# Shape the Universe.

Rainbow
I would just like to ask a very elegant question.
What is the shape of the universe?

Nexus555
I believe most cosmologist think it's spatially flat. Also the universe could as well be spherical shaped.

Gold Member
What is the shape of the universe?

Heart shaped!

But see Shape of the Universe

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

Nexus555 is correct, the present opinion within the community is the WMAP data is consistent with a flat universe, although there is a statistical preference for a slight over-closure-density, which would make it just spherical.

Garth

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Staff Emeritus
Gold Member
I would just like to ask a very elegant question.
What is the shape of the universe?

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.

Rainbow
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. Then, after the big bang, the matter and energy(or for that matter the universe) would have spread out symmetrical about every axis through the point of big bang singularity, thus giving the universe the shape of a sphere, which is now expanding.

Gold Member
Not a normal explosion

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...

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.

ray b
round
unless some other force limits a given direction
all directions should be equal
so it should be round unless something is changeing it!

Gold Member
round
unless some other force limits a given direction
all directions should be equal
so it should be round unless something is changeing it!
And that would be "round" in how many dimensions? Can you still call it a sphere if it has 11 dimensions?
And there's no rule that says the scale of those dimensions are the same.

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Gold Member
round
unless some other force limits a given direction
all directions should be equal
so it should be round unless something is changing it!

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

Depends which dimension you are looking in.

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

Gold Member
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. Then, after the big bang, the matter and energy(or for that matter the universe) would have spread out symmetrical about every axis through the point of big bang singularity, thus giving the universe the shape of a sphere, which is now expanding.
That only works if you assume the BB occurred 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.

Staff Emeritus
"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the drug store, but that's just peanuts to space."

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

Magister
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. Then, after the big bang, the matter and energy(or for that matter the universe) would have spread out symmetrical about every axis through the point of big bang singularity, thus giving the universe the shape of a sphere, which is now expanding.

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.

Staff Emeritus
Gold Member
Actually, it is space that is flat. In other words, ordinary Euclidean geometry should work in the universe, even on horizon scales. Spacetime is necessarily curved in general relativistic big bang models.

Magister
Actually, it is space that is flat. In other words, ordinary Euclidean geometry should work in the universe, even on horizon scales. Spacetime is necessarily curved in general relativistic big bang models.

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.

Staff Emeritus
Gold Member
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.

Quite right. It can be confusing, since our usual notion of flat is in two dimensions.

vld
It looks like the question was about the SHAPE of the universe but everybody here is discussing its GEOMETRY. The flat geometry can be attributed to different shapes. For example, a sphere of infinite radius has a flat geometry, but still it is a sphere (inlike a torus or a tea pot)

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

vld
I thought that geometry and topology are concerned with different questions. Of course, the geometry of the universe might be Euclidean or non-Euclidean but this does not liberate us from the question about its shape.

Gold Member
Dearly Missed
It looks like the question was about the SHAPE of the universe but everybody here is discussing its GEOMETRY. The flat geometry can be attributed to different shapes. For example, a sphere of infinite radius has a flat geometry, but still it is a sphere (inlike a torus or a tea pot)

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

I thought that geometry and topology are concerned with different questions. Of course, the geometry of the universe might be Euclidean or non-Euclidean but this does not liberate us from the question about its shape.

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).

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

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vld
In my whole life I never heard of a "sphere of infinite radius"

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?

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

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.

JEMZ000
I would just like to ask a very elegant question.
What is the shape of the universe?

It depends on what we can see blueshifts or no shifts
If we find blueshifts it is sphereical
If we find no shifts it is flat if we find one of those it is hyperbolic

John232
Most books will tell you that the true shape of the universe as a whole is an unkown. It would depend on the amount of matter in the universe. If there is enough matter to stop the expansion of space then the universe would be round. But, if there isn't enough matter in the universe and it keeps expanding then the universe as a whole would be saddle shaped. The thing is that the amount of matter in the universe is really close to about the same amount that would be inbetween the two. Then if the universe just expanded to a certain point and just stopped it would be flat. Its unkown if there is some connection that makes the universe stay close to the amount that would make it appear flat.

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.

Keep in mind that matter/energy determines the geometry of the universe, not its topology. Even a spatially flat universe can be of nontrivial topology, i.e. toroidal or planar. Einstein's Equations do not address the global topology of the universe.

AshsZ
As far as I understand, we know what we see.

When we look at the universe, we don't see the space. We see the objects. We see the distance between objects, but we don't 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?

Pesj
round
unless some other force limits a given direction
all directions should be equal
so it should be round unless something is changeing it!

I think that this point of view seems reasonable. As far as we know all directions are equal. This assumption makes it logical for me to think of the universe as a "surface" shaped like a 3-sphere. Perhaps that´s not what you mean by round but unless the universe or BB create a closed space you end up with difficult to explain edges of some sort.

Without an external point of reference, shape is a meaningless concept.

Chronos, I´ve come across this assertion quite a few times, and I don´t understand what it means. Could you please explain why this is? Wouldn´t the shape be meaningful to what you experience inside the universe? That is some observations can be different depending on what shape it has.

nap
The reality is no one absolutely knows here... just theories and guesses.

Gold Member
The reality is no one absolutely knows here... just theories and guesses.
That is science; nothing new to anyone here. We manage to plod on.

thinker5
the universe has the shape of thougt

Gold Member
the universe has the shape of thougt

A hypothesis that falls apart when one asks what shape the universe was before thought evolved.