# Centre of the Universe

1. Mar 20, 2012

### azizlwl

The universe is expanding as the cosmologists say and the analogy given as in the expanding balloon. But the balloon still has a center. The expanding universe after the big bang is also said taking place at nowhere and nowhen. Couldn't figure out how is something is expanding without referring to a point.

Thank You.

2. Mar 20, 2012

### Ravi Mandavi

We can't answer our position in universe, so how can we answer where the center of universe? Many things still needs to explore

3. Mar 20, 2012

### TheTechNoir

But we can answer where the middle of the universe is: there isn't one. Everything we've learned about cosmology implies that there is no center.

You're misunderstanding the balloon analogy. What I think your thinking is, is that you blow a balloon up and look at this 3-dimensional object and see that there is a center - true. However the analogy isn't intended for 3 dimensions, it is intended for 2. It is the surface of the balloon. Now, inflate a balloon, only consider it's exterior 2-d surface and tell me where the center is located. You can't, because there is no center. Every location has just as right a claim as any other to be the center and every location is just as invalid as any other claiming to be the center.

Please read the FAQ, this question is covered in-depth at least once a week on this forum. The FAQ also addresses your misconception that the big-bang is expanding into an already existing space.

4. Mar 20, 2012

### Nabeshin

Ah, so the analogy is a good one but you have to be really careful (and wherever you read it from has to be even more careful!) The key point is that we liken the expansion of our universe to the SURFACE of the balloon, not the physical 3-dimensional balloon, only its two-dimensional surface (Don't think the surface is two-dimensional? Ask yourself how many coordinates you need to describe (uniquely) a point on the surface. The answer is of course two, which we usually use to be latitude and longitude). Now, as the physical balloon blows up, its surface expands in a manner similar to the expansion of our own universe -- points recede from each other with velocity proportional to their distance. Imagine putting yourself in the position of a dot on the surface of the balloon -- everyone else is running away! No point on the SURFACE can claim to be the center of the expansion. Furthermore, if we run the clock back and suck air out of the physical balloon, this is equivalent to contracting the universe, or running the clock back in time for our physical universe. Things get closer together. You can make the physical balloon as small as you like, and the points on the surface of the balloon get arbitrarily close together -- this is the big bang.

OK so hopefully the above discussion makes sense, now some caveats. Of course, our own universe is 3-dimensional, not 2, so the surface of a balloon cannot be a real analog of our expanding universe. Indeed, the mathematical object would be the 3-dimensional surface of a 4-dimensional sphere! Don't try too hard to picture this, it's likely impossible. The other big caveat is that in the balloon analogy, there still exists a PHYSICAL balloon, which has of course a center in the usual 3-dimensional space. The abstract jump you need to make is that the geometry embodied by the SURFACE of that balloon can exist independently of the physical object we know as the balloon. Specifically, if you imagine yourself living on the surface of the balloon, you can run experiments by drawing triangles and summing their angles. If your results show that the sum of the angles is always greater than 180 degrees, you have shown that you are living on a curved surface like the surface of the balloon -- But these properties (sum of angles > 180) are INTRINSIC too the world you're living in, you don't need to mention any larger 'balloon' that you're sitting on the surface of. This is the abstract step that trips a lot of people up, and it's quite a tough one. The take-away message here is that although when we visualize a 2-D surface of a balloon, we necessarily also picture the 3-D balloon object, this is only a limitation of our imagination as human beings, not a mathematical necessity. The conclusion, then, is that the universe, as a 3-D object, does NOT imply the existence of some 4-D object it is sitting on the surface of.

(Note: Everywhere I've been speaking strictly of the dimension of SPACE.)

5. Mar 20, 2012

### azizlwl

Thank you very much. The analogy is taken from Discovery Channel about the universe. Maybe too technical(mathematically) for me to grasp the idea. Another silly question. Since the space is expanding, does the scale time(i mean 1 sec. today will be longer in next billion years) or it remains constant.

6. Mar 20, 2012

### phinds

Time does not change as the universe expands. You are perhaps getting confused with the concept of time dilation, which you can find discussed extensively both on this forum and on the internet in general

7. Apr 1, 2012

### azizlwl

Today i watch Discovery Chanel. One professor says the universe might be infinitely big. So understandably there will be no center of the universe.

The other he says that the universe might be finite but when we see very, very far and finally we see the back of our head. Surely will have the same vision with everybody and nobody is at the center of the universe.

What rules/laws govern the expansion of universe into the new space(empty/before occupied by the universe).

8. Apr 1, 2012

### phinds

That is not correct. According to modern cosmology there WAS no space before the singularity. The universe did not expand INTO anything at all. The universe is all there is and there is nothing "outside" of it.

There ARE completely hypethetical and unproven (and possibly unprovable) theories that there WAS something before the singluarity but this is not standard cosmology, just fanciful thinking.

9. Apr 1, 2012

as long as I know 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.

10. May 6, 2012

The balloon analogy generates confusion in the following ways:

1. It expands into something else-the space surrounding it.
2. The dots expand as well

The first contradicts the idea that the universe isn't expanding into anything outside itself.

The second ignores the gravity holding galaxies and clusters and super clusters together.

11. May 21, 2012

### cristo

Staff Emeritus
Check out the FAQ's in the cosmology forum for answers to commonly asked questions: https://www.physicsforums.com/forumdisplay.php?f=206 [Broken]

Last edited by a moderator: May 6, 2017