# The Center of the Universe

1. Jul 12, 2006

### Vogue

I was reading about the center of our Universe, Here is a FAQ.

"[URL [Broken] Where is the center of the Universe?

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Often when people are told that galaxies are receding from us, they
assume that means we are at the center of the Universe. However,
remember that the Universe is homogeneous and isotropic. No matter
where one is, it looks the same in all directions. Thus, all galaxies
see all other galaxies receding from them. Hubble's relationship is
compatible with a Copernican view of the Universe: Our position is not
a special one.

So where is the center? *There isn't one*. Although apparently
nonsensical, consider the same question about the *surface* of a
sphere (note the *surface*). Where's the center of a sphere's
surface? Of course, there isn't one. One cannot point to any point
on a sphere's surface and say that, here is the center. Similarly,
because the Universe is homogeneous and isotropic, all we can say is
that, in the past, galaxies were closer together. We cannot say that
galaxies started expanding from any particular point.[/URL]

------------------------- Question ---------------------------
If every Galaxy acts as the center of the Universe would this be strong evidence for the infinite Universe Theory, If the Universe was infinite wouldn't all Galaxies appear to be the center relative to the observer?
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Last edited by a moderator: May 2, 2017
2. Jul 12, 2006

### Jorrie

"Evidence" means that we have some observation that directly supports it. IMO there is no evidence that "every Galaxy acts as the center of the Universe". All we have is that it appears homogeneous and isotropic on large scales, for as far (distance wise) as we can observe.

Further, observation indicates that the universe is open, so we make an assumption that it is infinite, but we cannot be certain. If the universe is open but finite, then, in a way, there must be a center.

What we can however say for sure is that we are very near the CENTER of OUR observable universe!

3. Jul 12, 2006

### MeJennifer

Well doesn't it seem that the question as to what is the center we have to take into account that your space is not an euclidian space?
It is instead a 4-dimensional space that is curved and not euclidian. That it is not euclidian means space and time axes are distinctly different.

So what would "center" even mean in a 4-dimensional Minkowski manifold?

Last edited: Jul 12, 2006
4. Jul 12, 2006

### Jorrie

I don't think we live in "a 4-dimensional space" - did you mean "a 4-dimensional space-time"? "Center" as I used it means a "spatial center" and not a space-time center!

5. Jul 12, 2006

### MeJennifer

But isn't that the problem?

For instance, if I understand it correctly, in space-time the light cones can be bent by the distribution of mass, hence when one is to take a time slice of space-time at the local frame of reference one is not at all guaranteed to have a view of space only.

Isn't it true that one cannot take an absolute slice of time in space-time since that simply does not work in a Minkowski space. Each local frame of reference determines time. And in GR it only gets worse since the cones are turned by the curvature of space-time.

So are you not trying to ask or answer a question about space as if it were something different as what it really is?

Last edited: Jul 12, 2006
6. Jul 12, 2006

### Jorrie

I think you are moving into GR on local scales, where much of what you say is correct, except that in GR one should rather think Schwarzschild spacetime and not Minkowski!

However, cosmology (and the original question of a center of the universe) works only on cosmological scales, where matter is taken to be spread homogeneously.

7. Jul 12, 2006

### MeJennifer

Well you seem to appear much more knowledgable about all this than me, but my understanding is that our spacetime is definitly a 4-dimensional Minkowski manifold. I never heared of a Schwarzschild spacetime, Schwarzschild was simply the guy who provided a solution to Einstein's field equations for a spherical non-rotating mass. As far as I know he did not invent any different kind of space-time.

Yes and how does that invalidate the ideas of GR?

Are you saying that the question as to where is the center of the universe has any meaning?
So all the GR concepts of space-time are no longer valid because it is "cosmological"?

8. Jul 12, 2006

### George Jones

Staff Emeritus
What do you mean by

and by

9. Jul 12, 2006

### VikingF

Ok, I'm probably ignorant, but anyway...: If the universe is infinite, how come it started expanding from one point about 13.7 billion years ago? In other words, if it is expanding, then how can it be defined as "infinite"? Can something "infinite" have a beginning and be expanding? Or does that really mean that the universe is expanding towards infinity?

10. Jul 12, 2006

### Vogue

Isn't saying the Universe has an edge just as bad as saying it is infinite if they haven't the optical power to prove it?

I think we need to increase our light amplification Quadrillions fold in telescope technology, If our optical power runs out by square laws then the
Universe should be called infinite because no edge was found strictly by evidence of no edge. I think an edge should only be called if an edge is found and no other systems exist outside the edge.

11. Jul 12, 2006

### chroot

Staff Emeritus
Regardless of our observational powers, we cannot see objects that are so distant their light has yet to reach us. The "edge" of the observable universe is not a physical barrier; it's just the limit of what objects are close enough to us to be visible.

- Warren

12. Jul 12, 2006

### matt.o

no theory can ever be "proven without a doubt".

We do actually see to the edge of the observable (in terms of photons) universe when the cosmic microwave background is observed. Beyond this, the universe is opaque to photons, however we should be able to see further back with gravitational wave detectors (once they become sensitive enough) and neutrino detectors.

13. Jul 12, 2006

### Arian

warning: (these things migt have been said before)

If we view the universe from a view of string theory then the universe has to be finite.
This is because, according to string theory or whatever they call it now, the universe is like 10 demensions (or 11 i haven't read on it for a long time), f our universe was infinite, then we would assume all of the dimensions to be infinte. So, we should be able to go to every single one if we wanted to, but we can't, because 6 are rapped in a little, little, curl.

So, why should 4 be infinite and six not?

The Big Bang theory also dissallows the infinite universe idea. Since there was a big bang, time has started, meaning, 3-infinite demensions, one-expanding, and 6 curled up? No, 4 expanding and 6 curled up is more realisitc.

That finishes the infinite part.

Now, galaxies don't help it. Lets make it from 3 space, to 2 space. Now, lets say we have a bunch on dots in a circles. The space between them begins to expand. The circle that holds them is expanding also because we find, the circle is actually a 3-d dohnut. There for, our dots are around the entire hyperdonut. The ones at the top, push the ones at the bottom away. The ones at the left push the ones at the right away. This causes an illusion that the universe is infinite.
Because its a hyper-torus (dohnut). And a circles is seemingly infinite.

14. Jul 13, 2006

Staff Emeritus
I'm no expert on this but I believe physicists actually do have some suggestions about why this should be so. In any case the problem is not why the four dimensions should be "extended" but why the others should be "compact". And since the four ARE extended (we see them to be so), they might be infinite without straining the theory at all.

15. Jul 13, 2006

### DaveC426913

Let's go back to the sphere analogy posed in the answer and see what we get:

"If every point on the sphere acts as the center of the sphere, would this be strong evidence for the sphere being infinite in size?"

It now becomes obvious that symmetry-of-viewpoint provides no evidence at all about the size of the object in question.

16. Jul 13, 2006

### Jorrie

Infinite universe, or not?

It seems this (not so off-topic) question is still unanswered! Let me give it a try:

The Big Bang does not necessarily start from a single point - it is thought to have started from an infinitely dense state, which may, or may not, have been infinite in size already! Say it was infinite in size - can it still expand? Yes, for sure! Twice infinity is still infinity, or is my engineering mind missing something?

17. Jul 13, 2006

### MeJennifer

Why so you say we cannot be certain?

Seems to me that an open universe if by definition infinite and a close universe is finite.

18. Jul 13, 2006

### loseyourname

Staff Emeritus
19. Jul 13, 2006

### VikingF

I thought the singularity was infinitely small at t=0?

Well, no. If it expands, it expands from a certain size, and if it has a certain size, it's not infinite... If my engineering mind isn't missing something, of course.

20. Jul 13, 2006

### EL

Jorrie is right. Have a look at the link loseyourname provided where I tried to explain this by an analogy...(post #9)

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21. Jul 13, 2006

### MeJennifer

Well that is not correct.
Something that has an infinite size can most definitly expand.

Well I checked and I did not see the relevance.

A few comments, because perhaps all this is based on my misunderstanding of the matter.

A space (at least the kind of continuous spaces we are talking about in GR) is either open or closed correct?
For instance a 3-D Euclidean space is open and infinite. However the space on a 4-D sphere is closed and thus cannot be infinite.
Both open and closed spaces can expand and contract.

So, one either has an open or a closed universe, correct?
It it is open it must be infinit.

Now that does not imply that there is an infinite amount of mass in it. Inifinity simply referers to the structure of the universe and not to the number of objects it contains.

Now it is true that the amount of mass and the distribution of mass has a direct effect on how space looks, but once it is open it must be infinite.

Or am I seeing things wrong here?

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22. Jul 13, 2006

### VikingF

Thanks for the answer! I have read your post #9, and I think we understand the BB theory in different ways.

You wrote: "Now, by shrinking this infinite plane (i.e. going backwards in time) more and more, the points will get closer and closer, representing an increasing density. However, the plane will of course still remain infinite.
After infinitely many "shrinkings" we'll reach the Big Bang singularity (that is infinte density) without having stuffed all matter into a single point!"

This is how I looked at the BB theory a while ago, but after some (cosmological) reading, it seems to me that the BB theory describes an expanding of both matter and space, i.e. that even space was "stuffed into a single point" back at t=0, and in that way is not infinite... The way I understand your analogy, the 2D-plane represents space, which is infinite all the time, and the dots represent matter, which is expanding?

I will be the first one to admit that I may be wrong, because I have no cosmological background, except being very interested in the topic, but this is atleast how I have understood the theory.

How? Being infinite means that it is being endless (with no beginning and/or end) in size, doesn't it? And if it's already endless in size, how can that be expanding? Are you sure you are not talking about something boundless? Something boundless could still be finite. Could you please explain a bit further?

Regards,
A confused VikingF
(who tries his best to learn some cosmology)

23. Jul 13, 2006

### MeJennifer

Consider an infinite Cartesian 2D plane on a piece of clay.
Now start to stretch it in both directions, it will expand.
Or consider a balloon, then put more air in it, the surface will expand.
An open universe is more like the Cartesian 2D plane, it is open and infinite, a closed universe is more like the balloon, closed and finite.
Both, as I hope you see, both can expand or contract.

Last edited: Jul 13, 2006
24. Jul 14, 2006

### EL

Basically our universe is considered either open, flat or closed. In a closed universe the angles in a triangle sums up to more than 180 degrees, in a flat to exactly 180 degrees, while in an open to less than 180 degrees.
A closed universe is spatially finite, while both the flat and the open are spatially infinite.

It's flat (and infinite).
I guess you're really talking about the surface of a 4D ball, something we call a 3-sphere. That space is closed and finite.
Correct. (And the same is true for flat spaces.)

Or flat. Basically.
Yes, basically. (The resaon why I keep on writing "basically", is because there's something called non-trivial geometries, which I don't know much about, and I anyway just think would be confusing at this stage.)

True, however if we assume a homogenous distribution of matter in such universe, there must be an infinite amount of matter in it.

Last edited: Jul 14, 2006
25. Jul 14, 2006

### MeJennifer

Of course if you assume that.
But is that realistic?

The way I see it, and greater minds feel free to correct me on my mistakes, is that in order to have a homogenous distribution of matter in an open and thus infinite universe one cannot assume it has a finite age AND that there was a big bang singularity or pseudo singularity.

It would take matter an infinite amount of time to occupy a homogenous and infinite universe. Expansion would not help since the pre-expansion universe must have been closed the expansion would not be able to expand a close space into an infinite homogenous space in a finite amount of time.

The only way I could see this work is that the expansion caused the creation of matter rather than that all matter came from the big bang singularity or pseudo singularity.

But perhaps I am missing something?