<p>The first principle of cosmology is called the "cosmological principle", which asserts that the universe is more or less the same everywhere spatially, but changes with time.  This is a fundamentally new way to think of the history of the universe, and has revolutionized our ability to understand long-range observations.  We do not, however, know that this principle is true, we only know it is both useful and consistent with all observations.  So we adopt it as a working hypothesis, and all of cosmology follows.  Some day we may discover it is inadequate, but until then, we will continue to use it.  The same could be said for other principles, like conservation of energy-- we don't know that's true either, but it's useful and consistent with observations.  The cosmological principle rules out that the universe has a unique center.</p>

<p>Your question is whether the apparent lack of a unique center stems from our location not being special enough to be able to tell that it does have such a center (say, by encountering a boundary).  Ironically, the cosmological principle turns that question on its ear, and points to the possibility that one's perspective always puts themselves at the apparent center.  So we would say the illusion of <em>having</em> a center is all about perspective-- not the illusion of <em>not</em> having a center!</p>

CC BY-SA 3.0
2016-10-20T04:39:49.083Z
I understand the purpose of the cosmological principle to suggest ultimately that our universe is the "same" anywhere, on a large enough scale (the universe is the same for all observers). Yet the universe being homogeneous and isotropic does nothing to dispute the possibility for a center. I tub of yogurt can be homogeneous and isotropic, but it clearly has a center, right? I guess that is my point regarding the perspective. Are we in a position where, because of our position in the observable universe, we cannot ever determine where the center is/or could be?
LaserYeti
CC BY-SA 3.0
2016-10-20T04:39:52.600Z
Or is there the possibility that there truly is no center? And if so, then how can that actually be?
LaserYeti
CC BY-SA 3.0
2016-10-22T04:20:32.473Z
There would be no center if the universe were spatially infinite, or if it were  finite but closed, and those are more or less the only possibilities allowed by the cosmological principle.  A closed universe is often described as being like the surface of a sphere in a lower-dimensional analog, which has no center on that surface.
Ken G
CC BY-SA 3.0
2016-11-18T21:44:10.097Z
@LaserYeti "I understand the purpose of the cosmological principle", it doesn't seem so. The only purpose is to try to predict the results of future observations based on past observations, as accurately as possible. Real actual results, affecting you, your family, your cat. Does your hypothesis predict some new interesting observation (i.e. is it *falsifiable*)? Standard scientific method.
kubanczyk
CC BY-SA 3.0
2016-11-18T21:52:10.670Z
There is still the possibility that the universe does have a unique center, and there is the possibility that it does not.  You can even see this from your own analogy of birds floating in the ocean.  If there are continents that you cannot see, then there could be some meaning to the "center of the Atlantic ocean," for example, defined by the continents that define "the Atlantic ocean."  But take those continents away, and poof, you are back to an ocean with no unique center.  How would we know if we can't see the continents?  So we model the ocean with only what we can see-- no center.
Ken G
CC BY-SA 3.0
2016-11-20T04:03:51.677Z
@KenG I completely agree with your point and understand what you are saying... that is **why** I was asking the question in the first place. Coming back to the problem with the cosmological principle, it was predicated on our locality. If we had another point of view to observe the universe from (from the "outside"), we would have no reason to ask questions like this in the first place. That is why I still ask them. Those that blindly follow are easily lead astray.
LaserYeti
CC BY-SA 3.0
2016-11-20T19:50:06.890Z
@LaserYeti The cosmological principle is an assertion that is actively tested. There are occasional reports of small violations on large scales (e.g. in the cosmic microwave background). A flat but finite universe would have a centre. An infinite universe or a closed universe does not have a centre.  We are obviously not anywhere near an edge of a finite universe since the cosmological principle holds to a very high degree of accuracy for the observable universe and we cannot see beyond the observable universe.
ProfRob
CC BY-SA 3.0
2016-11-20T20:48:24.010Z
@RobJeffries Thank you, I understand, but I still like to ponder the what ifs. One of my research projects was a 212 orbit HST-STIS observation of the HLQG to determine if structures of that scale can disprove the current cosmological model, or if there is a misunderstanding of the homogeneity scale. Because there are several greater than $260 h^{-1}Mpc$ sized structures, I look for reasons why we assume the universal design that we do. I guess I'm just curious like a cat.
LaserYeti
CC BY-SA 3.0
2016-11-21T23:15:33.593Z
There is certainly no harm in curiosity, and all scientific principles, including the cosmological principle, must be constantly tested and attacked with our best possible vigor.   It is only a principle that stands up to withering assault that can be regarded as good-- and so far, the cosmological principle has.  That only makes it a good principle, not a true one.
Ken G
CC BY-SA 3.0
2016-10-19T14:37:19.143Z
2016-10-19T14:37:19.143Z
Ken G
<p>I get this question a lot: </p>

<blockquote>
  <p>Where is the center of the Universe?</p>
</blockquote>

<p>I see this question on the internet all of the time:</p>

<blockquote>
  <p>Where is the center of the Universe?</p>
</blockquote>

<p>Now I have a question regarding all of the answers that I always either give or <a href="https://astronomy.stackexchange.com/questions/13999/how-does-the-concept-of-a-universe-with-no-center-work">read</a>:
The question linked is the last question and answer that I saw before posing this question here, and is in no way the same question, as this question asks how the concept of a centerless universe can work, whereas the question below asks about how a universe can be perceived to be centerless even when it might not be.</p>

<p><strong>Is the Universe centerless? Or does it simply appear to be because of our perspective?</strong>  </p>

<p>$\\\\$<br>
This is about to be a bad example, but please bear with me.  </p>

<p>If you were a bird floating in the ocean, among a myriad of other birds, and you let time pass such that every bird floated away from the other birds in some random fashion. Would you, at some later time when everybirdy was roughly separated from one another, say that based upon your observations of the present conditions, and some evidence that at one point all of the birds were together, that the ocean is centerless? Or could you assume that because of your position in the flock, that from your perspective you can't tell where the center is, nor do you think that from a different birds position would you be able to tell? Creating the illusion of a centerless ocean.</p>

<p>Obviously this requires you not be at or close to the very edge of the flock.</p>

<p>Edit: Also see examples in KenG's posed answer and comments.</p>

CC BY-SA 3.0
2016-11-18T19:12:53.973Z
Also: http://astronomy.stackexchange.com/questions/8802/why-would-all-places-appear-to-be-at-the-center-of-the-universes-expansion?rq=1
ProfRob
CC BY-SA 3.0
2016-11-20T04:00:42.777Z
This question is nothing like the link you provided @RobJeffries... The assumption here is that all known physical concept are obviously legit, and now take a step back and look at the universe from a "different perspective." Do you come to the same conclusion? or do you see a different universe? **Not**  centerless! ... say what?...
LaserYeti
CC BY-SA 3.0
2016-11-20T15:17:35.010Z
You might try asking over at math stack. There's probably a theorem somewhere stating that every closed physical manifold has a center, in the classical sense, but that info is not going to be easy to find in human readable format.
Wayfaring Stranger
<universe><expansion>
Centerless expansion or a matter of perspective
LaserYeti
# Centerless expansion or a matter of perspective

(Asked by: username_0 on 2016-10-19)

I get this question a lot:

> Where is the center of the Universe?

I see this question on the internet all of the time:

> Where is the center of the Universe?

Now I have a question regarding all of the answers that I always either give or [read](https://astronomy.stackexchange.com/questions/13999/how-does-the-concept-of-a-universe-with-no-center-work):
The question linked is the last question and answer that I saw before posing this question here, and is in no way the same question, as this question asks how the concept of a centerless universe can work, whereas the question below asks about how a universe can be perceived to be centerless even when it might not be.

**Is the Universe centerless? Or does it simply appear to be because of our perspective?**

$\\\\$  
This is about to be a bad example, but please bear with me.

If you were a bird floating in the ocean, among a myriad of other birds, and you let time pass such that every bird floated away from the other birds in some random fashion. Would you, at some later time when everybirdy was roughly separated from one another, say that based upon your observations of the present conditions, and some evidence that at one point all of the birds were together, that the ocean is centerless? Or could you assume that because of your position in the flock, that from your perspective you can't tell where the center is, nor do you think that from a different birds position would you be able to tell? Creating the illusion of a centerless ocean.

Obviously this requires you not be at or close to the very edge of the flock.

Edit: Also see examples in KenG's posed answer and comments.

**Tags:** universe, expansion
> **Comments:**
> **username_1**: Also: http://astronomy.stackexchange.com/questions/8802/why-would-all-places-appear-to-be-at-the-center-of-the-universes-expansion?rq=1  
> **username_0**: This question is nothing like the link you provided @RobJeffries... The assumption here is that all known physical concept are obviously legit, and now take a step back and look at the universe from a "different perspective." Do you come to the same conclusion? or do you see a different universe? **Not**  centerless! ... say what?...  
> **username_2**: You might try asking over at math stack. There's probably a theorem somewhere stating that every closed physical manifold has a center, in the classical sense, but that info is not going to be easy to find in human readable format.  

---

## 1 answers

### Answer by username_3 on 2016-10-19. Score: 1 | Accepted answer

The first principle of cosmology is called the "cosmological principle", which asserts that the universe is more or less the same everywhere spatially, but changes with time. This is a fundamentally new way to think of the history of the universe, and has revolutionized our ability to understand long-range observations. We do not, however, know that this principle is true, we only know it is both useful and consistent with all observations. So we adopt it as a working hypothesis, and all of cosmology follows. Some day we may discover it is inadequate, but until then, we will continue to use it. The same could be said for other principles, like conservation of energy-- we don't know that's true either, but it's useful and consistent with observations. The cosmological principle rules out that the universe has a unique center.

Your question is whether the apparent lack of a unique center stems from our location not being special enough to be able to tell that it does have such a center (say, by encountering a boundary). Ironically, the cosmological principle turns that question on its ear, and points to the possibility that one's perspective always puts themselves at the apparent center. So we would say the illusion of *having* a center is all about perspective-- not the illusion of *not* having a center!

> **Comments:**
> **username_0**: I understand the purpose of the cosmological principle to suggest ultimately that our universe is the "same" anywhere, on a large enough scale (the universe is the same for all observers). Yet the universe being homogeneous and isotropic does nothing to dispute the possibility for a center. I tub of yogurt can be homogeneous and isotropic, but it clearly has a center, right? I guess that is my point regarding the perspective. Are we in a position where, because of our position in the observable universe, we cannot ever determine where the center is/or could be?  
> **username_0**: Or is there the possibility that there truly is no center? And if so, then how can that actually be?  
> **username_3**: There would be no center if the universe were spatially infinite, or if it were  finite but closed, and those are more or less the only possibilities allowed by the cosmological principle.  A closed universe is often described as being like the surface of a sphere in a lower-dimensional analog, which has no center on that surface.  
> **username_4**: @LaserYeti "I understand the purpose of the cosmological principle", it doesn't seem so. The only purpose is to try to predict the results of future observations based on past observations, as accurately as possible. Real actual results, affecting you, your family, your cat. Does your hypothesis predict some new interesting observation (i.e. is it *falsifiable*)? Standard scientific method.  
> **username_3**: There is still the possibility that the universe does have a unique center, and there is the possibility that it does not.  You can even see this from your own analogy of birds floating in the ocean.  If there are continents that you cannot see, then there could be some meaning to the "center of the Atlantic ocean," for example, defined by the continents that define "the Atlantic ocean."  But take those continents away, and poof, you are back to an ocean with no unique center.  How would we know if we can't see the continents?  So we model the ocean with only what we can see-- no center.  
> **username_0**: @KenG I completely agree with your point and understand what you are saying... that is **why** I was asking the question in the first place. Coming back to the problem with the cosmological principle, it was predicated on our locality. If we had another point of view to observe the universe from (from the "outside"), we would have no reason to ask questions like this in the first place. That is why I still ask them. Those that blindly follow are easily lead astray.  
> **username_1**: @LaserYeti The cosmological principle is an assertion that is actively tested. There are occasional reports of small violations on large scales (e.g. in the cosmic microwave background). A flat but finite universe would have a centre. An infinite universe or a closed universe does not have a centre.  We are obviously not anywhere near an edge of a finite universe since the cosmological principle holds to a very high degree of accuracy for the observable universe and we cannot see beyond the observable universe.  
> **username_0**: @RobJeffries Thank you, I understand, but I still like to ponder the what ifs. One of my research projects was a 212 orbit HST-STIS observation of the HLQG to determine if structures of that scale can disprove the current cosmological model, or if there is a misunderstanding of the homogeneity scale. Because there are several greater than $260 h^{-1}Mpc$ sized structures, I look for reasons why we assume the universal design that we do. I guess I'm just curious like a cat.  
> **username_3**: There is certainly no harm in curiosity, and all scientific principles, including the cosmological principle, must be constantly tested and attacked with our best possible vigor.   It is only a principle that stands up to withering assault that can be regarded as good-- and so far, the cosmological principle has.  That only makes it a good principle, not a true one.  

---

