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I always wonder that if universe has a boundary to it then what else exits after it? or

Does the universe has an end/boundary!!!

What does exists after 100000 light years kilo meters distance?

Am i the only one having this kind of doubts?

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    $\begingroup$ 100,000 light years is only barely outside the Galaxy. You've got a Hike ahead of you if you're trying to get to Andromeda. Luckily, I hear she's coming over anyhow. Also, wrap your mind around this: what is the boundary of a ball's surface? That said, this is not a worldbuilding question. $\endgroup$ – Serban Tanasa Mar 31 '15 at 11:54
  • $\begingroup$ You could have good luck with this question on the Physics forum. The phrasing "the boundary of the universe" is a hotly debated concept in physics with many answers. However, it may need some rephrasing to satisfy their wording needs: instead of asking "what exists after the boundary" as "what do the models predict exists after the boundary." That variant of the phrasing is closer to the kind of phrasing Physics would answer (and they'd like it even better if you named which model you wanted them to answer with, but I don't know the models well enough to suggest one for you) $\endgroup$ – Cort Ammon - Reinstate Monica Mar 31 '15 at 14:42
  • $\begingroup$ Regarding real universe, see here: physics.stackexchange.com/questions/93319/… In short, closer to the universe's boundary (the cosmic even horizon) time is diliated and length contracted so that as an object approaches it it gets slowed down, without noticing it. $\endgroup$ – Anixx Apr 1 '15 at 9:38
  • $\begingroup$ ...universe as such has no boundaries (or if it does, we'll never learn of them). The problem is space expansion which creates the concept of observable universe - the section of universe which can send light that will reach us ever - the "universal event horizon" is an [entirely non-distinct] area of universe, that thanks to space expansion moves at speed of light away from us. So, light emitted by anything beyond it won't ever reach us (and anyone traveling from Earth at speed of light will never catch up to it). $\endgroup$ – SF. Apr 1 '15 at 11:08
  • $\begingroup$ See also worldbuilding.stackexchange.com/questions/30957/… $\endgroup$ – JDługosz May 15 '17 at 15:06
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The mathematical answer to that question is that you can define a manifold without embedding it into an n-dimensional vector-space. But this may be a bit too mathematical, so I will elaborate.

The usual example is to look at the circle: if you live inside the circle, you can go forward (or backward) as much as you like and you will never find a boundary. You will tell me: "Yes, but when I look at the circle from the outside, I can see that there are boundaries."

The reason why you think that is that you are used to see every object inside our usual 3-dimensional world. But mathematically speaking, there is nothing stopping a ring to exist "on its own", without a 3-dimensional space around it.

Let's go up by one dimension. Let's say you live on a sphere. You can go as far as you want forward, backward, left or right. Once again there are no boundaries. The boundaries only exist if you try to embed the sphere into our usual 3-dimensional world. [Note that you can replace the sphere with a more complicated example (like a torus, or the surface of the Klein bottle) if you like.]

What happens if I add again an other dimension? Well now you have what is called a 3-sphere. If you go in the same direction long enough you will end up in the same spot. If you have ever played Portal, you can imagine what it is like: get a big room, place portals on the floor, on the ceiling and on every wall and you obtain almost the same thing. Why do I say almost? Well by doing so you are using the fact that there are walls. However, just like in the sphere or the circle example, mathematically speaking you can define such a geometry without any reference to an outside world.

About the distance problem: It can be confusing to talk about distance in such a geometry ("I walked 200km and I end up in the same spot. How can I be both 200km away and in the same spot?"). But if you think about it, we already are doing this every day. Indeed we are living on a sphere: Earth. The good thing is that the Earth is big enough that as long as we don't move much, it doesn't really matters if the Earth is round or flat, and distances make sense. However if you walk far enough (ie. 45 000 km) you do end up in the same place.

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  • $\begingroup$ So it's possible to define a 3D space that is finite and unbounded. That doesn't mean that the universe does such a thing. What bothers me about such a closed space is being both flat and lacking a preferred axis, but that might be due to poor/conflicting explainations. More seriously, if two objects part in opposite directions and then meet again at the antipode without ever accelerating, that introduces the twin paradox. $\endgroup$ – JDługosz Apr 1 '15 at 7:15
  • $\begingroup$ @JDługosz I agree with the beginning of your comment: This doesn't prove anything about the shape of the universe, merely that this possibility exists. I do not really understand the end of your comment though: what do you mean by 'flat' and 'lacking preferred axis'? Also AFAIK the twin paradox is about speed, I am not sure what acceleration has to do with it... $\endgroup$ – Maxime Lucas Apr 1 '15 at 8:48
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What exists outside the universe? Nothing, no matter, no space, no time.

Our galaxy is about 80 - 100 thousand light years across so if you go that far in any direction your still going to be in space somewhere. Milky Way

Nah, lots of people wonder about this stuff, casually, then there are astrophysicists who actually study it. At some point i concluded that the universe must be infinitely large because nothing could exist outside it (therefore nothing like a wall at the edge of the universe) but that's not really the view of physics which points to the big bang as the origin of all things, including apparently space and time. (I think this covers it but i have not read it, Stephen Hawking is a good place to start anyway: The Origin of the Universe )

I don't think the question fits in World Building and you will probably get down voted, but i think it's interesting anyway.

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  • $\begingroup$ If the universe contains infinite volume, there could still be points that are not in it. That is, being infinite doesn't avoid the problem of there being an outside. $\endgroup$ – JDługosz Apr 1 '15 at 7:19
  • $\begingroup$ I think you just broke my brain. But if there was a single point outside the universe then wouldn't the universe be infinity - 1? Or is it that you are thinking of the universe as a gigantic container which you can take something out of, in which case what would that something become if it didn't have space to occupy? $\endgroup$ – Dylan Apr 1 '15 at 7:38
  • $\begingroup$ There are an infinite number of even numbers. Call that the universe. Odd numbers exist too, and that doesn't make the cardnality of the set of even numbers any less: it's still infinite. Infinite does not mean there is nothing that is not part of that set. $\endgroup$ – JDługosz Apr 1 '15 at 7:55

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