In what direction is the center of the universe?

[CaptainToonces]

If the universe is finite, then it must have a center of gravity. In which direction is it?

[Jens]

It doesn't have a center of gravity, as far as we know. From our point of view, we seem to be in the center of the universe. But we presume that this would be true for anybody, i.e. that we are not in a privileged position.

[01101001]

If the universe is finite, then it must have a center of gravity.

Does not follow from the assumption.

[Shaula]

Think of Pacman space - when you go off the side of the screen you reappear on the other one. In a spacetime like that (which is perfectly possible) there is no centre of mass. You can only define one by imposing artificial boundaries on which particular way to dice the repeating volume - so the CoM is where you put it!

[Jeff Root]

If the universe is finite, then it must have a center of gravity.

Does not follow from the assumption.

It does follow if you don't realize that space might not be
Euclidean.

-- Jeff, in Minneapolis

[swampyankee]

GR would also break if there was a definable center of mass for the Universe. At least that's what I think: a definite c/m means there is a point from which an absolute coordinate system could be defined.

[astromark]

The only way to answer this question is... Not to answer it at all. 'IF' the universe is finite 'AND' was born from a single point. Then it should be possible to pinpoint a center of mass. As we seem to be unable to do that because of eccelorating expansion and shear size....
Then if we can not find such center of mass there might not be one and all bets are off.
Regarding the singular Big Bang of initial creation...
I can except that there are some things that we may never actually know. The deeper you dig into this. The harder to find is a answer.
If you consider that as time is part of the universe and thus did not exist pre universe. Time and space are part of the whole thing so finding a start point or time is never possible... Where is the center of the Universe ? ... Everywhere.

[Van Rijn]

The only way to answer this question is... Not to answer it at all. 'IF' the universe is finite 'AND' was born from a single point. Then it should be possible to pinpoint a center of mass.

Why should it be possible? You seem to be thinking of the misconception of the Big Bang as an explosion in space.

From here:

http://www.xs4all.nl/~johanw/PhysFAQ...GR/centre.html

There is no centre of the universe! According to the standard theories of cosmology, the universe started with a "Big Bang" about 14 thousand million years ago and has been expanding ever since. Yet there is no centre to the expansion; it is the same everywhere. The Big Bang should not be visualised as an ordinary explosion. The universe is not expanding out from a centre into space; rather, the whole universe is expanding and it is doing so equally at all places, as far as we can tell.

[DrRocket]

If the universe is finite, then it must have a center of gravity. In which direction is it?

Even if the universe is finite, and there is no proof that such is the case, it does not follow that it has a center of gravity.

Think about an irregularly shaped body, or even a hollow sphere. The center of gravity does not lie on the body, but rather it is a point in the space in which the body is embedded.

So far as anyone knows the universe is an intrincic manifold and it is not embedded in anything.

[DrRocket]

The only way to answer this question is... Not to answer it at all. 'IF' the universe is finite 'AND' was born from a single point. Then it should be possible to pinpoint a center of mass.

Wrong. In fact it would be difficult to be more wrong on so many points.

If the universe is finite and was born from a single point, then everywhere in space is the point of origin "now". There still may be no center of mass, depending on the topology of the universe. And if there is it might even be the case that, under the assumption of homogeneity, every point is the center of mass.

On the other hand if, as you seem to be suggesting one looks at the entire spacetime manifold, and one assumes that it began as a point (and the singularity predicted is not just some simple point but we'll make your assumption anyway), then that point is the point of origin of both space and time. However, the way the question is usually framed is in terms of some point in a space-like slice of spacetime which is a different kettle of fish. see above.

[astromark]

What is wrong with your understanding Mark ? much, these people would be telling me... and they not me are in error.
... Did they see the 'IF' and the 'AND'... No they did not., or if they did they ignored it... which is a shame because I agree with them. That is what I have said. There is NO center to be found. Its all of it.
Energy became matter and is and has been expanding ever since. I used the word everywhere.... and am right to do so. Read my post again.

[DrRocket]

What is wrong with your understanding Mark ? much, these people would be telling me... and they not me are in error.
... Did they see the 'IF' and the 'AND'... No they did not., or if they did they ignored it... which is a shame because I agree with them. That is what I have said. There is NO center to be found. Its all of it.
Energy became matter and is and has been expanding ever since. I used the word everywhere.... and am right to do so. Read my post again.

I read it, and my response stands.

[Jeff Root]

GR would also break if there was a definable center of mass for
the Universe. At least that's what I think: a definite c/m means
there is a point from which an absolute coordinate system could
be defined.

The Universe having a center would not break general relativity.
General relativity shows that it is possible to accurately describe
the Universe without a center. But general relativity does not
require that the Universe has no center. The Earth has a center,
and that doesn't break GR. The Milky Way has a center, and that
doesn't break GR. The Universe could have a center, too, without
breaking GR. A center just isn't isn't seen, and isn't required by
what is seen.

-- Jeff, in Minneapolis

[Jeff Root]

'IF' the universe is finite 'AND' was born from a single point.
Then it should be possible to pinpoint a center of mass.

Why should it be possible? You seem to be thinking of the
misconception of the Big Bang as an explosion in space.

From here:

http://www.xs4all.nl/~johanw/PhysFAQ...GR/centre.html

There is no centre of the universe! According to the standard theories of
cosmology, the universe started with a "Big Bang" about 14 thousand million
years ago and has been expanding ever since. Yet there is no centre to the
expansion; it is the same everywhere. The Big Bang should not be visualised
as an ordinary explosion. The universe is not expanding out from a centre
into space; rather, the whole universe is expanding and it is doing so equally
at all places, as far as we can tell.

I will argue with the quote. The expansion can be accurately described in
terms of Reimannian geometry as centerless, but it might be described just
as accurately as like an explosion in Euclidean space. That would require
some unknown physics, though. The unknown physics has been shown to
actually exist: It is the acceleration of the expansion, discovered 12 years
ago. Without the unknown physics, Reimannian geometry is needed. With
the unknown physics, Euclidean geometry may suffice. However, since we
don't see any indication of a center or an edge, a centerless Universe is still
the preferred description.

There is no reason the Universe -- or the part of the Universe that came out
of the Big Bang and is still participating in the expansion -- couldn't have a
center. If it has a center, evidence of it may or may not be visible to us. But
the evidence so far doesn't hint at the existence of a center or its location.

-- Jeff, in Minneapolis

[undidly]

If the universe is finite, then it must have a center of gravity. In which direction is it?

If you are on the surface of Earth then it is obvious that the center of the Earth is at right angles to the two directions of movement on the surface.

To explain more fully I must be in ATM.

[astromark]

I want so much for you to understand me... looking at the whole posting i have quoted below. I HAVE HIGHLIGHTED THE SENTENCE YOU SEEM TO HAVE NOT SEEN....

The only way to answer this question is... Not to answer it at all. 'IF' the universe is finite 'AND' was born from a single point. Then it should be possible to pinpoint a center of mass. As we seem to be unable to do that because of eccelorating expansion and shear size....

* ''' Then if we can not find such center of mass there might not be one and all bets are off.''' *

Regarding the singular Big Bang of initial creation...
I can except that there are some things that we may never actually know. The deeper you dig into this. The harder to find is a answer.
If you consider that as time is part of the universe and thus did not exist pre universe. Time and space are part of the whole thing so finding a start point or time is never possible... Where is the center of the Universe ? ... * Everywhere.*

[Aik4on]

In reply to the OP, if the universe is and always has been 'everything' then, from an observer's point of view, the direction in which the centre of universe lies is always 'everywhere'.

It's not profitable to visualise the universe as a 3D shape with a 'centre' as this implies it is expanding in to some form of higher dimensional space. Saying that, it's very hard not to visualise it in this way as our perception and reality is based on observing 3D objects and the 'space' around them.

[CaptainToonces]

Thanks to Jeff Root for the only sensible answer of them all

What a lot of you seem to be overlooking, barring perhaps the introduction of extra dimensions, is that the word "expands" itself implies a center, because expands means to gain volume, and anything with volume has a center, so you cannot say that the universe is expanding without saying it has a center. I think the correct answer to my question, is really, "we don't know."

[CaptainToonces]

Think of Pacman space - when you go off the side of the screen you reappear on the other one. In a spacetime like that (which is perfectly possible) there is no centre of mass. You can only define one by imposing artificial boundaries on which particular way to dice the repeating volume - so the CoM is where you put it!

Pacman space is simply a torus. Or since there are no passages in Pacman that go up/down, only left/right ones, Pacman is a cylinder. If you could have up/down passages too, it would be a torus. Both these things do have centers. In fact it is impossible to define a 3d shape with volume that does not have a center.

[Shaula]

What a lot of you seem to be overlooking, barring perhaps the introduction of extra dimensions, is that the word "expands" itself implies a center, because expands means to gain volume, and anything with volume has a center, so you cannot say that the universe is expanding without saying it has a center.

That is wrong. All expansion means is that if you take two points in space then over time they end up further apart as space itself has expanded. That can happen in an infinite universe, in a finite but unbounded universe and so on. As for the centre - why assume it is inside the shape it is associated with? As has been said - a torus has a centre but that centre is not within the torus. So if the universe were that shape then the 'centre' of it would be outside the universe.

[Shaula]

Pacman space is simply a torus. Or since there are no passages in Pacman that go up/down, only left/right ones, Pacman is a cylinder. If you could have up/down passages too it would be a torus. Both these things do have centers. In fact it is impossible to define a 3d shape with volume that does not have a center.

No, Pacman space is 2D. It is flat. So it is a simple tiled space.

[CaptainToonces]

That is wrong. All expansion means is that if you take two points in space then over time they end up further apart as space itself has expanded. That can happen in an infinite universe

No, that cannot happen uniformly for all points in an infinite universe, BARRING THE INTRODUCTION OF EXTRA DIMENSIONS, because for every two points you move apart, they become closer to other points in an equal and opposite way.

[CaptainToonces]

No, Pacman space is 2D. It is flat. So it is a simple tiled space.

You used Pacman as an example because of the passages that link left and right, correct? This shape indeed defines a cylinder. It is not 2d.

[Strange]

Thanks to Jeff Root for the only sensible answer of them all

What a lot of you seem to be overlooking, barring perhaps the introduction of extra dimensions, is that the word "expands" itself implies a center, because expands means to gain volume, and anything with volume has a center, so you cannot say that the universe is expanding without saying it has a center. I think the correct answer to my question, is really, "we don't know."

Despite the cryptic final comment, undidly's analogy is a good one: say your existence is confined to the surface of a sphere (i.e. a 2D analogy for a 3D universe). If the sphere is expanding; the surface (which is all you know) is also expanding but it has no center. All points on the surface of the sphere are moving apart from one another. Except for some local motion, none are moving closer together. The surface is finit, expanind and has no center. That may be the way the universe is.

[CaptainToonces]

That may be the way the universe is.

yes, it may be. But notice i said "barring the introduction of extra dimensions." When you offer the example of a 2d perception of a 3d environment (the 2d people on the surface of the balloon who don't know it's a 3d balloon) you are introducing the concept of the unknown dimension. But even in that example, there IS a center of the universe, even if that center is NOT IN the universe!

[CaptainToonces]

I think the correct answer to my question, is really, "we don't know."

Or maybe the answer could be: in the direction of spacetime that points directly into the past

[Strange]

yes, it may be. But notice i said "barring the introduction of extra dimensions." When you offer the example of a 2d perception of a 3d environment (the 2d people on the surface of the balloon who don't know it's a 3d balloon) you are introducing the concept of the unknown dimension. But even in that example, there IS a center of the universe, even if that center is NOT IN the universe!

I would be argue that the third dimension in the balloon analogy has no counterpart in the "real world" - this is where the analogy runs out of steam. (All analogies have a limit; that is why they are analogies.) On the other hand, you could consider the inside of the balloon as the past (where all the smaller balloons existed) in which case, the "center of the universe" is in the past; at the big bang.

[astromark]

I like my science to be a bit of this, that, and the other. " In what direction would the center of the Universe be ?"
Can not and does not fit my demanding criteria. You can not go look at the beginning of the universe. You can not even show me all of it now.
That more than 70% of it is some sort of unknown 'dark mater'. That the expansion rate is eccelorating by a force we call 'Dark Energy'
It would seem that a awful lot of things are unknown to us...
It makes perfect sense to me that as time did not begin until the universe had begun to expand. That nano second of time when expansion driven by I know not what began. Was also the beginning of time. To be a part of that whole makes it impossible to define a center. There simply could not be one.
I want good science to show me why I might be wrong. I am looking for a argument that has more than ifs and maybe's. On that base we can not answer this question with anything other than we have little idea. Until we know of other possible conclusions we can only best guess from what we do...

[Aik4on]

The problem in these discussions is always that questions like 'in what direction is the centre of the universe' or 'what shape is the universe?' are inappropriate in the same way that to ask 'what shape is water?' or 'how long is air?' would be generally inappropriate. It seems the structure of the universe cannot be measured, described or accurately visualised in terms of a 3D object. Analogies like the balloon analogy are only slightly useful in demonstrating certain properties of the universe, like expansion.

[Shaula]

You used Pacman as an example because of the passages that link left and right, correct? This shape indeed defines a cylinder. It is not 2d.

I intended you to be able to move off the screen in any direction and still come back in the corresponding place on the other side of the screen. Maybe I should have said Kwik Snax space but that would have been showing my age. Let's not get caught up in this analogy, OK? I intended it as a 2D analogue of what was going on where to the person embedded in the universe it looks like an infinite tiling series. You could warp it to a 3D shape to 'explain' how it is not truly a tiled universe but that would be introducing extra dimensions that are not visible in the universe postulated...

In essence your question had too many assumptions in it. Or assumptions you would have to make to get an answer. Personally I think from the cosmology I have done that most of the ones you are making to get an answer are wrong (that expansion works as you say it does is a critical one - the universe is not expanding into something and just saying that you don't want there to be higher dimensions doesn't make that go away). The answer to your question is that as far as we understand the universe doesn't have a centre.