## Where is the center of the universe?

And this is where you lose most of us astrophysical midgits, right there. It's possible to observe??? that there is no center but it is impossible to contemplate the possiblility that there is a center but we just cant find it........YET. Bill has, what seems to me, to be a good analogy. Why is it not possible.

If there was an expansion as it is usually described, it had to expand from somewhere, to somewhere. Either we should at least try to find the center of the expansion as a matter of curriosity or quit using the word 'expansion' to describe whatever it was that happened.

But that might step on a few theories, dog gone it.

tex

 Quote by thetexan And this is where you lose most of us astrophysical midgits, right there. It's possible to observe??? that there is no center but it is impossible to contemplate the possiblility that there is a center but we just cant find it........YET. Bill has, what seems to me, to be a good analogy. Why is it not possible.
I've just shown why. If, from any vantage point in the universe, you see the same thing - galaxies at the edge of your observation bubble moving away, with their recession proportional to their distance, then all points are equally privileged. What point can claim to be the centre?

I refer you back to the expanding balloon analogy. A hundred ants on it surface all see the other ants receding from it with their recession proportional tot heir distance. Which one can claim to be at the centre of the surface of the balloon?
 Recognitions: Gold Member Science Advisor Just to emphasize what Dave said here. The balloon analogy is a 2D analog. In that toy universe, all existence is concentrated on the 2D surface and the point which WE see the balloon expanding outwards from does not exist in that universe. None of the 2D critters slithering around in that 2D (with no thickness) universe would be able to point a finger in the direction of the center-of-expansion. They can only point in 2D level directions in their world. Lightrays in their world travel along greatcircle routes in their 2D world, always staying in the balloon surface. A light beam is never observed to take a "shortcut" (go out of existence, or into some "higher dimension" and come back into existence somewhere else). The 2D denizens of that 2D universe have no visible evidence that their world is immersed in a 3D one. That is how WE see it, but that is not how it is for them. It sometimes helps if you watch the brief animation of an expanding 2D universe with galaxies and little colored packets of light traveling between them. I put the link to it in my signature at the end of the post. This animation helps some people get the concept.
 I agree that you cannot, UP TO THIS POINT, determine where the center is due to the fact that from where ever you make the observation it seems to be the center of the universe. Then, all that has been asserted at this point is that...you cannot observe where the center is because all receding objects seem, from that vantage point, to be receding from that observation point. This doesnt prove, in and of itself that there is no center. 1. It may be true that we cant, now, prove where it is. 2. This inability does not, in and of itself, prove there is no center. 3. If the possibility exists that there is a center (a point that closely approximates where the big bang took place, or from where true expansion is radiating) why doesnt anyone make the attempt to try to find a new way to go looking for it, such as what Bill suggested? 4. In fact, there seems to be such an aversion for even contemplating the possibility that there seems to be something else at play here, especially since what we are dealing with here are THEORIES. And I think it is that no one wants to risk his or here peer respect for suggesting such a thing or for making the attempt at locating it. And, one does not want to ask a question they cant stand the answer to...meaning....what would be the ramifications of finding it? tex

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 Quote by thetexan I agree that you cannot, UP TO THIS POINT, determine where the center is due to the fact that from where ever you make the observation it seems to be the center of the universe.
A center is a point of symmetry, a point about which you can rotate the system and have the system stay more or less the same. And crucially, it is a unique point.

There is no such unique point of symmetry for our universe.
 I seemed to have stirred up a hornets' nest. On an Easter day as well. I used the balloon analogy as Stephen Hawking used it. If the universe has no centre then it is infinite, but this disagrees with Stephen Hawking's Big Bang theory. However, I concede that the observable universe is not the universe, in which case my method for locating the centre of the universe would not work. Indeed the diametrical opposite light source, which I was relying on, would not be available, However, I still maintain that one can find the centre of the observable universe by the method stated. I do not think that what I said was at odds with Davec426913's view. Yes, the ants on the surface of the balloon will move away from each other as the balloon expands. Agreed. However, the question posed is "where is the centre of the universe?" Not, where is the centre of the skin of the universe? The balloon has a centre and it is the balloon's centre, which I attempted to locate. "the point at which we see the balloon expanding outwards from does not exist" . Well, I would be obliged to be directed to where this theory was stated and survived peer review. There may be some astronomical observations to support this but I do not know them. I presume the proponents of this theory must believe that a void pervaded the centre of the "balloon" after the material of the Big Bang had passed through. A "void" being literally that. That is, it is not even empty space and in which case the light from my diametrically opposite galaxy will never be able to reach the observer in the Milky Way.

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 Quote by marcus Just to emphasize what Dave said here. The balloon analogy is a 2D analog. In that toy universe, all existence is concentrated on the 2D surface and the point which WE see the balloon expanding outwards from does not exist in that universe. None of the 2D critters slithering around in that 2D (with no thickness) universe would be able to point a finger in the direction of the center-of-expansion. They can only point in 2D level directions in their world. Lightrays in their world travel along greatcircle routes in their 2D world, always staying in the balloon surface. A light beam is never observed to take a "shortcut" (go out of existence, or into some "higher dimension" and come back into existence somewhere else). The 2D denizens of that 2D universe have no visible evidence that their world is immersed in a 3D one. That is how WE see it, but that is not how it is for them. It sometimes helps if you watch the brief animation of an expanding 2D universe with galaxies and little colored packets of light traveling between them. I put the link to it in my signature at the end of the post. This animation helps some people get the concept.
 Quote by Bill Crean ... "the point at which we see the balloon expanding outwards from does not exist" . Well, I would be obliged to be directed to where this theory was stated and survived peer review. There may be some astronomical observations to support this but I do not know them. I presume the proponents of this theory must believe that a void pervaded the centre of the "balloon" after the material of the Big Bang had passed through. A "void" being literally that. That is, it is not even empty space and in which case the light from my diametrically opposite galaxy will never be able to reach the observer in the Milky Way.
Bill, you seem to be gradually getting the idea even tho you still doubt it. You are getting closer and clearer to a standard mainstream cosmology view.

I'm trying to communicate to you the view that the overwhelming majority of peer review literature is based on. To understand it you probably need to go here and watch carefully for a while. http://www.astro.ucla.edu/~wright/Balloon2.html
(while watching remember to think that in this 2D toy model all existence is concentrated on the 2D surface, it is all space and all points of space are on that surface)

If you think about it, what are the possibilities for a 2D spatial universe? Two of the simplest are:
an infinite flat sheet of paper (with no thickness)
a 2D "skin" wrapped about a non-existent 3D ball (again no thickness, the balloon universe idea, no inside or outside).

there are others but they tend not to be so simple and symmetrical, a 2D creature plopped into some random location on, say, the skin of a donut, might notice some odd optical effects that wouldn't be the same in all directions.

So we take the two simplest ideas of 2D space, and pick one, the sphere, and study it.

Now then for COSMOLOGY, to do the analogous thing for 3D, we have to imagine either infinite 3D space (analogous to the flat piece of paper) or a 3D skin wrapped around a non-existent 4D ball.

Those are both convenient models of 3D space to work with (mathematically speaking) and people work with both. We can say what the angles of triangles add up to in either case. We can write formulas for how volume depends on radius etc etc. How many galaxies to expect to count within a certain distance? What angular sizes to expect things to have at various distances. etc.

It's pretty commonsense, what other possibilities for 3D space do you seriously want to consider? And thinking of space in these two basic ways goes back to the 1920s and the work of Alex Friedmann. He studied several possible expanding-distances models of cosmos based on Einstein's 1915 general theory of geometry. He could have told you about the 3D universe which can be thought of as the skin wrapped around a nonexistent 4D ball, even before the Belgian priest Father LeMaître. Of course many people would credit LeMaître, but he actually thought of it later, in 1927.

And space still could be infinite! We have to keep our options open until there is enough evidence to decide which. So people continue to use both models and fit data and calculate with either. In neither simple model case does 3D space contain a point which you can point your finger at or aim a lightbeam at which is the "center-of-expansion". Friedmann's and LeMaître's formulas can be adapted to EITHER the infinite or the wraparound case just by adjusting parameters.

Happy Easter by the way! And you didn't stir any hornets nest Everybody is just trying to help you. Sometimes it takes a while. Watch the little 2D animation.
http://www.astro.ucla.edu/~wright/Balloon2.html

 Quote by thetexan ...especially since what we are dealing with here are THEORIES.
Okay, here we go...

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 Quote by Bill Crean I seemed to have stirred up a hornets' nest. On an Easter day as well. I used the balloon analogy as Stephen Hawking used it. If the universe has no centre then it is infinite, but this disagrees with Stephen Hawking's Big Bang theory.
Whether the universe is infinite or not has no bearing on the Big Bang theory, nor does it have any bearing on if there is a center.

 I do not think that what I said was at odds with Davec426913's view. Yes, the ants on the surface of the balloon will move away from each other as the balloon expands. Agreed. However, the question posed is "where is the centre of the universe?" Not, where is the centre of the skin of the universe? The balloon has a centre and it is the balloon's centre, which I attempted to locate.
You are missing the key element, which is that it is an ANALOGY. Of course we can see that the balloon has a center. We live in a 3d world which the balloon is part of. However the hypothetical 2d ants do NOT live in a 3d world but in a 2d world. There is no center of the balloon to them because there isn't a third dimension that they can interact with. There is only the 2d surface, which is expanding in our analogy.

 "the point at which we see the balloon expanding outwards from does not exist" . Well, I would be obliged to be directed to where this theory was stated and survived peer review.
It isn't a theory, it's an analogy.