# Where is the edge of the universe

welcome to the forum.

Your example while good on the 4 dimensions, lacks further explanation in regards to a finite, flat geometry. Take that piece of paper you just described and fold it into a Klein bottle. Now it has no edge.

http://www.math.osu.edu/~fiedorowicz.1/math655/Klein2.html

there are other possible flat shapes that do not have an edge.

torus
Moebius strip. are two other examples.

images and write up on universe geometry is discussed here.

http://abyss.uoregon.edu/~js/cosmo/lectures/lec15.html

as far as the universe being more than 4 dimensions is a string theory view point. Unfortunately string theory does not have a lot of successes. Its still a relatively new theory though.

phinds
Gold Member
2019 Award
Mordred, I believe the Moebius strip DOES have an edge, it just has a single surface so you don't to cross the edge to get to the other side as you would on an unfolded piece of paper which has two sides

Mordred, I believe the Moebius strip DOES have an edge, it just has a single surface so you don't to cross the edge to get to the other side as you would on an unfolded piece of paper which has two sides
It depends on how you define the Mobius strip. If you define it as a compact surface, then it will indeed have an edge (=manifold with boundary). But if you remove the edge, then you have some kind of Mobius strip which does not have an edge. If you want to work with manifolds without boundary, then you don't want this edge.

phinds
Gold Member
2019 Award
I am getting the impression that people dont believe time is real.
How can a universe expanding out from a singularity become anything other than at least roughly, spherical in shape, as far as I can see any other shape would require wierd starting conditions.

I am getting the impression that people dont believe time is real.
How can a universe expanding out from a singularity become anything other than at least roughly, spherical in shape, as far as I can see any other shape would require wierd starting conditions.
From images it looks like observable Universe is far from expected 3D sphere, it looks more like 2D ellipse.

So, why is it flat?

mfb
Mentor
What you see is a projection of the whole sky on a surface (paper, or a computer screen). This does not work perfectly, and the problem is similar to the issue how to make a map of earth: you need a globe to do it right. An ellipse is a way to graph the data without too much deformation.
It is not a 3D-map.

How can a universe expanding out from a singularity become anything other than at least roughly, spherical in shape, as far as I can see any other shape would require wierd starting conditions.
I don't think a torus or an infinite universe would be weird.

phinds
Gold Member
2019 Award
I am getting the impression that people dont believe time is real.
.
HUH? Why do you say that?

when we say the universe is flat, curved etc we are talking about a relation between the ratio of the universes energy-mass density $\rho$ compared to the hubble constant

http://www.ams.org/notices/199811/cornish.pdf

Chronos
Gold Member
The CMB is depicted using Mollweide projection, which is a way of mapping a spherical surface onto a flat sheet of paper. The resulting image is egg shaped.

Ryan_m_b
Staff Emeritus
How can a universe expanding out from a singularity...
AFAIK this notion is erroneous, it implies that the Big Bang was an explosion in pre-existing space. If this was so we would be able to identify a centre but no matter where you are in the universe if you try to do this you end up measuring yourself as the centre. This is because the Big Bang happened everywhere at once as strange as that sounds. Perhaps someone more knowledgeable can explain this better.

Chronos
Gold Member
Ever tried to make a mobius tube? You get a torsion factor that is hugely complicated to model.

What you see is a projection of the whole sky on a surface (paper, or a computer screen). This does not work perfectly, and the problem is similar to the issue how to make a map of earth: you need a globe to do it right.
The CMB is depicted using Mollweide projection, which is a way of mapping a spherical surface onto a flat sheet of paper. The resulting image is egg shaped.
So, our Universe (at least observable one) is a nicely formed sphere (it's not flat and it's not ellipse)?

... no matter where you are in the universe if you try to do this you end up measuring yourself as the centre. This is because the Big Bang happened everywhere at once as strange as that sounds.
I can 'visually' explain this to myself only if I 'imagine' more dimensions than those we are able to perceive, and that Big Bang and expansion of Universe has to do with changes within dimensions (their 'relation'; maybe Big Bang was an event where other dimensions curved into themselves and became very tiny, what is what String Theory is saying I guess, possible?).

Perhaps someone more knowledgeable can explain this better.
Yes, please. The more explanations the better.

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The "edge"of the universe could well be in the very fundamentals of the nature itself(Which we don't comprehend yet)I believe no one is able to answer this question ; "yet"!

I found this interesting:

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mfb
Mentor
The observable universe is so flat that experiments could not observe a curvature yet. If the universe is curved, then the scale of that is really large.

That is unrelated to the graph of the CMB, however. Independent of the curvature, you can look in all directions in the sky.

The observable universe is so flat that experiments could not observe a curvature yet. If the universe is curved, then the scale of that is really large.
OK thanks, I think I get it a bit better now. Though, if most consider that whole (not just observable) Universe is infinite then there is nothing strange if the scale of curvature of observable Universe is really large, right?

phinds
Gold Member
2019 Award
OK thanks, I think I get it a bit better now. Though, if most consider that whole (not just observable) Universe is infinite then there is nothing strange if the scale of curvature of observable Universe is really large, right?
If the whole universe is in fact infinite, then it is almost certainly flat and the fact that we measure the observable universe as almost flat and only within the tolerance of our ability to measure it will just turn out to be a lack of accurate measurement.

In regards to that last question. You have it right. However the universe can be so big that even a finite universe would look flat in the observable portion.
If the South pole data on the curvature proved accurate. They calculated that a photon would take 880 billion years to cirmumvent the universe if and only if expansion were to stop.
At current expansion rates that photon may never be able to transverse the universe.

Keep in mind the above applies only if the South pole data is correct on the universe being finite.

I'm currently taking an Astronomy class (I'm a high school senior so it's not a very in-depth class) and this question comes up a lot. After thinking about the "edge of the universe" for a while, I came to a conclusion and "answer" that makes logical sense to me (although my teacher seems confused when I try to explain it to him). I personally don't understand the confusion but I will share my view on the "edge" problem... Again I'm no Physicist so please don't assume that I'm giving (or trying to give) the "correct" answer. The "correct answer" I feel could never really be found (at least not for many lifetimes) due to the distance and speed one would have to travel to find (or not find) an "edge". Anywho, here it goes;

Imagine a central point in the middle of a piece of paper. The piece of paper representing all of spacetime and this "central point" will be the "singularity" that is the big bang before expansion. At the point of expansion (the expansion of the matter and energy inside of the singularity), the matter and energy will begin to move across spacetime (time starts here) and keep moving forever or until the expansion rate slows enough for gravity to bring it all back in on itself. Now, you draw a circle (doesn't necessarily matter how large or small because this is purely a visual to make sense of it) around the "central point". This circle represents the current expansion in age and in space (how far the expansion has moved across spacetime). Outside of that circle is the future and therefore we can only reach it when the expansion has reached it. You cannot "go over" that "edge" unless you go into the future. I personally think that the Science-Fiction idea of "jumping into the future (or the past)" is silly, but that is debatable and isn't the point of my example so I digress. As far as the present goes, what is on the other side of the "edge" (besides the future) doesn't matter because you will never reach it outside of waiting until the future is here. Unless you could go faster than the expansion rate and faster than light, but that just isn't possible... as far as we know any who, but I like to think that Einstein was correct. If you try to vision an "edge" that is purely physical (does not include time in the "spacetime"), you will find yourself confused and frustrated trying to understand it. This is because the other side of that edge must be "nothing" otherwise it would be included in the "universe", and "nothing" doesn't make sense to current-human logic. For this reason, I find that one should include the "time" in "spacetime" to make more sense of it. Of course, there are multiple ways to think about the "edge" problem so, just accept the one that makes the most sense to you (even if someone claims that it "makes no sense" or is "wrong").

If I did not make myself clear, feel free to ask questions. Again, this is just my thought on the whole shibang so don't quote me as claiming to have the perfect answer or any of that jazz... I am not a Physicist, and I like to think that I'm not that arrogant.

mfb
Mentor
but I like to think that Einstein was correct.
I don't think your model is possible in General Relativity.

Just throwing this out there, but is it not logical to think that the big bang would have projected matter out in all directions? Creating a 360 degree universe? Also is it not logical to think that the universe is a sphere but it's just so so big that even with our most poweful telescopes we can not see the horizon... Because the universe is expanding faster than the speed of light, light can not make a full circle and come back to us.

If the universe was not expanding, eventually light would complete a full circle. I think the curve is just so small and the universe is expanding faster than the speed of light so we can never see the horizon. It's just getting further away with every passing second.

I think the curve is just so small and the universe is expanding faster than the speed of light so we can never see the horizon. It's just getting further away with every passing second.
Others more knowledgeable can answer this well. I'd just like to say that if expansion stops all light will eventually reach us (later than sooner I guess) and (future) we might see birth of Universe :-)

phinds