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## Main Question or Discussion Point

I would just like to ask a very elegant question.

What is the shape of the universe?

What is the shape of the universe?

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I would just like to ask a very elegant question.

What is the shape of the universe?

What is the shape of the universe?

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Garth

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Heart shaped!What is the shape of the universe?

But see Shape of the Universe

(The present Wilipedia article to which I have given a permanent link looks reasonable enough)

Garth

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SpaceTiger

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The basic answer is that nobody knows. The universe appears to be locally flat, but on scales much larger than the horizon size, it could have any of a large number of shapes.I would just like to ask a very elegant question.

What is the shape of the universe?

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As any good cosmology book, even popular ones, will tell you: the big bang cannot be similar to a normal explosion! So forget that notion, it will only lead you further from understanding. Rather study what the current accumulated wisdom says.What I think is that, the universe started with a big bang right? Now, let us take the big bang to be similar to a normal explosion...

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unless some other force limits a given direction

all directions should be equal

so it should be round unless something is changeing it!

DaveC426913

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And that would be "round" in how many dimensions? Can you still call it a sphere if it has 11 dimensions?

unless some other force limits a given direction

all directions should be equal

so it should be round unless something is changeing it!

And there's no rule that says the scale of those dimensions are the same.

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You may perhaps call our observable universe "approximately round", but it is only speculation as to what lurks far beyond that observable horizon!round

unless some other force limits a given direction

all directions should be equal

so it should be round unless something is changing it!

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3 dimensions: Sphere

4 dimensions of movement : (Feels more like a flat plane)

Chronos

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That only works if you assume the BB occured inside some sort of preexisting space. According to the mainstream model, the BB created not just energy and mass, it also created all the spacetime dimensions as well. Without an external point of reference, shape is a meaningless concept.

Astronuc

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from The Hitchhiker's Guide to the Galaxy, Douglas Adams

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What is flat is not the space but the space-time!

Space-time can be seem as a sphere with positive or negative curvature and flat, just to speak of the most popular space-time geometries.

SpaceTiger

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Right! I just wanted to point out the meaning of a flat universe. Flat doesn t mean that the shape of the universe is a flat disc.

SpaceTiger

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Quite right. It can be confusing, since our usual notion of flat is in two dimensions.Right! I just wanted to point out the meaning of a flat universe. Flat doesn t mean that the shape of the universe is a flat disc.

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DaveC426913

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This is still geometry - it's just not **Euclidean** geometry.

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marcus

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I'm a retired mathematician. In my whole life I never heard of a "sphere of infinite radius"

You are using words in a confusing way. There is no such thing as a sphere of infinite radius---such a thing would not be a sphere

You can define a limit as R -> infty, but the limit is not a sphere.

I see no clear distinction between discussing the geometry of space and discussing the shape.

In NEITHER case would one be merely discussing the topology. If you simply want to talk about the topology of space, then please don't call it the "shape".

that is another confusing use of words and will just muddy the discussion

You seem to be slinging words around without a clear understanding. When one asks about the geometry of space one is not asking "Is it Euclidean or non-Euclidean?"

there are a vast range of different geometries, each defined by a metric (a distance function).

when one asks about the geometry (which is the same as asking about the shape) one is asking about the METRIC.

It is not as if there were just two geometries "Euclidean and non-Euclidean"

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It is quite unusual to see a mathematician denying a special case of a mathematical entity to belong to that entity. A plane, as well as a point, are two special cases of sphere. Are you denying this?In my whole life I never heard of a "sphere of infinite radius"

You are right that geometry is identified in cosmology with metric. But shape isn't. In terms of cosmology shape is equivalent to topology. Then, a cube have essetially the same shape as a sphere but a different metric. Locally we cannot say, which shape we are looking at. That is why it makes much more sense speaking about geometry (metric) when discussing the local properties of space and about shape (topology) when discussing its global properties (as in cosmology). Anyway, locally the geometry of the universe is always Minkovskian.I see no clear distinction between discussing the geometry of space and discussing the shape.

... when one asks about the geometry (which is the same as asking about the shape) one is asking about the METRIC.

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It depends on what we can see blueshifts or no shiftsI would just like to ask a very elegant question.

What is the shape of the universe?

If we find blueshifts it is sphereical

If we find no shifts it is flat if we find one of those it is hyperbolic

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One thing I have wondered is how would we be able to tell if the universe was not contracting on the other side of it. Then wouldn't it only appear that everything was expanding away from us even though it was contracting in on the opposite side if the universe was round? So, I don't think we would ever really be able to know for sure.

bapowell

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When we look at the universe, we dont see the space. We see the objects. We see the distance between objects, but we dont see the space between them. When one speaks to the shape of the universe, they can only conclude based on the objects that are visible, right?

As far as I understand, what we can see at the furthest distances in any direction implies that objects are placed in a spherical shape around us. That is all we know for sure. What is beyond them is purely speculation, right?