What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand equally everywhere in all directions.
But they also raise some questions, like if you are on the surface of a balloon and travel in 1 direction, you will eventually return to your starting point, is it possible our universe have this feature?
If it has, or had, would this be a symmetry of sorts ($\psi(x)=\psi(x+R)$), and as such have a conserved quantity associated with it (by Noether)?
Assuming "small curled up dimensions" wouldn't these dimensions have this type of symmetry, what are the associated conserved quantities?
Is it known exactly what the geometrical shape of the universe is? (on a large scale) (I am not talking about only the observable universe).
How does one define the "size" of a dimension, is this scale only applicable to curled up ones?
Is it possible to describe to a layman the shape of the universe without resorting to inept analogies?
Answer
There are a bunch of questions here. Let me try to take them in order:
- Is it possible that our Universe has the feature that if you travel far enough you return to where you started?
Yes. The standard Big-Bang cosmological model is based on the idea that the Universe is homogeneous and isotropic. One sort of homogeneous spacetime has the geometry of a 3-sphere (like a regular sphere, but with one more dimension). In these cosmological models, if you travel far enough you get back to where you started.
However, the best available data seem to indicate that the Universe is very nearly spatially flat. This means that, if we do live in a 3-sphere Universe, the radius of the sphere is very large, and the distance you'd have to travel is much larger than the size of the observable Universe. Even if that weren't true, the fact that the Universe is expanding would make it hard or impossible to circumnavigate the Universe in practice: no matter how fast you went (short of the speed of light), you might never make it all the way around. Nonetheless, 3-sphere Universes, with the geometrical property you describe, are definitely viable cosmological models.
- Does this give rise to a symmetry by Noether's theorem?
Not really. Noether's theorem is generally applied to continuous symmetries (i.e., ones that can be applied infinitesimally), not discrete symmetries like this. The fact that space is homogeneous gives rise to a symmetry, namely momentum conservation, whether or not space has the 3-sphere geometry, but the symmetry you're talking about here doesn't give rise to anything extra.
- Would small curled up dimensions have the same sort of symmetry?
I'll leave this for someone else, I think. Not my thing.
- Is it known exactly what the geomtrical shape of the universe is?
No, and don't let anyone tell you otherwise! Sometimes, especially in pop-science writing, people imply that we know a lot more about the global properties of the Universe than we do. We often assume things like homogeneity to make our lives simpler, but in fact we have precisely no idea what things are like outside of our horizon volume.
- How to describe the "size" of a dimension?
If the Universe's geometry has enough symmetries, it makes sense to define an overall time coordinate everywhere. Then it makes sense to imagine a "slice" through spacetime that represents the Universe at an instant of time. If some of those slices have the geometrical property you're talking about, that traveling a distance R in a certain direction gets you back to your starting point, then it makes sense to call R the "size" of the corresponding dimension. If you can travel forever, then we say the size in that dimension is infinite.
- Is it possible to describe to a layman the shape of the universe without resorting to inept analogies?
All analogies are imperfect. I think the best you can do is use a bunch of them and try to convey the limitations of each.
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