# Center of the universe

1. Nov 4, 2009

### superdave

So every physics/astro student is told there is no center of the universe, imagine it like a balloon and every point expands equally from every point.

Except, there is a center of a balloon. Right in the middle where the air is. The universe isn't the flat surface of a balloon, it is 3 dimensions and expands in 3 dimensions.

Now, if we are measuring from Earth, and pick a direction, there must be a galaxy out there that is the farthest from Earth. Unless space curves completely back in on itself. There is some debate about that. Now, this galaxy is likely too far for us to actually ever see, but it exists.

And if we measure in every direction, there should be similar 'farthest galaxies'. The distances to these farthest galaxies won't necessarily be the same. But there should be a point in space where the distance to these 'farthest galaxies' is roughly the same in all directions. That point would then be the center of the universe.

I'm sure this has been thought of and holes poked through it, but I'd like to hear others' thoughts.

2. Nov 4, 2009

### mgb_phys

The balloon analogy is for a 2d universe.
Imagine you are a 2d flat creature on the surface of the balloon there is no centre.

3. Nov 4, 2009

### xxChrisxx

It all depends on definitions really. There is a centre of the universe with regard to the fact that it's the point where it expanded from.

The centre doesn't exist in any of the three dimensions we can comprehend, so it's all a bit moot.

4. Nov 4, 2009

### WhoWee

If a center could be identified, wouldn't it need to be plotted over time?

5. Nov 5, 2009

### Molydood

I have always struggled with this one
I am not arrogant enough to challenge the statement that there is no centre, as I have heard it stated before, and I am sure by much brainier people than me, but in the interests of discussion and provoking the right answers via my own stupidity, here is my (incorrect) thought process on it anyway:

At t=0s, all matter is in the centre
At t=1s all matter is heading off in different directions
At t=1s some matter is at one side of the universe, and some matter is on another side of the universe
At t=1s the universe now has a finite size with edges/boundaries

Therefore, why is there not a point within the universe, perhaps occupied by nothing, that is no more towards one side than the other? And the same in all dimensions thus a true 3 dimensional centre point?

6. Nov 5, 2009

### xxChrisxx

But its 3D spacetime that is expanding. You could set off in 1 direction, and so long as you went straight you'd end up back where you started.

Just like blowing up the balloon with the 2D universe, for the 2D creatures there is no centre in the 2 dimensions they are aware of.

Lenth, width.

To be aware of the fact the centre of the baloon lies in a 3rd dimension, you need to be aware of depth.

Moving this concept to a 3D universe. The centre lies in an nth spatial dimension.

Length, width, height, sometihng else.

It acutally can't have centre defined by the 3 dimensions.

7. Nov 5, 2009

8. Nov 5, 2009

### Nebozilla

I think its because our perception of what the big bang was is whats confusing. It wasnt an explosion with a center but it was the expansion of space itself. Also you couldnt even find a center since there is no definite point of complete rest. Its all relative.

9. Mar 11, 2010

### bannerman100

Wasn't the three dimensional expansion of the universe discovered by observing the red shift, then logically reckoning that by working backwards, everything must have come from some definite point in space ?
Big Bang matter and light radiated outward in all directions, so there must be a very big dark hole left behind. (Just kidding. Or am I ? ;)

Light emittted from the Big Bang must still be expanding the universe by continually creating more space (at the speed of light), and that light is continually creating the ever expanding "edge" of the universe. Physical matter (unable to keep up with C) is surely being left far behind.

May I take the original question a little further ?

We know gravity can bend light, but can it slow it down ? Yes, because black holes stop it entirely. So the gravitic pull of the mass of the universe must be slowing down its expansion by decelerating the light from the Big Bang. (Imagine gravity's effect if you travelled from the Earth's centre to it's surface.)
So the further you are from the centre of the universe, yet still within the sphere of expanding matter, the slower light will travel in the outward direction. (Until it goes beyond the mattersphere.)(Did I just invent a new word ?)(I bet someone got there first.)
But does this gravity also speed up light going toward the centre of the universe ?

If so, could we construct an instrument that would compare the two speeds, and thereby indicate our distance from the centre ?

If it's not actually heresy, can I ask:
Has anyone figured out which direction from our solar system is "inward", back toward the origin of the Big Bang ?
Or does that question naively ignore the extra dimensions ?

10. Mar 11, 2010

### Dmitry67

no, working backwards, all objects were closer and closer, and everyhting was more and more dense
BUT
there is no 'definite point in space'

So the rest of your post is wrong.

11. Mar 11, 2010

### mgb_phys

No - this is a fundemental (and common) misunderstanding of the big bang.

There wasn't an empty black space where everything exploded out into.
The big bang created the space.
It was the actual universe (ie space-time and everything) being created and expanding

12. Mar 11, 2010

### bannerman100

So where is the universe expanding from ? The concept of expansion requires a centre as a starting point.

13. Mar 11, 2010

### Dmitry67

No it does not.

Imagine INFINITE line (or surface).
Now you expand (or contract it, if we "go backwards")
You divide all distances by 1000, and now all objects are 1000 times closer. But still it is infinite.
Go go backwards even deeper, dividing all by 10000000000000000000. Now it is extremely dense and still it is infinite and there is no 'center'.

14. Mar 11, 2010

### bannerman100

But that would mean there was no Big Bang, locatable in time, and everything just expanded, and always has been expanding, from an infinitely small space ?

That would mean time would have no beginning.

Please patiently correct me if I have misunderstood you.

15. Mar 11, 2010

### Dmitry67

Why?
Big Bang has definite location in time, but not in space.

If Universe is Infinite now (which is very likely) then it was ALWAYS infinite.

here si a mathematical model
At t=1 we have an infinite line (all real numbers) from -infinity to +infinity
Going back in time, say, t=0.01 we have all distances 100 times smaller, but our line is infinite.
It is also infinite at t=0.00000001 or t=0.00000000000000000001
So it was always infinite at any t>0
However, the location in time of Big Bang is clear, it is at t=0.

16. Mar 11, 2010

### Hoku

Saying that "the big bang created space" is a little presumptious. I know that many people are using that as an *assumption* based on relativity, but the idea is only an interpretation - and it may not be right. This is one of the things that the Gravity Probe B is out to answer.

James Overduin, currently assistant professor at Towson University in Maryland, is keeping in close contact with the project and has published the statement that, "spacetime behaves relationally but exists absolutely", based on information that is coming in from the probe.

On another note, I don't understand why people make the statement that time existed before space did. Why do we constantly forget that spacetime is a singularity? You cannot isolate one from the other. (This is another problem with Verlinde's idea. He based it on the assumption that time existed before space).

17. Mar 12, 2010

### Chronos

Eternal time is another confounding fallacy, time is irrelevant prior to the big bang. As Einstein noted, time is what clocks measure. No clocks, no time.

18. Mar 12, 2010

### Mihael@@/&

EPIC FAIL

how can a Science Advisor post somthing like that?? Time probably won't exist only in absolutely homogeneous universe.

19. Mar 12, 2010

### Hoku

Mihael, you have misunderstood Chronos, who is saying that time didn't exist before the big bang.

I'm the one who suggested that spacetime is absolute, and since I'm a layman, I guess I won't get such harsh criticism for it. However, as I said in my last post, evidence is mounting in support of the absolute view and more scientists are begining to take it seriously. Check this out if you're interested - http://www.springerlink.com/content/k0htmwr32m4wd7kv/ . I will purchase the full article myself next week.

I know that the relational spacetime view compells us to say that "spacetime was irrelevant prior to the big bang", but since nobody actually knows what preceeded the big bang, that makes us unqualified to make such a statement.

Perhaps a way to bridge the absolute/relational spacetime controversy is by comparing it with the two kinds of energy, kinetic and potential. Both kinds of energy are very real, they're just in different phases of expression. Might this also apply to spacetime? Before the big bang was "potential spacetime" and afterwards it became "kinetic spacetime". Hmm...

20. Mar 13, 2010

### Mihael@@/&

It happens from time to time, not to understend somebody here:) Did not ment to be rude, sorry!

21. Mar 13, 2010

### Kronos5253

But it's 3D... So wouldn't it make more sense to say that you are a 3D creature inside the balloon? The way I see it, if the balloon is our universe, and the bug is us, shouldn't we be on the inside of the balloon, not on the outside? In that case everything expands from a central point, and you have a direct center of the "universe".

22. Mar 13, 2010

### marcus

Hoku that is a strange post! You refer to a 2001 article by James Overduin. If you want to know what Overduin was saying almost 10 years ago, you don't need to purchase that article. You can read this 2001 review article by him for nothing:
http://arxiv.org/abs/astro-ph/0101484

In any case how does what a not-too-prominent guy says in 2001 show that "evidence is mounting and more scientists are taking seriously"?

Why do you say that "relational spacetime view" entails believing that spacetime was irrelevant before the start of expansion?
It is people who explicitly adopt the relational view who have been constructing and studying models where spacetime goes back before the start of expansion. They say their view is relational and their computer models of evolving cosmic geometry go back before.

I'm talking about research proceeding at an increasing rate on the order of 100 papers a year. And current efforts to find ways to test the models.
This reality totally does not square with your statement.

Michael, that is an interesting statement. I think I understand. And I agree with what I think is the meaning you intended. The other issue is "does time go back before the big bang?"
On that question I think it is irrational, perhaps silly, to make statement Yes or No. We do not know. Do you perhaps agree with me here?

We have models where time stops and we have models where it continues on back before. The models have testable consequences and we should be able to test. But we have not yet tested. So there is no scientific reason to believe either one thing or the other.

Do you know the Einstein-Online website? I recommend reading A Tale of Two Big Bangs on the issue of does time have a beginning at the big bang.
http://www.einstein-online.info/en/spotlights/cosmology/index.html

http://www.einstein-online.info/en/spotlights/big_bangs/index.html

It's a public outreach branch of the Max Planck Institute. Hope you check it out.

Last edited: Mar 13, 2010
23. Mar 13, 2010

### Hoku

Marcus, thanks for your objections. Actually, the free article that you recommended is not the same as the one on Springerlink. I know this because I read the first page of the Springerlink article, which is free, and it is completely different from the arvix article. Still, it might have enough of the same info to save some money.

The arvix article was updated in 2002 but that doesn't mean the idea died out shortly thereafter. In fact, Stanford picked up on it and included the idea in their website http://einstein.stanford.edu/index.html, which is maintained to this day. Overduin wrote the "spacetime" section, which was current as of September 2007. His statement that "Spacetime behaves relationally but exists absolutely" can be found at the bottom of the "Einstein's Spacetime" subsection.

If there are any other objections to Overduin's publications let it be based on the specifics of what he is saying and not on date of publication or whether he's attained a high enough position at a prestigious enough university. An assistant professor position at a university in Maryland is respectable enough for a man as young as Overduin.

You are right about my incorrect use of the phrase, "relational spacetime view". I tend to incorrectly say that when the proper phrase I should be using is "emergent spacetime view". From an emergent spacetime view, spacetime IS irrelevant before the big bang because, based on that view, it did not yet exist.

24. Mar 13, 2010

### marcus

I don't see how that follows. There are theories where geometry emerges from some more fundamental degrees of freedom, which however can also exist before the beginning of expansion. I'm not sure what you mean by "emergent". It is a buzzword that people use various different ways. I don't see any reason why if spacetime emerges after the BB moment it could not emerge before the BB moment. No rational reason to believe that the same underlying stuff doesn't exist before. Fact is we don't know.

I'm not disrespecting James Overduin. As I say he's just not very prominent. In no way would I take him as an authority. And of course those two 2001 articles are different. I looked at the Springer sample too The free one is a long review article with lots of references and my guess is that it gives a good idea of what he thought and said around that time.

You cite a general audience outreach 2007 article by him at the GravityProbeB website. http://einstein.stanford.edu/SPACETIME/spacetime2.html
I don't altogether agree with it, but heck! Why not copy a sample excerpt here and see if anyone else has questions or objections?

==quote Overduin 2007==

Relational or Absolute?

In 1918, Einstein described Mach's principle as a philosophical pillar of general relativity, along with the physical principle of equivalence and the mathematical pillar of general covariance. This characterization is now widely regarded as wishful thinking. Einstein was undoubtedly inspired by Mach's relational views, and he hoped that his new theory of gravitation would "secure the relativization of inertia" by binding spacetime so tightly to matter that one could not exist without the other. In fact, however, the equations of general relativity are perfectly consistent with spacetimes that contain no matter at all. Flat (Minkowski) spacetime is a trivial example, but empty spacetime can also be curved, as demonstrated by Willem de Sitter in 1916. There are even spacetimes whose distant reaches rotate endlessly around the sky relative to an observer's local inertial frame (as discovered by Kurt Gödel in 1949). The bare existence of such solutions in Einstein's theory shows that it cannot be Machian in the strict sense; matter and spacetime remain logically independent. The term "general relativity" is thus something of a misnomer, as pointed out by Hermann Minkowski and others. The theory does not make spacetime more relative than it was in special relativity. Just the opposite is true: the absolute space and time of Newton are retained. They are merely amalgamated and endowed with a more flexible mathematical skeleton (the metric tensor).

Nevertheless, Einstein's theory of gravity represents a major swing back toward the relational view of space and time, in that it answers the objection of the ancient Stoics. Space and time do act on matter, by guiding the way it moves. And matter does act back on spacetime, by producing the curvature that we feel as gravity. Beyond that, matter can act on spacetime in a manner that is very much in the spirit of Mach's principle. Calculations by Hans Thirring (1888-1979), Josef Lense (1890-1985) and others have shown that a large rotating mass will "drag" an observer's inertial reference frame around with it. This is the phenomenon of frame-dragging, whose existence Gravity Probe B is designed to detect. The same calculations suggest that, if the entire contents of the universe were to rotate, our local inertial frame would undergo "perfect dragging" — that is, we would not notice it, because we would be rotating too! In that sense, general relativity is indeed nearly as relational as Mach might have wished. Some physicists (such as Julian Barbour) have gone further and asserted that general relativity is in fact perfectly Machian. If one goes beyond classical physics and into modern quantum field theory, then questions of absolute versus relational spacetime are rendered anachronistic by the fact that even "empty space" is populated by matter in the form of virtual particles, zero-point fields and more. Within the context of Einstein's universe, however, the majority view is perhaps best summed up as follows: Spacetime behaves relationally but exists absolutely.

==endquote==

Last edited: Mar 13, 2010
25. Mar 14, 2010

### Flatland

In a closed universe space curves back itself. What this means is that if you travel in a straight line in any direction you will eventually end back at your starting point. Now tell me what is the center to that?