What is the shape of Universe?

What we can assume by the shape of universe. As I know when we talk about universe we talk about multi dimension structure but that structure should have some shape. I wonder that if we look the Universe from a far distance, how it will look like?

I know people will tell that its infinite we can't go outside universe and see, but assume if its true that universe has some end and its too far and out of our reach due to our limitation but exists.

I feel that its a kind of myth or assumption that universe is infinite everything is finite in this universe.

Do you remember in early times around 1000 years before humans used to think that earth is flat and infinite?

But after many years we reach to conclusion that earth is round.

Don't you think we are misunderstanding universe the same way we did before for Earth?

• What it currently looks, is a planar (curvature zero), infinite 3D plane. This is how does it look, but note: no one can be sure from anything out of our vision ($\approx$ 40billion light years). Aug 28, 2020 at 15:51

When we talk about the "shape" of the universe. It is its intrinsic shape, not the embedding of the universe within a larger space.

It does make sense to talk about the intrinsic shape. Properties like "curvature" can be measured from inside a shape, you don't need suppose that the universe is "in" something to talk about its shape.

Now the space of the universe is three dimensional, this creates visualisation problems. So I'm going to first talk about an imaginary 2d universe:

A 2d universe could be flat, with a curved boundary (like a disc) Or it could be flat with no boundary (an flat infinte plane). Or a 2d universe could be intrinsically curved. If it has positive curvature it could be like the surface of a ball (positive curvature, finite with no boundary) Or shaped like a bowl (positive curvature) Alternatively it could have negative curvature: it could be shaped like a "Pringles crisp" These have negative curvature (this can happen with or without a boundary). Finally there could be regions with different curvature: positive in some parts, negative in others.

The same possibilities exist for our universe: It could be flat (with or without boundary) It could be positively curved (This is intrinsic curvature and your brain can't visualise this for a 3d shape) Or it could be negatively curved (again, don't try to visualise this for a 3d shape, you can't). In the case of positive curvature, it could be finite and unbounded, but for negative curvature, it must either have an edge, or be infinite.

So mathematically these are the possibilities. What does the science say?

We can measure the curvature on large scales. Our measurements are not perfect, there is some room for error. We measure the large scale mass/energy density of the universe, since curvature is caused by gravity, and gravity is caused by mass and energy. If the mass/energy density is > 1 then space would be positively curved. We actually find that the energy density is 1.00±0.02 That is the universe is either flat, or very nearly flat.

No there are profound problems with supposing that the universe has a boundary. But there are also problems with supposing it to be infinite. Scientifically, no edge has ever been detected. So when we build models of the universe, we will generally suppose to have no edge.

Thus our "best guess" is that the universe is uncurved and infinite.

• congratulations on such a nice and educative answer on a question I deemd quite strange :) Aug 27, 2020 at 6:59
• @James K If I assume it be both like postive curvature and negative curvature , A model -> kind of a hollow sphere, assume yourself inside the big hollow sphere having no physical boundaries. M I correct with my assumptions or not? Aug 28, 2020 at 10:02
• If you are going to "assume", then assume it to be uncurved (on the large scale, there are little wrinkles caused my local mass concentrations like stars) and infinite, having no boundaries. If this is hateful to you, then you might choose to believe in a positively curved three-dimensional space that is analogous to the two-dimensional surface of a sphere. This is boundariless and finite. There is no evidence for this shape, but it is still consistent with the observed universe. Aug 28, 2020 at 20:45
• This is a beautifully simplified answer. However, as James hints at in his comment, if we assume the universe is both homogeneous and isotopic at the largest scale, it would be almost impossible to observe any difference between a flat infinite universe and a closed finite universe with an extremely small curvature (ie a universe many orders of magnitude larger than the observable universe). @kriti, the latter quite untestable option might give you some comfort. Aug 29, 2020 at 3:42