The Shape of Spacetime.

Howde all.

With reference to matter originating from the big bang. Nothing else. No multiverse, no pbranes - nothing. Just the Big Bang.

Ok - given that galaxies are moving away from each other - then there is an overall outer edge shape created by these galaxies.

Lets say now that you can hold this "shape" in your hand. What does it look like?

a) A donut?
b) A soccer ball?
c) A rugby ball/american football?
d) Saturn?
e) A spiral galaxy?
f) Science does not know?

Cheers all.

Related Astronomy and Astrophysics News on Phys.org
I think the general consensus is that it's shape is similar to a black hole , hyperbolic. Geometry is dependent on which model universe we are speaking of.. there are open/close systems...

P.S: Open system ( hyperbolic) has the following condition : k<1 , on the other hand a closed system (k>1) would collapse back to a singularity over the passage of time (cyclic universe model).

Last edited:
Ok - but which of my options would it most look like at this point in time - today - if I were able to hold all the matter in my hand in the shape it had come to form.

First of all, this isn't the shape of matter in the universe. This is something topologically equivalent to the shape of space we're talking about. Also, these are just two-dimensional analogies for the shape of three-dimensional space.

AlbertE said:
Ok - given that galaxies are moving away from each other - then there is an overall outer edge shape created by these galaxies.
No.

But, again, let's assume you're wondering what the Universe is topologically equivalent to.

Science does not know?
That's pretty much it, though we have a good amount of evidence that the observable universe is flat.

A donut?
Assuming you're talking about a 3-Torus.

A soccer ball?
A rugby ball/american football?
The previous two are topologically equivalent.

Saturn?
Well, this would be two Universes, one topologically equivalent to a 3-sphere, one topologically equivalent to a ... something.

A spiral galaxy?
Too confused by what exactly you mean. There are a bunch of interpretations of what you said.

Hi Whovian.

Ok - today - right now - out there - there are galaxies.

If I wrapped the whole lot of them in a big cloth, that cloth would have a shape. Period.

Where am I going wrong :)

The only thing you're getting wrong is assuming you could wrap all of them in a cloth (of course, assuming you could produce a large enough cloth, it would stand up to the enormous amount of energy being blasted into bits of it by GRBs and so on, and you would have an achievable way to wrap up the galaxies, all without disrupting them, but this is a though experiment.) While there is a limit to the size of the Observable Universe (I can't remember it, it was something like 40-something billion Ly,) we have no idea if the Universe itself is infinite. If it is, there's obviously no way to wrap the infinite galaxies up in a cloth.

phinds
Gold Member
2019 Award
Ok - given that galaxies are moving away from each other - then there is an overall outer edge shape created by these galaxies.
That is an unsupportable assumption. It MAY be true but most likely is not. The universe is almost certainly either infinite or finite but unbounded and in either case there is no "shape" such as you are looking for.

I disagree.

Yup - its a thought experiment - so I have strong and large bits of cloth :)

Ok - thinking along here.....

We know the big bang occurred around 13.7? billion years ago.

If this is true - and I believe it to be the case, then unless it expanded at an infinite speed, it must be finite in size - this is not debateable and is absolutely set in stone.

As the above is not debateable, and we are happy with the timescales give or take - then there is no possibility of the matter being unable to form an outer shape.

Therefore there IS an outer shape which is formed by the fastest moving objects (as they are the outer objects).

LOL - still - where is this theory going wrong? I fear my convictions in simplicity are hindering my search!!!!!

George Jones
Staff Emeritus
Gold Member
We know the big bang occurred around 13.7? billion years ago.

If this is true - and I believe it to be the case, then unless it expanded at an infinite speed, it must be finite in size - this is not debateable and is absolutely set in stone.
No, this is wrong. There are many situations to which everyday physical intuition does not apply. The Big Bang was not a place in space from which all matter emerged.

phinds
Gold Member
2019 Award
unless it expanded at an infinite speed, it must be finite in size - this is not debateable and is absolutely set in stone.
No, as George said, this is totally wrong. It happened everywhere at once, not a some point. You should check out the FAQ sticky in the cosmology section, or try this:

www.phinds.com/balloonanalogy

Sorry - "what" happened everywhere at once? There was at the beginning of time, no everywhere!!!

Sorry - "what" happened everywhere at once? There was at the beginning of time, no everywhere!!!
The Big Bang happened everywhere at once. It did not start at a point and the Universe came into existence as the spherical explosion propagated outward. The Big Bang occurred at every point in space simultaneously.

George Jones
Staff Emeritus
Gold Member
This may not be completely accurate, but it might be better to think along the lines of
Very good stuff guys.
But it started from a singularity.Which is a single point .Hence a center.
No, a singularity is not necessarily a single point. A singularity is usually more like an edge. In the case of standard cosmological models, this "edge" is located in all directions.
Of course the singularity is nothing but the most Super Massive Black Hole there ever was.
No, the cosmological singularity is not a supermassive black hole.

"The Big Bang occurred at every point in space simultaneously."

How big was space when the big bang occurred?

George Jones
Staff Emeritus
Gold Member
"The Big Bang occurred at every point in space simultaneously."

How big was space when the big bang occurred?
Observationally, we can't pin it down, but but it is theoretically possible that space has always been infinite, and current observations are consistent with both finite and infinite space.

This would contradict the following statement - with which I was "brought up" as it were.

"Galaxies are flying apart, this means that at one time in the past, they were all together in one place".

Is the above statement no longer true?

This would contradict the following statement - with which I was "brought up" as it were.

"Galaxies are flying apart, this means that at one time in the past, they were all together in one place".

Is the above statement no longer true?
Check this out...
http://www.astro.ucla.edu/~wright/BBhistory.html

marcus
Gold Member
Dearly Missed
... given that galaxies are moving away from each other - then there is an overall outer edge shape created by these galaxies.
Albert if you want to gradually get to understand standard expansion cosmology I susect you first need to get rid of the highlighted idea. I don't want to say it's WRONG (today's models are not final and can always be revised, they are just the most reliable accurate picture we've been able to construct so far) but it is a misconception from the standpoint of modern cosmology. It has nothing to do with the way working cosmologists think.

Current models they use to fit observational data to are not that of an explosion outward from some point in empty space.

... which of my options would it most look like at this point in time - today - if I were able to hold all the matter in my hand in the shape it had come to form.
Since matter is not supposed to be traveling outwards into empty space, away from some central point, there would not be any "overall edge shape" that it had "come to form".

So you are asking about something that is simply not part of our conception of the universe.
The question is based on a false premise and does not make sense.
====================

There are ANALOGIES that people use to illustrate expansion with one-dimensional or two-dimensional toy models. They can be very helpful but analogies tend to be imperfect and require care.

One analogy is a CIRCULAR RING with no surrounding space. All existence concentrated in this infinitely thin ring. Galaxies and stars are one-dimensional, little dots and dashes specklend along this ring.

As the ring expands it describes a flaring cone-shape, or a bell shape. The ring represents SPACE and the bell which it describes as it expands is SPACETIME, in this toy analog picture.

that particular analogy doesn't appeal to me personally, but you see picturesque versions of it around and about. Some NASA outreach documents use it as a kind of impressionistic illustration.

Another analogy is where today's space and the galaxies in it are infinitely thin two-dimensional, like all existence (all space all matter) concentrated in the surface of a spherical balloon.

In cosmology we do not assume there is any "space around space" or any "boundary" or border to space, or any "central point" from which things spread out. Therefore to get the good from this analogy we must imagine no inside or outside of the balloon. There is no surrounding 3D space, and therefore of course no center. Only the pure infinitely thin surface of the balloon exists. The mental concentration involved in thinking the balloon analogy can take some time to get used to.
=============

Those are just lower dimensional ("infinitely thin") analogies, that may or may not help you.
The goal is to be able to think of edgeless boundaryless THREE-dimensional space, with no "space outside of space"---i.e. all existence concentrated in this full-bodied 3D space that we experience.

And no center from which it is expanding. And since there is no "outside" it can have no shape as seen from the outside. No person looking from the outside, or "holding it in my hand" as you said.

We experience the expansion and the curvature of this 3D space INTERNALLY, by witnessing large triangles that do not add up to 180 degrees (as they would in a zero curvature space).
We do not stand outside to view the curvature, it is something experienced by creatures within the space. Likewise expansion. We witness it in several ways, not only in the enlargement of wavelengths of light and the cooling of background temperature but also in the curious fact that beyond a certain distance objects actually look LARGER (take up wider angle in the sky) the farther away.

this strange beautiful optical effect of expansion is something to understand. It is as if the ancient sky was smaller and so objects of a given size (e.g. compression waves in a cloud of gas) took up a wider sector of the sky. and so they look bigger than more recent objects the same size would look.

I'm telling you to think of "shape" that is geometry as something experienced from the inside, from within the space that realizes that geometry, that curvature, that expansion.
Geometry and the change it undergoes are not something to visualize from the outside, because there is no outside.

(according to the normal cosmic model that folks use, and fit their observational data to.)

(of course as I think you know there are those more speculative models in which there is a higher-dimensional outside surrounding our space but they aren't needed to fit data to and aren't used in normal everyday cosmology.)

marcus
Gold Member
Dearly Missed
This would contradict the following statement - with which I was "brought up" as it were.

"Galaxies are flying apart, this means that at one time in the past, they were all together in one place".

Is the above statement no longer true?
That never was the idea. "flying apart" is a kind of poetical half-truth.
"one place" is a highly misleading expression since it suggests a point sitting in empty space.
That was certainly never the idea even if you go back to the beginnings of expansion cosmology in the 1920s.

Those words are more appropriate to irresponsible JOURNALISM than to an honest description of the usual expansion model cosmos.

"The goal is to be able to think of edgeless boundaryless THREE-dimensional space, with no "space outside of space"---i.e. all existence concentrated in this full-bodied 3D space that we experience."

I need pictures. :(

I need pictures - DEFINATELY :)

I need pictures - DEFINATELY :)
Not to sound rude but why are you so keen into visualizing things , I agree Einstein's mind worked that way however , he did extensive study . Marcus has given you an excellent detail even beyond what your typical first year UG books cover.

marcus
Gold Member
Dearly Missed
...Marcus has given you an excellent detail even beyond what your typical first year UG books cover.
Thanks for the positive evaluation, Ibysaiyan! A good comment like that now and then makes the effort to write about cosmology worthwhile.

Lets suppose for a moment the big bang DID begin at some point in space....like from a stick of dynamite...a discussion by Alan Guth [originator of Inflation Theory, this from THE INFLATIONARY UNIVERSE], 1997]:

..light is emitted and matter strewn outward. After a short time light would have moved beyond the matter we ride on....the light [radiation] would disappear, so why can we see it [all around us]??
He is referring here to that fact the cosmic micowave background radiation [CMBR]is all around us and very uniform in all directions.

he goes on to explain: ..The Friedman Robertson walker [FRW] cosmological model [which closely matches our astronomical observations] was constructed to be homogeneous and isotropic.

....This means matter is assumed to have uniformly filled all of space at all times, right back to the instant of the big bang. There is no edge and no center to the distribution of matter. Since matter fills all of space, it is impossible for the radiation to leave the matter filled region as it would for a stick of dynamite.
Why believe this model?
...because a localized explosion cannot explain the observed uniformity in the CMBR. Either we would have to be at the exact spot the explosion happened or else we would observe a local hotspot
from the heavens around us. And the extreme uniformity of the CMBR, about 1 part in 1000,000 variation, means the background was extremely uniform as modeled, and is a virtually perfect black body emitter as expected. Things must have been very uncomplicated, very uniform, when this ancient radiation was emitted....

marcus
Gold Member
Dearly Missed
"The goal is to be able to think of edgeless boundaryless THREE-dimensional space, with no "space outside of space"---i.e. all existence concentrated in this full-bodied 3D space that we experience."

I need pictures. :(
Yes you need pictures, we all use mental diagrams, sketches, which illustrate useful analogies. But you also need PATIENCE. Feed your brain honest concepts little by little and give yourself time for it to soak in.

The balloon model of expanding geometry is only a toy analog (in reduced dimensionality, no thickness) but it might be good for you to watch the actual movie.