# What is at the center of the universe?

by .ultimate
Tags: universe
 P: 45 What is at the center of the universe? after the Big bang?
 P: n/a there is no centre, or you can say everything is at the centre.. Because the universe began in a single point, the whole universe and its centre where the same point, so now everything is at the centre of the universe. This is also why no matter where in the universe you place yourself every thing seems to be moving away from you, like you were at the centre. Because of the expansion of the universe...
 P: 15,294 The venerable balloon analogy: Imagine a colony of ants living on the surface of a balloon that is expanding contrinuously. Which ant is nearest the centre of the ballon's surface? Every ant sees every other ant moving away from it. From its viewpoint, it is at the centre of the universe - the same as every other ant. They're ALL at the centre!
P: 34

## What is at the center of the universe?

Hi Dave,

I believe Balloon and ant example is a poor analogy. In this analogy, ant would see only other ants moving away in two dimensional space. But in actual universe, you would see matter moving away in three dimensional. Actuall shape of the universe is always debated and clear picture is not yet available. Ant and ballon example would just confuse the readers more than helping them to visualize the shape of the universe.

Hi Maxwell,

No can say with 100% confidence that universe doesnt has a center. You could be right in saying that whereever you go, you would see matter moving away from you but this doesn't mean that there is no center to the universe.

If there is center, at some place, you would see matter moving away in all directions with the same velocity (dependent on distance) but
concentration of matter would be higher on one side.

If there is no center, all the galaxies would be attracted equally by the surrounding galaxies. So there wont be any net gravitational effect on any Galaxy on a large scale (Ignoring short distance ones like Andromeda). But still
most of the astronomers believe in a big crunch due to gravitational attaction provided the matter density in the universe is less than critical density and also Dark energy is said to be of overcoming gravity. So here we are clearly accepting gravity on large scales which is not possible if the universe doesn't has a center.

I am not saying that there is a center to a universe but still its not clear.

Thank You
Talksabcd (Talks Basics)
P: 2,050
 Quote by talksabcd I believe Balloon and ant example is a poor analogy. In this analogy, ant would see only other ants moving away in two dimensional space. But in actual universe, you would see matter moving away in three dimensional.
Sure, the analogy is two dimensional, because we're used to curved two dimensional spaces. If you could already visualise curved three-or-four dimensional spaces, you wouldn't use the analogy. Main point is that the balloon's surface has no centre. And no reason why all the ant colonies couldn't exert attractive forces on each other (perhaps by running springs across the surface, with effects analogous to gravity), causing the entire space to recollapse, despite there still being no center of collapse (within the two-dimensional analogy).
P: 14
 Quote by .ultimate What is at the center of the universe? after the Big bang?
The earth! We can't see that now, because, I believe, the universe is in a temporary state, so it is not now obvious.
 P: 34 There is no problem in using two dimensional analogy if the same could be applied to three dimensional but this analogy utterly fails when comes to three dimensional to prove that there is no center. Please see my second paragraph in post #4
Sci Advisor
P: 1,250
 Quote by talksabcd If there is no center, all the galaxies would be attracted equally by the surrounding galaxies. So there wont be any net gravitational effect on any Galaxy on a large scale (Ignoring short distance ones like Andromeda). But still most of the astronomers believe in a big crunch due to gravitational attaction provided the matter density in the universe is less than critical density and also Dark energy is said to be of overcoming gravity. So here we are clearly accepting gravity on large scales which is not possible if the universe doesn't has a center. I am not saying that there is a center to a universe but still its not clear.
Sorry but this is all wrong. The solution to the equations of general relativity describing a homogeneous, isotropic and infinite are perfectly happy with the current observations. You do not need to have a center of the universe that everything else is moving away from. There is no net gravitational force on anything in the universe, since in general relativity gravity is not a force. Instead all the matter in the universe follows geo-desic paths determined by solving the equations. The solutions tell us that everything moves apart even though the infinite universe is homogeneous.

It all boils down to initial conditions. The Big Bang caused everything to begin moving apart and hence they continue to do so. It is a common misconception that the Big Bang happened at a point. This is not the case. It happened everywhere. There is no one point you can trace all the matter in the universe back to if you run time backwards.

By the way, the Big Crunch is not thought by anyone to be a reasonable prediction of the universes future. It is not supported by the observations at present. Even if it was, it dosn't require a center of the Universe to exist in order to occur. A Big Crunch is NOT where all the material in the Universe moves to a single point. What it is is that density of the universe goes to infinity, but there can still be an infinite (or indeed finite) distance between points in the infinitely dense universe.
P: 45
 Quote by Maxwells Demon there is no centre, or you can say everything is at the centre.. Because the universe began in a single point, the whole universe and its centre where the same point, so now everything is at the centre of the universe. This is also why no matter where in the universe you place yourself every thing seems to be moving away from you, like you were at the centre. Because of the expansion of the universe...
Does that mean that the Universe is constant? Does that mean while the outer shell is expanding ... the Universe is also contracting at the center?

Also, Isn't ther violation of conservation of mass during Big bang?
Sci Advisor
P: 1,250
 Quote by .ultimate Does that mean that the Universe is constant? Does that mean while the outer shell is expanding ... the Universe is also contracting at the center? Also, Isn't ther violation of conservation of mass during Big bang?
No, No and No are the answers I'm afraid.

The post you quote was almost correct although it also made the common mistake of thinking the Big Bang occurred at a single point.

The universe is homogeneous (on large enough scales) and hence is doing the same dance at all places at any one time. So if the universe is expanding then the whole universe is doing so, if the universe is accelerating or decelerating then again it does so at all places at the same time. There is no inner or outer shell.

There is no violation of mass conservation in the Big Bang since conservation laws describe the constancy of something with time. Since time begins with the Big Bang there is nothing to compare with previously! Mass is conserved as the universe expands since the density decreases. If the universe doubles in volume then the density of mass halves.
P: 34
 Quote by Wallace Sorry but this is all wrong. The solution to the equations of general relativity describing a homogeneous, isotropic and infinite are perfectly happy with the current observations. You do not need to have a center of the universe that everything else is moving away from. There is no net gravitational force on anything in the universe, since in general relativity gravity is not a force. Instead all the matter in the universe follows geo-desic paths determined by solving the equations. The solutions tell us that everything moves apart even though the infinite universe is homogeneous. It all boils down to initial conditions. The Big Bang caused everything to begin moving apart and hence they continue to do so. It is a common misconception that the Big Bang happened at a point. This is not the case. It happened everywhere. There is no one point you can trace all the matter in the universe back to if you run time backwards. By the way, the Big Crunch is not thought by anyone to be a reasonable prediction of the universes future. It is not supported by the observations at present. Even if it was, it dosn't require a center of the Universe to exist in order to occur. A Big Crunch is NOT where all the material in the Universe moves to a single point. What it is is that density of the universe goes to infinity, but there can still be an infinite (or indeed finite) distance between points in the infinitely dense universe.

I wouldn't argue with general theory of relativity. Here you are considering universe as infinite. So many things are possible under infinite universe. If the universe is infinite, no one would dare to say that there is a center to the universe. But how can anyone be so sure that universe is infinite ? and not finite ?. Is there any sound practical evidence that universe is infinite ? Infinite universe would also mean infinite energy which is beyond my grasp. Please explain.

You are saying that Big Bang happened everywhere. How can this be ?
Can you give brief explanation or direct me to some link ? I never heard of this..

You are saying that under big crunch, density of the universe goes to infinity.
We know that there is nothing higher than infinity. How one can measure distances in an infinitely dense universe ? Please explain. Infinite density means zero volume. How can one measure distances in zero volume ? Are you talking about extra dimensions ?
Sci Advisor
P: 1,250
 Quote by talksabcd I wouldn't argue with general theory of relativity. Here you are considering universe as infinite. So many things are possible under infinite universe. If the universe is infinite, no one would dare to say that there is a center to the universe. But how can anyone be so sure that universe is infinite ? and not finite ?. Is there any sound practical evidence that universe is infinite ? Infinite universe would also mean infinite energy which is beyond my grasp. Please explain.
It is true that we cannot be certain that the universe is infinite given that we only observe a finite portion. However even if somehow, in violation of present theories (the solutions of GR we use to model the universe require it to be infinite), the universe is finite there is nothing that make the center of the finite blob special. It just happens to be the center.

 You are saying that Big Bang happened everywhere. How can this be ? Can you give brief explanation or direct me to some link ? I never heard of this..
A quick google search found this and this which might be helpful. Read these and post any questions you still have.

 You are saying that under big crunch, density of the universe goes to infinity. We know that there is nothing higher than infinity. How one can measure distances in an infinitely dense universe ? Please explain.
I'm not sure what any of this question means. See below, I hope that clear up the confusion.

 Infinite density means zero volume.
This is only true assuming a finite mass, since the mass of the universe is infinite it can be infinitely large as well as being infinitely dense.

 How can one measure distances in zero volume ? Are you talking about extra dimensions ?
See above, the Big Bang is NOT a point of zero volume. It is a singularity so discussion of volume is meaningless.
 Sci Advisor P: 1,250 There is nothing that we have observed that suggests the Earth is the center of the Universe. There is no theory that requires this to be true to explain something else. If you have any evidence for this not being the case please share it with us all! [sorry Wallace - I'm trying to clean this up... -Russ]
 P: 183 Whats at the center, well, first answer whether universe in restricted to some expanse or not ? The point of bigbang needn't be the center (how do we know if the explosion was symmetrical in all dimensions) PS: Sorry if this sounds unconvincing,i am a novice in this field. I am just sharing my views.
Mentor
P: 21,658
 Quote by talksabcd So many things are possible under infinite universe. If the universe is infinite, no one would dare to say that there is a center to the universe. But how can anyone be so sure that universe is infinite ? and not finite ?.
Those questions are irrelevant, since the universe does not need to be spatially infinite to lack a center. That's one of the things that the balloon analogy tells us.
Mentor
P: 21,658
 Quote by f(x) Whats at the center, well, first answer whether universe in restricted to some expanse or not ? The point of bigbang needn't be the center (how do we know if the explosion was symmetrical in all dimensions) PS: Sorry if this sounds unconvincing,i am a novice in this field. I am just sharing my views.
The Big Bang was not an explosion and the unverse looks pretty much the same in every direction.
P: 34
 Quote by russ_watters Those questions are irrelevant, since the universe does not need to be spatially infinite to lack a center. That's one of the things that the balloon analogy tells us.
Could you please explain how a three dimensional universe could be finite but still has no center ?
I can easily visualize two dimensional finite space with no center using
ant and balloon analogy but it helped me nothing in visualizing a three dimensional finite space with no center.
 Mentor P: 21,658 Visualizing it isn't really possible because our eyes (and thus our brains) are limited to seeing in 3 dimensions, but in 1 dimension (the edge of a circle), the perimeter has no center, but finite length and is curved in a second dimension. In two dimensions (the surface of a sphere), the surface has no center, but a finite area and is curved over a third dimension. In three dimensions, then, the space would have no center but finite volume and there must be a curvature in higher dimensions. The three concepts are the same. You can't visualize why/how, but if you fly far enough in one direction, you'll end up back where you started, just like if you were flying around the earth.

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