<p>A good analogy which shows that the expansion has no centre is one of a 2D sheet covered with uniform dots - representing the galaxies where the one in the centre is us:</p>

<p><img src="https://i.sstatic.net/vwILH.gif" alt="enter image description here"></p>

<p>As the universe expands the dots (galaxies) get further away from each other:</p>

<p><img src="https://i.sstatic.net/sjgar.gif" alt="enter image description here"></p>

<p>Now if you overlay the future and present images lining up on the central galaxy (us) you can see that all the other galaxies appear to be moving away from us, and that the further away they are the faster they appear to be receding:</p>

<p><img src="https://i.sstatic.net/zS3sP.gif" alt="enter image description here"></p>

<p>However, if you line up on any other galaxy in the image you can see that all the other galaxies appear to be moving away from them:</p>

<p><img src="https://i.sstatic.net/TcluC.gif" alt="enter image description here"></p>

<p>As no two points can be the centre of the universe we can only conclude that there is no centre to the the universe.</p>

<p><a href="http://calgary.rasc.ca/universe_expansion.htm" rel="noreferrer">Image Source</a></p>

CC BY-SA 3.0
2013-09-28T23:07:31.200Z
2013-10-02T14:41:24.487Z
2013-10-02T14:41:24.487Z
ChrisF
<p>Basically, if two particles are placed with <em>no other interaction between them</em>, the distance between them will increase.</p>

<p>Imagine living on the surface of a balloon which is being blown up. Your size stays fixed, because you're more or less rigid, but items not attached to you will move further away. Your ruler, another rigid body, stays fixed in size (though it may bend to accommodate the new curvature &mdash; this isn't so important). But <em>two</em> rulers (which are not attached to each other) move further away.</p>

<p><img src="https://i.sstatic.net/ULNK3.jpg" alt="enter image description here"></p>

<blockquote>
  <p>...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D </p>
</blockquote>

<p>On the outset this seems true, however there are other forces at play here. Our measurement devices are held together by electromagnetic interactions, and the strength of these will not change. So the measurement device will hold itself together.</p>

<p>Imagine two faraway atoms. When space expands, the distance between the two atoms increases. However, the size<sup>1</sup> of the atom does not &mdash; this is determined by electrostatic equilibrium (and quantum mechanical considerations), and this remains unaffected. Even if the atom was stretched, it would rebound.</p>

<p>This scales up to measurement devices, so they don't get distorted either. Indeed, the expansion of space only really makes sense when you look at galaxies &mdash; these are pretty far away (when not in the same supercluster) and they don't have any interactions maintaining an equilibrium distance between them.</p>

<p><sup>1. Whatever closest analog we have to "size" for atoms; eg the area which contains 99% of the charge density; or the nth Bohr radius.</sup></p>

CC BY-SA 3.0
2014-03-26T19:43:19.460Z
The main thing I dislike about the balloon analogy is that it requires a topological change in thinking, looking at a 3-dimensional object changing 3-dimensionally, but discussing what a 2-dimensional observer would see
Jeremy
CC BY-SA 3.0
2014-03-26T19:55:06.600Z
@Jeremy sure, but it's hard to explain how metrics can change and all to a layman, the balloon thing works :)
Manishearth
CC BY-SA 3.0
2014-03-26T20:13:12.797Z
I'm not sure it does, most laymen are not used to the topological change in thinking and are considering it in 3-dimensions. They see it being blown up and everything moving away from the centre of the balloon.
Jeremy
CC BY-SA 3.0
2014-03-26T20:14:25.070Z
Hm. I'll look into improving this later, but I'm not really that convinced that there is something there that can be improved without losing the layman tone.
Manishearth
CC BY-SA 3.0
2014-04-02T22:07:31.423Z
I wonder if attention can be focused on the surface of the balloon by drawing a circle around several dots. Look at what happens inside the circle, ignore the rest of the balloon. But then the circle itself needs to represent something in this analogy, but what?
Jeremy
CC BY-SA 3.0
2013-10-02T12:42:06.863Z
[*Related* answer](http://physics.stackexchange.com/a/65560/11062) at Physics.SE which is also *yours* ;-)
Waffle's Crazy Peanut
CC BY-SA 3.0
2013-09-27T12:10:03.853Z
2013-09-27T19:25:24.370Z
2013-09-27T19:25:24.370Z
Manishearth
<p>So far I got to understand the expansion of space is not to be understood as stars drifting further apart through space. There's something more fundamental - e.g. you can't simply measure speed of it, nor tell where the center of the universe (the midpoint of expansion) is. Well, some smart people got me from knowing the answer that is "Simple, obvious, neat and wrong"  to "I have no clue what it is".</p>

<p>Is there a resource/method that would allow a layman to understand how to think about distortions of space like its expansion; to visualize and understand the concept correctly and without <em>erroneous</em> simplifications; understand it before trying to delve into underlying details of physics?</p>

CC BY-SA 3.0
2013-09-27T11:53:31.260Z
I saw a good example which showed a 2D sheet filled with uniform dots stretching in both directions. When you line the sheets up on any dot it appears that all the dot are moving away from the one you chose to line them up by.
ChrisF
CC BY-SA 3.0
2013-09-27T11:58:35.813Z
...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D
SF.
<expansion>
Space expansion in layman terms
SF.
# Space expansion in layman terms

(Asked by: username_0 on 2013-09-27)

So far I got to understand the expansion of space is not to be understood as stars drifting further apart through space. There's something more fundamental - e.g. you can't simply measure speed of it, nor tell where the center of the universe (the midpoint of expansion) is. Well, some smart people got me from knowing the answer that is "Simple, obvious, neat and wrong" to "I have no clue what it is".

Is there a resource/method that would allow a layman to understand how to think about distortions of space like its expansion; to visualize and understand the concept correctly and without *erroneous* simplifications; understand it before trying to delve into underlying details of physics?

**Tags:** expansion
> **Comments:**
> **username_1**: I saw a good example which showed a 2D sheet filled with uniform dots stretching in both directions. When you line the sheets up on any dot it appears that all the dot are moving away from the one you chose to line them up by.  
> **username_0**: ...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D  

---

## 2 answers

### Answer by username_2 on 2013-09-27. Score: 18 | Accepted answer

Basically, if two particles are placed with *no other interaction between them*, the distance between them will increase.

Imagine living on the surface of a balloon which is being blown up. Your size stays fixed, because you're more or less rigid, but items not attached to you will move further away. Your ruler, another rigid body, stays fixed in size (though it may bend to accommodate the new curvature — this isn't so important). But *two* rulers (which are not attached to each other) move further away.

![enter image description here](https://i.sstatic.net/ULNK3.jpg)

> ...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D

On the outset this seems true, however there are other forces at play here. Our measurement devices are held together by electromagnetic interactions, and the strength of these will not change. So the measurement device will hold itself together.

Imagine two faraway atoms. When space expands, the distance between the two atoms increases. However, the size1 of the atom does not — this is determined by electrostatic equilibrium (and quantum mechanical considerations), and this remains unaffected. Even if the atom was stretched, it would rebound.

This scales up to measurement devices, so they don't get distorted either. Indeed, the expansion of space only really makes sense when you look at galaxies — these are pretty far away (when not in the same supercluster) and they don't have any interactions maintaining an equilibrium distance between them.

1. Whatever closest analog we have to "size" for atoms; eg the area which contains 99% of the charge density; or the nth Bohr radius.

> **Comments:**
> **username_4**: [*Related* answer](http://physics.stackexchange.com/a/65560/11062) at Physics.SE which is also *yours* ;-)  
> **username_3**: The main thing I dislike about the balloon analogy is that it requires a topological change in thinking, looking at a 3-dimensional object changing 3-dimensionally, but discussing what a 2-dimensional observer would see  
> **username_2**: @Jeremy sure, but it's hard to explain how metrics can change and all to a layman, the balloon thing works :)  
> **username_3**: I'm not sure it does, most laymen are not used to the topological change in thinking and are considering it in 3-dimensions. They see it being blown up and everything moving away from the centre of the balloon.  
> **username_2**: Hm. I'll look into improving this later, but I'm not really that convinced that there is something there that can be improved without losing the layman tone.  
> **username_3**: I wonder if attention can be focused on the surface of the balloon by drawing a circle around several dots. Look at what happens inside the circle, ignore the rest of the balloon. But then the circle itself needs to represent something in this analogy, but what?  

---

### Answer by username_1 on 2013-09-28. Score: 13

A good analogy which shows that the expansion has no centre is one of a 2D sheet covered with uniform dots - representing the galaxies where the one in the centre is us:

![enter image description here](https://i.sstatic.net/vwILH.gif)

As the universe expands the dots (galaxies) get further away from each other:

![enter image description here](https://i.sstatic.net/sjgar.gif)

Now if you overlay the future and present images lining up on the central galaxy (us) you can see that all the other galaxies appear to be moving away from us, and that the further away they are the faster they appear to be receding:

![enter image description here](https://i.sstatic.net/zS3sP.gif)

However, if you line up on any other galaxy in the image you can see that all the other galaxies appear to be moving away from them:

![enter image description here](https://i.sstatic.net/TcluC.gif)

As no two points can be the centre of the universe we can only conclude that there is no centre to the the universe.

[Image Source](http://calgary.rasc.ca/universe_expansion.htm)


---

