<p>The answer to this question lies in cosmology.</p>

<p>It's because of relativity in positions.  Every galaxy appears to be moving away from us and the universe we see is only the observable universe.  Further points are more in the distant past, when you observe the deep field you observe the past.  This may seem irrelevant to your question, but the idea of a "center" to a universe where any position in the universe, be it from our galaxy or from a galaxy in the Sloan Great Wall is the true center is not consistent with observation.  That's because galaxies/clusters of galaxies are moving away from each other, relative to each other.  </p>

<p>Every galaxy is slowly being pulled apart from other galaxies, save for those in gravitationally bounded clusters.  (The whole architecture of the universe, with massive voids of absolute nothing and bunched up clusters of galaxies reflects this quite a bit)  If the universe had a central point, this would be easily observed because there would be variation in the color shift of galaxies that could be analyzed to deduce a central point where the galaxies are moving away from.</p>

<p>That is not what is observed.  Instead, pretty much most galaxies (save for exceptions like Andromeda or the Triangulum, who prove the rule since those galaxies are gravitationally connected to our galaxy via the local group) are moving away from us.  The Great attractor being not a path to the center of the universe, but the center of the recently discovered Laniakea super cluster.  So if there's attraction between galaxies, it's because of gravitational interactions.  The universe is also without any real edge unless you count the edge of the observable universe.  The observable universe being more a historical record than what the contemporary universe around us looks like and is a byproduct of light speed being so limited.  The speed of light in a vacuum is also the speed of information.  The whole universe is far more massive by comparison.</p>

<p>So to speak about a center of the universe would require the following:</p>

<ol>
<li><p>A region of space that galaxy is moving away from or towards. (Which does not exist, instead relative trajectories and gravitational clustering is all regional at most.)</p></li>
<li><p>Edges that are defined enough to pinpoint a center (If such edges exist, they are impossible to observe due to the limited speed information travels in the universe)</p></li>
<li><p>Absolute positioning to pinpoint where this center would be. (All positions are relative, the galaxy, the sun and our planet all are moving at extremely fast velocities we don't notice thanks to relativity.)</p></li>
</ol>

CC BY-SA 3.0
2015-12-23T05:43:25.007Z
2015-12-23T05:48:33.240Z
2015-12-23T05:48:33.240Z
InterstellarTK
<p>The centre of something ( anything ) has to be defined in some way by the properties of the something and a coordinate system.</p>

<p>So you can in principle define a centre to the universe, but it only has a meaning in the context of the chosen definition.</p>

<p>You could easily define a different measure that produces a different centre.</p>

<p>And you can define a centre even on the surface of your balloon, simply by arbitrary selection.</p>

<p>An edge to the universe requires a context.  How do we define it ?  How do we detect it ?  The only meaningful measure of this ( in a human sense ) that I know of is "how far can I see ?".  That keeps changing.</p>

<p>So at the moment I would say "centre is arbitrary" and "edge is unknown".</p>

CC BY-SA 3.0
2015-12-23T02:59:33.243Z
2015-12-23T02:59:33.243Z
StephenG - Help Ukraine
<blockquote>
  <p>Why does it make sense to say the universe has no centre?</p>
</blockquote>

<p>We can't be sure it does. </p>

<blockquote>
  <p>If we suppose we and everything in the universe is perfectly flat. That is, we have width and depth, but no height. And we are situated on the surface of a balloon blowing up with air and expanding "outwards". Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point. In other words, the surface of a balloon (the "universe") has no centre. </p>
</blockquote>

<p>This analogy is often used to justify that the claim that the universe has no centre. But there's no evidence of any higher dimensions. We simply don't know that there's any kind of "asteroids" curvature such that if you keep going one way you come back the other. </p>

<blockquote>
  <p>Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?</p>
</blockquote>

<p>I wouldn't say it's stupid. But I would say it's presenting hypothesis as some kind of fact. For all we know our universe does have a centre and an edge. </p>

CC BY-SA 3.0
2015-12-23T04:19:11.727Z
"But there's no evidence of any higher dimensions." -- this seems like a non sequitur, or else I don't understand what you're trying to say. A lack of any higher dimensions does not affect the analogy (indeed, most acknowledge its limitations in this regard), nor does it indicate anything one way or the other about whether a hypothetical spatially finite but center-less universe is right or wrong.
Stan Liou
CC BY-SA 3.0
2015-12-23T08:54:21.630Z
The balloon analogy is not used to justify the claim of no centre, it is used to illustrate a space/manifold which is finite and yet has no centre, that is to show it is possible. Many popular models of space time are not finite but still have no "centre". Also you do not need extra dimensions to have even a closed space time with no centre, the closure and curvature are intrinsic properties of the space/manifold and do not refer to anything outside the manifold.
Conrad Turner
CC BY-SA 3.0
2015-12-23T09:15:37.383Z
@Stan Liou : it's not a non-sequitur, it's simple fact. As far as we know the universe is flat. Planck didn't find an evidence of any toroidal topology, see http://arxiv.org/abs/1303.5086 , and there's no actual evidence that the universe is infinite. So it could be finite, like the raisin-cake, with a centre. For all we know some guy 46 billion light years away could be looking up wondering why half the sky is black. Conrad: with no evidence of higher dimensions or intrinsic curvature, the balloon analogy just isn't science, it's speculation.
John Duffield
CC BY-SA 3.0
2015-12-23T14:48:44.767Z
@JohnDuffield: I'm not questioning those things by themselves. What's a non-sequitur, though, is the suggested connection between lack of evidence of other dimensions and lack of evidence of the universe being finite. Those are independent issues.
Stan Liou
CC BY-SA 3.0
2015-12-23T15:41:28.153Z
@Stan Liou : I think they're very much connected. It goes like this: _in olden times people could not conceive of a world that didn't have an edge. In modern times people cannot conceive of a world that does_. So they invent fairy tales about ants and balloons which just don't stand up to scrutiny. And yet these tales are peddled as if they're scientific fact. IMHO the scientific thing to say is _we don't know_.
John Duffield
CC BY-SA 3.0
2015-12-23T19:14:20.387Z
@JohnDuffield: That doesn't have anything to do with science, though. There are many possible spatial geometries, both finite and infinite--we don't really know which; it doesn't have anything to do with this balloon thing, but rather mathematics of GTR. Even under assumptions of homogeneity and isotropy, there's two finite and two infinite geometries (none of which are a torus). And without those assumptions, the number of possibilities explodes. A finite boundaryless universe is just one class of possibilities among many; it's not fact, but *no competent cosmologists claim that it is fact*.
Stan Liou
CC BY-SA 3.0
2015-12-22T18:52:03.913Z
2015-12-22T18:52:03.913Z
John Duffield
<p>I suspect it's just that we can't see the full extent of the universe. We don't know its boundaries. Possibly some day radio telescopes will see the boundary but we may never know its extent and possibly that's because of the red-shift, that the light from distant stars becomes more red-shifted in the radio frequencies. Or possibly because the signals are so weak at the edge of the universe.</p>

CC BY-SA 4.0
2023-12-08T12:05:26.330Z
This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/33425)
GrapefruitIsAwesome
CC BY-SA 4.0
2023-12-04T23:45:42.067Z
2023-12-04T23:45:42.067Z
John Baker
<p>If we suppose we and everything in the universe is perfectly flat.  That is, we have width and depth, but no height.  And we are situated on the surface of a balloon blowing up with air and expanding "outwards".  Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point.  In other words, the surface of a balloon (the "universe") has no centre.  Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?</p>

CC BY-SA 3.0
2015-12-23T09:32:32.783Z
There's simply no evidence of any higher dimensions or intrinsic curvature. The evidence from WMAP and elsewhere suggests that the universe is flat. We have no evidence whatsoever to support the "ant on a balloon" analogy. Whilst the idea of a closed universe isn't stupid, what is, is people telling you about it as if it's scientific fact, and "explaining" it in terms of something that isn't explained. As if the balloon makes everything OK. As if we really are 2D creatures on a 3D surface. Only last time I looked, we're not.
John Duffield
CC BY-SA 3.0
2015-12-23T12:36:53.807Z
The balloon analogy appears three dimensional, but it's really two dimensional with all things having width and depth, but absolutely no height. Now all the points on the surface of this balloon are equidistant from a "centre" that doesn't exist except in a mathematical way, therefore one point is no more central than any other.  Suppose I stand at the south pole, in our analogy no matter which way I point, and no matter how big the world is, I'll always be pointing north.  Isn't this why we receive low frequency background radiation from the big bang in all directions?
Michael Lee
CC BY-SA 3.0
2015-12-23T15:55:30.050Z
The thing is Mike, is that the Earth isn't flat. It's three-dimensional. And as far as we can tell, so is everything else, including this universe. The lower-dimensional balloon analogy is nice to think about, but it has no foundation in fact. We receive the [CMBR](https://en.wikipedia.org/wiki/Cosmic_microwave_background) from all directions because it was being emitted from all locations in all directions, and circa 380,000 years after the Big Bang the universe became transparent to such radiation.
John Duffield
CC BY-SA 3.0
2015-12-23T17:00:18.950Z
Fair enough John, thank you for popping my balloon.  lol
Michael Lee
CC BY-SA 4.0
2020-07-18T21:24:19.127Z
Perhaps we are within the skin of the ballon or bubble, which is itself expanding. The center of this universe bubble would be in an unaccessible point inside it. Since we and everything even spacetime no longer occupies the center, it wouldn't exist yet somehow still be there.
Armondo Villaescuza
CC BY-SA 4.0
2023-12-05T14:53:37.733Z
Since the big bang basically created everything wouldn't where ever it happened be the center?
cybernard
<universe>
Why does it make sense to say the universe has no centre?
Michael Lee
# Why does it make sense to say the universe has no centre?

(Asked by: username_0 on 2015-12-22)

If we suppose we and everything in the universe is perfectly flat. That is, we have width and depth, but no height. And we are situated on the surface of a balloon blowing up with air and expanding "outwards". Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point. In other words, the surface of a balloon (the "universe") has no centre. Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?

**Tags:** universe

> **Comments:**
> **username_1**: There's simply no evidence of any higher dimensions or intrinsic curvature. The evidence from WMAP and elsewhere suggests that the universe is flat. We have no evidence whatsoever to support the "ant on a balloon" analogy. Whilst the idea of a closed universe isn't stupid, what is, is people telling you about it as if it's scientific fact, and "explaining" it in terms of something that isn't explained. As if the balloon makes everything OK. As if we really are 2D creatures on a 3D surface. Only last time I looked, we're not.  
> **username_0**: The balloon analogy appears three dimensional, but it's really two dimensional with all things having width and depth, but absolutely no height. Now all the points on the surface of this balloon are equidistant from a "centre" that doesn't exist except in a mathematical way, therefore one point is no more central than any other.  Suppose I stand at the south pole, in our analogy no matter which way I point, and no matter how big the world is, I'll always be pointing north.  Isn't this why we receive low frequency background radiation from the big bang in all directions?  
> **username_1**: The thing is Mike, is that the Earth isn't flat. It's three-dimensional. And as far as we can tell, so is everything else, including this universe. The lower-dimensional balloon analogy is nice to think about, but it has no foundation in fact. We receive the [CMBR](https://en.wikipedia.org/wiki/Cosmic_microwave_background) from all directions because it was being emitted from all locations in all directions, and circa 380,000 years after the Big Bang the universe became transparent to such radiation.  
> **username_0**: Fair enough John, thank you for popping my balloon.  lol  
> **username_2**: Perhaps we are within the skin of the ballon or bubble, which is itself expanding. The center of this universe bubble would be in an unaccessible point inside it. Since we and everything even spacetime no longer occupies the center, it wouldn't exist yet somehow still be there.  
> **username_3**: Since the big bang basically created everything wouldn't where ever it happened be the center?  

---

## 4 answers

### Answer by username_4 on 2015-12-23. Score: 4 | Accepted answer

The answer to this question lies in cosmology.

It's because of relativity in positions. Every galaxy appears to be moving away from us and the universe we see is only the observable universe. Further points are more in the distant past, when you observe the deep field you observe the past. This may seem irrelevant to your question, but the idea of a "center" to a universe where any position in the universe, be it from our galaxy or from a galaxy in the Sloan Great Wall is the true center is not consistent with observation. That's because galaxies/clusters of galaxies are moving away from each other, relative to each other.

Every galaxy is slowly being pulled apart from other galaxies, save for those in gravitationally bounded clusters. (The whole architecture of the universe, with massive voids of absolute nothing and bunched up clusters of galaxies reflects this quite a bit) If the universe had a central point, this would be easily observed because there would be variation in the color shift of galaxies that could be analyzed to deduce a central point where the galaxies are moving away from.

That is not what is observed. Instead, pretty much most galaxies (save for exceptions like Andromeda or the Triangulum, who prove the rule since those galaxies are gravitationally connected to our galaxy via the local group) are moving away from us. The Great attractor being not a path to the center of the universe, but the center of the recently discovered Laniakea super cluster. So if there's attraction between galaxies, it's because of gravitational interactions. The universe is also without any real edge unless you count the edge of the observable universe. The observable universe being more a historical record than what the contemporary universe around us looks like and is a byproduct of light speed being so limited. The speed of light in a vacuum is also the speed of information. The whole universe is far more massive by comparison.

So to speak about a center of the universe would require the following:

1. A region of space that galaxy is moving away from or towards. (Which does not exist, instead relative trajectories and gravitational clustering is all regional at most.)
2. Edges that are defined enough to pinpoint a center (If such edges exist, they are impossible to observe due to the limited speed information travels in the universe)
3. Absolute positioning to pinpoint where this center would be. (All positions are relative, the galaxy, the sun and our planet all are moving at extremely fast velocities we don't notice thanks to relativity.)


---

### Answer by username_5 on 2015-12-23. Score: 2

The centre of something ( anything ) has to be defined in some way by the properties of the something and a coordinate system.

So you can in principle define a centre to the universe, but it only has a meaning in the context of the chosen definition.

You could easily define a different measure that produces a different centre.

And you can define a centre even on the surface of your balloon, simply by arbitrary selection.

An edge to the universe requires a context. How do we define it ? How do we detect it ? The only meaningful measure of this ( in a human sense ) that I know of is "how far can I see ?". That keeps changing.

So at the moment I would say "centre is arbitrary" and "edge is unknown".


---

### Answer by username_1 on 2015-12-22. Score: -1

> Why does it make sense to say the universe has no centre?

We can't be sure it does.

> If we suppose we and everything in the universe is perfectly flat. That is, we have width and depth, but no height. And we are situated on the surface of a balloon blowing up with air and expanding "outwards". Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point. In other words, the surface of a balloon (the "universe") has no centre.

This analogy is often used to justify that the claim that the universe has no centre. But there's no evidence of any higher dimensions. We simply don't know that there's any kind of "asteroids" curvature such that if you keep going one way you come back the other.

> Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?

I wouldn't say it's stupid. But I would say it's presenting hypothesis as some kind of fact. For all we know our universe does have a centre and an edge.

> **Comments:**
> **username_6**: "But there's no evidence of any higher dimensions." -- this seems like a non sequitur, or else I don't understand what you're trying to say. A lack of any higher dimensions does not affect the analogy (indeed, most acknowledge its limitations in this regard), nor does it indicate anything one way or the other about whether a hypothetical spatially finite but center-less universe is right or wrong.  
> **username_7**: The balloon analogy is not used to justify the claim of no centre, it is used to illustrate a space/manifold which is finite and yet has no centre, that is to show it is possible. Many popular models of space time are not finite but still have no "centre". Also you do not need extra dimensions to have even a closed space time with no centre, the closure and curvature are intrinsic properties of the space/manifold and do not refer to anything outside the manifold.  
> **username_1**: @username_6 : it's not a non-sequitur, it's simple fact. As far as we know the universe is flat. Planck didn't find an evidence of any toroidal topology, see http://arxiv.org/abs/1303.5086 , and there's no actual evidence that the universe is infinite. So it could be finite, like the raisin-cake, with a centre. For all we know some guy 46 billion light years away could be looking up wondering why half the sky is black. Conrad: with no evidence of higher dimensions or intrinsic curvature, the balloon analogy just isn't science, it's speculation.  
> **username_6**: @JohnDuffield: I'm not questioning those things by themselves. What's a non-sequitur, though, is the suggested connection between lack of evidence of other dimensions and lack of evidence of the universe being finite. Those are independent issues.  
> **username_1**: @username_6 : I think they're very much connected. It goes like this: _in olden times people could not conceive of a world that didn't have an edge. In modern times people cannot conceive of a world that does_. So they invent fairy tales about ants and balloons which just don't stand up to scrutiny. And yet these tales are peddled as if they're scientific fact. IMHO the scientific thing to say is _we don't know_.  
> **username_6**: @JohnDuffield: That doesn't have anything to do with science, though. There are many possible spatial geometries, both finite and infinite--we don't really know which; it doesn't have anything to do with this balloon thing, but rather mathematics of GTR. Even under assumptions of homogeneity and isotropy, there's two finite and two infinite geometries (none of which are a torus). And without those assumptions, the number of possibilities explodes. A finite boundaryless universe is just one class of possibilities among many; it's not fact, but *no competent cosmologists claim that it is fact*.  

---

### Answer by username_8 on 2023-12-04. Score: -2

I suspect it's just that we can't see the full extent of the universe. We don't know its boundaries. Possibly some day radio telescopes will see the boundary but we may never know its extent and possibly that's because of the red-shift, that the light from distant stars becomes more red-shifted in the radio frequencies. Or possibly because the signals are so weak at the edge of the universe.

> **Comments:**
> **username_9**: This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/33425)  

---

