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I was thinking of a world with a closed 3-dimensional space that would loop on itself. To describe it in another way: Forever traveling west, you came back to your starting point from east, same thing going south and north and forever. More importantly : digging the ground, you would end up falling from the sky.

To keep it simple, let's assume this universe is a cube. And Day/Night cycles, Seasons cycles would still be almost the same (even though I don't really know how it would work)

EDIT : Talking about scale of this universe, i would say something like 40,075km a side. I also forgot to mention that it would be a flat "earth" with a ceiling. (which would the same object as the ground).

How would that affect history, civilizations, religions, fauna, flora and physics in general ?

I'm not into reality-check, so please do not answer "It's impossible".

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    $\begingroup$ Specify the size of the cube. If its small, e.g. solar system sized (or smaller), all manner of weird effects would occur. If its sufficiently huge, on the other end, its indistinguishable from our universe. $\endgroup$ – Durandal Jan 5 '17 at 19:47
  • $\begingroup$ If sufficiently small for these effects to be felt, the question is too broad, as our whole world would be turned on its head (no pun intended). If sufficiently large (solar system sized), we wouldn't notice anything until very, very late in the game (sending probes out of the solar system), as it were, and even then, it wouldn't affect us too much. Last but not least, "not being into reality check" is irrelevant. People have a right to voice their opinions. And mine is that this question is too broad / opinion-based. $\endgroup$ – AndreiROM Jan 5 '17 at 19:57
  • $\begingroup$ Along Durandal's thinking if when you say "assume this universe is a cube," you mean something the size of the observable universe that science explores, the effects would be imperceptible (as in would have no meaningful effect on humanity or even our planet). If you are assuming the universe is much much smaller (say, a few hundred kilometers), the story changes. $\endgroup$ – Cort Ammon - Reinstate Monica Jan 5 '17 at 20:16
  • $\begingroup$ "How would that affect history, civilizations, religions, fauna, flora and physics in general ?" This is way too broad. Even if you just picked one (e.g., "How would that affect fauna?") it's way too broad. Worldbuild.SE is for when you have a specific question you need answered. $\endgroup$ – Azuaron Jan 5 '17 at 20:25
  • $\begingroup$ This is what is usually a finite bounded space. The surface of a sphere is also a finite bounded space. Keep going in any one direction and you will return to your starting point, eventually. Our universe maybe such a space. If was scaled down to the size of a planet, people could feel shut in. However, the pre-Copernician universe didn't bother people. That was a finite cosmos. Perhaps, it would be as if they never abandoned the pre-Copernician worldview. See Arthur Clarke's short story "Wall of Darkness". $\endgroup$ – a4android Jan 6 '17 at 4:11
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What you describe is possibly the geometry of our actual universe. It's hard (impossible?) to tell because the universe is larger than the observable universe and even if it is cyclic, light from stars hasn't had time to go more than a fraction of the distance around it since there were any stars to make light in persistently observable patterns. Also our universe seems to be expanding, so.light might never make it all the way round.

A toroidal universe does permit Maxwell's equations to be accurate without requiring the existence of at least two magnetic monopoles. OTOH there's plenty of universe in which to hide a handful of monopoles so the fact that we haven't ever seen one may not be meaningful.

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  • $\begingroup$ Did you mean that a toroidal universe requires two magnetic monopoles? It's not what you said, but my intuition was always that a typical cartesian universe required no monopoles, so I'm wondering if that was a typo or if I just need to learn something about the magnetic field on a cartesian manifold! $\endgroup$ – Cort Ammon - Reinstate Monica Jan 5 '17 at 21:33
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    $\begingroup$ No, it's a non-toroidal universe that requires monopoles. You can't get all the hairs on a tennis ball to lie flat to its surface at once. Now imagine that the hairs are tiny sections of magnetic field lines which (absent monopoles) must never meet or cross over, and that the surface is a universe. You can make all the hairs on a fuzzy torus lie flat to its surface at once. $\endgroup$ – nigel222 Jan 5 '17 at 21:44
  • $\begingroup$ Ahh, that makes sense. Can you avoid the monopoles in an infinite flat universe as well? $\endgroup$ – Cort Ammon - Reinstate Monica Jan 5 '17 at 21:52
  • $\begingroup$ BTW I was told that it generalises from a flatland universe embedded on the surface of a 3D object, to a 3D universe embedded on a 4-sphere or 4-torus. My maths is not up to proving that. $\endgroup$ – nigel222 Jan 5 '17 at 21:55
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    $\begingroup$ I'd guess that you could put the monopoles infinitely far away in an infinite flat universe.. But if there was a big bang beginnIng then the universe can't be an infinite Euclidean plane. It must be cyclically closed, either like the surface of a sphere, or a torus, or something topologically more complex. $\endgroup$ – nigel222 Jan 5 '17 at 22:01
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There had been astronomers suggesting, that our universe is what you described in the beginning. So if your cube is so very large that the whole universe fits into it, the answer is: Nothing changes.

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