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I’m trying to figure out rules for my Sci-fi universe and I need to know what type of universe, the universe needs to be for the existence of magnetic monopoles to exist. I read somewhere that the universe is generally believed to be flat and that that is why we can’t find magnetic monopoles. Yet magnetic monopoles are predicted by string theory and I very much like string theory and want to believe it is true. However I WANT my model of the universe to be 100% accurate so I will discard magnetic monopole if the real shape of the universe forbids it.

Basically is the universe flat and has no monopoles and for magnetic monopoles to exist; would the universe have to be curved?

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    $\begingroup$ String theories tend to allow almost anything... that is their tragedy. $\endgroup$
    – James K
    Oct 17, 2021 at 18:00
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    $\begingroup$ I’m voting to close this question because it is appropriate to Worldbuilding where they do deal with science and hard-science questions like this. $\endgroup$ Oct 17, 2021 at 19:20
  • $\begingroup$ It’s for worldbuilding but I am asking for “real” theoretical science that people on Worldbuilding most likely have no clue about; so please don’t close. $\endgroup$
    – Max
    Oct 17, 2021 at 20:25
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    $\begingroup$ The physics of when monopoles are allowed or even necessary (the Dirac quantization condition unfortunately goes the other way) is very real theoretical physics and has played a significant role in cosmology, so it is an appropriate question. $\endgroup$ Oct 17, 2021 at 22:14
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    $\begingroup$ I don't think you'll get a definitive answer to this (which is what you want). We don't actually know if monopoles exists and current theories are essentially trying to explain why they are not detectable (by being rare now). See The Monopole Problem on Wikipedia. That's my understanding of the state of things. $\endgroup$ Oct 18, 2021 at 1:10

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I think this alludes to the "solution" to the monopole problem given by cosmic inflation.

Grand Unified Theories predict the existence of magnetic monopoles: lots of them. So many that they would be easily observed. But we haven't observed a single one. Cosmic inflation solves this problem: the monopoles would be spread out by inflation, so there could be vast distances between them. Cosmic inflation also can explain why spacetime seems (nearly) flat.

So a universe with inflation could have rare monopoles and nearly flat space.

But there could just be more monopoles, so even with inflation they are still common, or monopoles could form in other ways (string-theories also predict monopoles and universes with tightly curved dimensions) There is nothing logically inconsistent between a universe in which monopoles are common and a flat universe.

We don't know if the theories that predict monopoles are correct, and there are grand unified theories and string theories that don't have monopoles.

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