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This question already has an answer here:

Due to curiosity, it made me wonder what is outside of the universe, is it a new chemical or just empty space?

But empty space have protons appearing and disappearing in it, it basically still has something in it.

(It might have something to do with string theory.)

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marked as duplicate by Qmechanic Feb 6 '14 at 13:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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The universe isn't expanding "into" anything, it's the space itself that is expanding, or "stretching" if you will! Besides, it's the space between the galaxies that's expanding mostly. Inside them, gravity is overcoming the expansion, so it's really only on the larger scales.

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  • $\begingroup$ Nice answer. However it is wishful thinking to assume that all galaxies are maintaining their rigidity. Some very loosely bound systems are dispersing slowly due to inflation. $\endgroup$ – dj_mummy Sep 17 '13 at 12:37
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    $\begingroup$ @dj_mummy: Some very loosely bound systems are dispersing slowly due to inflation. Do you really mean inflation, or did you just mean to write "expansion?" I think Schlomo's answer is accurate, since it states the case correctly for galaxies (with the appropriate qualifier "mostly"), and he does note that expansion does affect structures on larger scales. $\endgroup$ – Ben Crowell Sep 17 '13 at 15:53
  • $\begingroup$ @BenCrowell You're right and his answer is accurate too. I meant to say expansion. I'll rephrase: 'it is significant from an experimental point of view'. At least that's what some research papers have argued. I just wanted to draw attention to this. I think this area has not been studied enough, so I am trying to subtly nudge budding scientists in this direction. You can't blame me for trying :D $\endgroup$ – dj_mummy Sep 17 '13 at 16:38
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A long time ago, Gauss discovered that the curvature of a surface depends only on distances and angles within the surface. He called this the Remarkable Theorem. For instance, a sheet of paper can be rolled on various cylinders or cones, or simply stay flat on the table, but its curvature will be in all cases 0. So, Gauss understood that the geometry of the surfaces is independent on how the surface is embedded in the three-dimensional space, or whether it is embedded at all. This is intrinsic geometry, because of course we can study the way an embedded surface stands in the ambient space (extrinsic geometry). Assuming that the surface is populated by some (fictional) two-dimensional people, as in the book Flatland, they can't determine, by measuring distances and angles or anything else for that matter, how the surface they live in is embedded in an ambient space, or if it is embedded at all.

These findings were generalized by Riemann to any number of dimensions, when he developed Riemannian geometry. A slighty modified version of this geometry, called semi- or pseudo-Riemannian geometry is used to describe spacetime in general relativity.

So, when physicists say that spacetime is curved, or that it expands, they don't mean this with respect to an ambient space in which our spacetime is embedded. They can measure whatever they can in their universe, and they will find no evidence about an environment in which our universe is embedded, something that we would call "outside the universe". So, in general, they will not assume the existence of such an universe.

Some physicists think that there may be an "outside", with more dimensions, of course, and that some physical forces which we observe and even the big bang may be explained by this ambient universe. So far there is no direct evidence of such a thing.

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    $\begingroup$ OMG this is totally awesome answer! I never really stopped and thought about that, with the embedding and all I'm going to save this answer for future use :D $\endgroup$ – dingo_d Sep 18 '13 at 13:27

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