Why does it make sense to say the universe has no centre?

If we suppose we and everything in the universe is perfectly flat. That is, we have width and depth, but no height. And we are situated on the surface of a balloon blowing up with air and expanding "outwards". Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point. In other words, the surface of a balloon (the "universe") has no centre. Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?

• There's simply no evidence of any higher dimensions or intrinsic curvature. The evidence from WMAP and elsewhere suggests that the universe is flat. We have no evidence whatsoever to support the "ant on a balloon" analogy. Whilst the idea of a closed universe isn't stupid, what is, is people telling you about it as if it's scientific fact, and "explaining" it in terms of something that isn't explained. As if the balloon makes everything OK. As if we really are 2D creatures on a 3D surface. Only last time I looked, we're not. – John Duffield Dec 23 '15 at 9:32
• The balloon analogy appears three dimensional, but it's really two dimensional with all things having width and depth, but absolutely no height. Now all the points on the surface of this balloon are equidistant from a "centre" that doesn't exist except in a mathematical way, therefore one point is no more central than any other. Suppose I stand at the south pole, in our analogy no matter which way I point, and no matter how big the world is, I'll always be pointing north. Isn't this why we receive low frequency background radiation from the big bang in all directions? – Michael Lee Dec 23 '15 at 12:36
• The thing is Mike, is that the Earth isn't flat. It's three-dimensional. And as far as we can tell, so is everything else, including this universe. The lower-dimensional balloon analogy is nice to think about, but it has no foundation in fact. We receive the CMBR from all directions because it was being emitted from all locations in all directions, and circa 380,000 years after the Big Bang the universe became transparent to such radiation. – John Duffield Dec 23 '15 at 15:55
• Fair enough John, thank you for popping my balloon. lol – Michael Lee Dec 23 '15 at 17:00
• Perhaps we are within the skin of the ballon or bubble, which is itself expanding. The center of this universe bubble would be in an unaccessible point inside it. Since we and everything even spacetime no longer occupies the center, it wouldn't exist yet somehow still be there. – Armondo Villaescuza Jul 18 '20 at 21:24

The answer to this question lies in cosmology.

It's because of relativity in positions. Every galaxy appears to be moving away from us and the universe we see is only the observable universe. Further points are more in the distant past, when you observe the deep field you observe the past. This may seem irrelevant to your question, but the idea of a "center" to a universe where any position in the universe, be it from our galaxy or from a galaxy in the Sloan Great Wall is the true center is not consistent with observation. That's because galaxies/clusters of galaxies are moving away from each other, relative to each other.

Every galaxy is slowly being pulled apart from other galaxies, save for those in gravitationally bounded clusters. (The whole architecture of the universe, with massive voids of absolute nothing and bunched up clusters of galaxies reflects this quite a bit) If the universe had a central point, this would be easily observed because there would be variation in the color shift of galaxies that could be analyzed to deduce a central point where the galaxies are moving away from.

That is not what is observed. Instead, pretty much most galaxies (save for exceptions like Andromeda or the Triangulum, who prove the rule since those galaxies are gravitationally connected to our galaxy via the local group) are moving away from us. The Great attractor being not a path to the center of the universe, but the center of the recently discovered Laniakea super cluster. So if there's attraction between galaxies, it's because of gravitational interactions. The universe is also without any real edge unless you count the edge of the observable universe. The observable universe being more a historical record than what the contemporary universe around us looks like and is a byproduct of light speed being so limited. The speed of light in a vacuum is also the speed of information. The whole universe is far more massive by comparison.

So to speak about a center of the universe would require the following:

1. A region of space that galaxy is moving away from or towards. (Which does not exist, instead relative trajectories and gravitational clustering is all regional at most.)

2. Edges that are defined enough to pinpoint a center (If such edges exist, they are impossible to observe due to the limited speed information travels in the universe)

3. Absolute positioning to pinpoint where this center would be. (All positions are relative, the galaxy, the sun and our planet all are moving at extremely fast velocities we don't notice thanks to relativity.)

The centre of something ( anything ) has to be defined in some way by the properties of the something and a coordinate system.

So you can in principle define a centre to the universe, but it only has a meaning in the context of the chosen definition.

You could easily define a different measure that produces a different centre.

And you can define a centre even on the surface of your balloon, simply by arbitrary selection.

An edge to the universe requires a context. How do we define it ? How do we detect it ? The only meaningful measure of this ( in a human sense ) that I know of is "how far can I see ?". That keeps changing.

So at the moment I would say "centre is arbitrary" and "edge is unknown".

Why does it make sense to say the universe has no centre?

We can't be sure it does.

If we suppose we and everything in the universe is perfectly flat. That is, we have width and depth, but no height. And we are situated on the surface of a balloon blowing up with air and expanding "outwards". Us flat people can move around on the surface of our balloon but not through it, in much the same way we navigate around the earth, in a plane say. No matter where something is situated on the surface of the balloon, one point is no more central than any other point. In other words, the surface of a balloon (the "universe") has no centre.

This analogy is often used to justify that the claim that the universe has no centre. But there's no evidence of any higher dimensions. We simply don't know that there's any kind of "asteroids" curvature such that if you keep going one way you come back the other.

Is this why we say it makes no sense to say the universe has a centre, or edge for that matter, or is this analogy stupid?

I wouldn't say it's stupid. But I would say it's presenting hypothesis as some kind of fact. For all we know our universe does have a centre and an edge.

• "But there's no evidence of any higher dimensions." -- this seems like a non sequitur, or else I don't understand what you're trying to say. A lack of any higher dimensions does not affect the analogy (indeed, most acknowledge its limitations in this regard), nor does it indicate anything one way or the other about whether a hypothetical spatially finite but center-less universe is right or wrong. – Stan Liou Dec 23 '15 at 4:19
• The balloon analogy is not used to justify the claim of no centre, it is used to illustrate a space/manifold which is finite and yet has no centre, that is to show it is possible. Many popular models of space time are not finite but still have no "centre". Also you do not need extra dimensions to have even a closed space time with no centre, the closure and curvature are intrinsic properties of the space/manifold and do not refer to anything outside the manifold. – Conrad Turner Dec 23 '15 at 8:54
• @Stan Liou : it's not a non-sequitur, it's simple fact. As far as we know the universe is flat. Planck didn't find an evidence of any toroidal topology, see arxiv.org/abs/1303.5086 , and there's no actual evidence that the universe is infinite. So it could be finite, like the raisin-cake, with a centre. For all we know some guy 46 billion light years away could be looking up wondering why half the sky is black. Conrad: with no evidence of higher dimensions or intrinsic curvature, the balloon analogy just isn't science, it's speculation. – John Duffield Dec 23 '15 at 9:15
• @Stan Liou : I think they're very much connected. It goes like this: in olden times people could not conceive of a world that didn't have an edge. In modern times people cannot conceive of a world that does. So they invent fairy tales about ants and balloons which just don't stand up to scrutiny. And yet these tales are peddled as if they're scientific fact. IMHO the scientific thing to say is we don't know. – John Duffield Dec 23 '15 at 15:41
• @JohnDuffield: That doesn't have anything to do with science, though. There are many possible spatial geometries, both finite and infinite--we don't really know which; it doesn't have anything to do with this balloon thing, but rather mathematics of GTR. Even under assumptions of homogeneity and isotropy, there's two finite and two infinite geometries (none of which are a torus). And without those assumptions, the number of possibilities explodes. A finite boundaryless universe is just one class of possibilities among many; it's not fact, but no competent cosmologists claim that it is fact. – Stan Liou Dec 23 '15 at 19:14

My opinion and 20 Cents:

Humanity did confused many things in history.

Back to the days of the holy bonfires on the squares of Europe, nobody didn't understood that the Earth - is not a center of universe.

We are still believing that speed of light is a maximum speed of information, while there is discovered quantum teleportation with higher speed, infinity speed. Infinity looks much better than 299 792 458 meters per second, is not?

Moreover, we can not describe why does photon getting its super-speed without any boost, immediately, as a part of its existence.

That is why, I'm thinking that we are confused with this great theory about center of universe, which is everywhere, there and here, anywhere...

It is too beauty and too hard to understand, that is why everybody simply believing in it, without tries to imagine anything else.

You could read about Cosmic microwave background. There is a redshift and Milky-Way is moving to the one side of this big-bang sphere.

There is a sphere. There is a redshift. We are inside.

Do you still believe that center of universe is "everywhere"?