most depictions I read mention something to the effect of "at 1^-20 seconds the universe was the size of <strong>_</strong>". (Essentially mentioning a finite size at a very small length of time). So it came from a singularity, and had a finite size at some point. No matter how fast inflation went, any multiple of that finite size is NOT infinity though. If it was finite once, it is still finite.</p>

<p>If it's infinite then it was always infinite which means it was infinite density, infinitely large, etc. That's too crazy to swallow.</p>

<p>Isn't it more likely that the universe is really really big but not infinite? And we are at the center of the observable universe (to us), however we are off center in some grander universe that's maybe 1 trillion light years across? And nothing is past that because space and time are the same thing. Therefore if there is no light/matter/events, there is no time and hence no space.</p>

CC BY-SA 3.0
2014-08-13T22:40:38.177Z
I have to say, I don't think the community is leaning one way or the other. I can say that you're obviously right about us being in the center of the observable universe, but that because there is no center to the universe (see other questions on this stack exchange), we're definitely not in the center.
HDE 226868
CC BY-SA 3.0
2014-08-14T13:27:00.630Z
If it were finite in size then there is certainly a center. We just can't see it.
Paul
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2014-08-14T13:31:44.740Z
In the universe (on the largest scales), everything is moving away from everything else. I can assure you, there is no center. See this: http://physics.stackexchange.com/questions/25591/does-the-universe-have-a-center
HDE 226868
CC BY-SA 3.0
2014-08-14T13:34:53.170Z
Whoops, sorry, no single point that is the center.
HDE 226868
CC BY-SA 3.0
2014-08-14T19:05:35.477Z
@HDE226868 If it's finite in size, then there certainly is a center. We just don't know where it is because we are the center of our observable one. If it's infinite in size then there is of course no center. But if the universe is 1 trillion light years across, then there's a very obvious center which may or may not be in our own observable universe.
Paul
CC BY-SA 3.0
2014-08-14T20:00:20.653Z
You didn't understand the muffin/balloon analogy? There is no center.
HDE 226868
CC BY-SA 3.0
2014-08-14T20:26:50.910Z
If the universe is finite in size and flat then it must have a center. There's no way around that. Say it's a sphere of 300 billion light years across. There's a clear and obvious center. Now say we're sitting at an arbitrary point inside it. We can't see an edge and our center is us. Our observable center.
Paul
CC BY-SA 3.0
2014-08-14T20:31:21.253Z
And yes, I read the other page you linked to. But the balloon explanation fails for me. No point on the balloon inflating is the center. I get that. But that's a 2D representation. The universe as we see it is 3 dimensional. And that inflating balloon HAS a center. Not on the balloon, but inside it. A flat, finite universe must have a center. This is my hangup.
Paul
CC BY-SA 3.0
2014-08-14T20:33:11.537Z
We could be on the surface of a 3-manifold, i.e. on the surface of a 4-d object.
HDE 226868
CC BY-SA 3.0
2014-08-14T20:33:34.483Z
And no, the universe is not necessarily flat. Locally flat? Yes. Globally flat? Not necessarily.
HDE 226868
CC BY-SA 3.0
2014-08-15T00:22:57.787Z
Your main error is in the following: "So it came from a singularity, and had a finite size at some point." This implication is completely wrong about the universe at large. Your subsequent error is this: "That's too crazy to swallow." That's just appeal to personal incredulity.
Stan Liou
CC BY-SA 3.0
2014-08-17T18:03:01.190Z
@Paul, your ending sentence is illogical. Lack of matter and energy do not imply that there is no time or space.
HDE 226868
<universe><observable-universe>
Infinite universe? How?
Paul
# Infinite universe? How?

(Asked by: username_0 on 2014-08-13)

So it seems the overall thought is that the universe is infinite. I don't get how.

Most depictions I read mention something to the effect of "at 1^-20 seconds the universe was the size of **\_**". (Essentially mentioning a finite size at a very small length of time). So it came from a singularity, and had a finite size at some point. No matter how fast inflation went, any multiple of that finite size is NOT infinity though. If it was finite once, it is still finite.

If it's infinite then it was always infinite which means it was infinite density, infinitely large, etc. That's too crazy to swallow.

Isn't it more likely that the universe is really really big but not infinite? And we are at the center of the observable universe (to us), however we are off center in some grander universe that's maybe 1 trillion light years across? And nothing is past that because space and time are the same thing. Therefore if there is no light/matter/events, there is no time and hence no space.

**Tags:** universe, observable-universe
> **Comments:**
> **username_1**: I have to say, I don't think the community is leaning one way or the other. I can say that you're obviously right about us being in the center of the observable universe, but that because there is no center to the universe (see other questions on this stack exchange), we're definitely not in the center.  
> **username_0**: If it were finite in size then there is certainly a center. We just can't see it.  
> **username_1**: In the universe (on the largest scales), everything is moving away from everything else. I can assure you, there is no center. See this: http://physics.stackexchange.com/questions/25591/does-the-universe-have-a-center  
> **username_1**: Whoops, sorry, no single point that is the center.  
> **username_0**: @HDE226868 If it's finite in size, then there certainly is a center. We just don't know where it is because we are the center of our observable one. If it's infinite in size then there is of course no center. But if the universe is 1 trillion light years across, then there's a very obvious center which may or may not be in our own observable universe.  
> **username_1**: You didn't understand the muffin/balloon analogy? There is no center.  
> **username_0**: If the universe is finite in size and flat then it must have a center. There's no way around that. Say it's a sphere of 300 billion light years across. There's a clear and obvious center. Now say we're sitting at an arbitrary point inside it. We can't see an edge and our center is us. Our observable center.  
> **username_0**: And yes, I read the other page you linked to. But the balloon explanation fails for me. No point on the balloon inflating is the center. I get that. But that's a 2D representation. The universe as we see it is 3 dimensional. And that inflating balloon HAS a center. Not on the balloon, but inside it. A flat, finite universe must have a center. This is my hangup.  
> **username_1**: We could be on the surface of a 3-manifold, i.e. on the surface of a 4-d object.  
> **username_1**: And no, the universe is not necessarily flat. Locally flat? Yes. Globally flat? Not necessarily.  
> **username_2**: Your main error is in the following: "So it came from a singularity, and had a finite size at some point." This implication is completely wrong about the universe at large. Your subsequent error is this: "That's too crazy to swallow." That's just appeal to personal incredulity.  
> **username_1**: @Paul, your ending sentence is illogical. Lack of matter and energy do not imply that there is no time or space.  

---

## 2 answers

### Answer by username_3 on 2014-08-14. Score: 1

Depending on the mass of the universe, the universe has three different possible shapes.
<http://en.wikipedia.org/wiki/Shape_of_the_universe>

It can be finite or infinite depending on the shape it has, and there are some ways we can discard possible shapes (if it started to shrink instead of expand).

Hope this helps

> **Comments:**
> **username_1**: Well, it technically has three different curvatures, no three different shapes.  
> **username_3**: Universe topologies, if you like :)  

---

### Answer by username_4 on 2014-08-14. Score: 0

Space and time are total different things.

These citations are about the observable universe, and not from the whole.

We can see only a 13.7billion light year sphere from our spacetime. It is not very big.

[The current measurement of the WMAP satellties](https://physics.stackexchange.com/questions/88786/lower-limit-of-the-size-of-the-universe-wmap) says, the universe is probably infinite big.

Your questions can be explained with the Friedman equations. These equations are the solutions of the Einstein Field Equations for the infinite vacuum of our spacetime.

*And its spacelike sections of our Friedman-spacetime is planar*, at least in the limits of the precision of the measurement. What currently can be said, is that "if the whole universe is finite, and has a (hyper-)spherical geometry, then its radius must be at least 300billion light years".

> **Comments:**
> **username_0**: I will look into the Friedman equations.  
> **username_2**: The Friedmann equations are much more general than either the vacuum condition or being spatially infinite. The whole section is more than a bit confused. @HDE226868 The implication, which this answer alludes to but fails to communicate clearly, is that the WMAP observations indicate spatially flatness, which under the modeling assumptions of the FRW family of solutions (global isotropy and homogeneity) would imply an infinite universe.  
> **username_5**: `We can see only a 13.7billion light year(radius or diameter?) sphere`, nah the universe, as many observations seem to show is about 13.5 billion years old, but the sphere of our visible universe is much bigger, [almost 50 billion lightyears in radius](http://en.wikipedia.org/wiki/Observable_universe)  
> **username_1**: @StanLiou Thanks for the information about WMAP.  

---

