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Apr 3, 2020 at 19:56 comment added Will Sawin @GTA To relate these two notions, justifying why locally symmetric spaces are the special fiber, you have to specialize at $\infty$, then turn the Galois representations into Hodge structures, then split the Hodge structures into 1-forms and conjugate 1-forms, then view the $1$-forms as generalized functions. This Hodge theory story doesn't work the same way as function fields, so there's no reason for the two kinds of spaces to have such a direct relationship.
Apr 3, 2020 at 19:54 comment added Will Sawin @GTA From the point of view of Langlands, the key property of modular curves (for concreteness, but the same applies to other Shimura varieties) is that their cohomology splits as a sum over automorphic forms of two-dimensional Galois representations associated to them. On the other hand, the key property of locally symmetric spaces is that (generalized) functions on them are modular curves.
Apr 3, 2020 at 19:52 comment added Will Sawin @GTA No, the fibers will all have the same dimension, which will almost never be the dimension of $\operatorname{Bun}_G$. The analogy might not be perfect in all respects - although maybe there is an explanation here.
Apr 3, 2020 at 19:32 comment added GTA @WillSawin I am confused how this could happen in the context of shtukas. Can a fiber of the moduli of shtukas with some number of legs at a point of the base be just Bun_G?
Apr 3, 2020 at 16:33 comment added Will Sawin @GTA Shimura varieties are not locally symmetric spaces. They are schemes over the integers whose fiber over the complex numbers has an underlying manifold which is a locally symmetric space. Shtukas with one leg at a point $p$ are the analogue of the reduction of the Shimura variety mod $p$, and Shtukas with one varying leg are the analogue of the Shimura variety as a scheme over $\mathbb Z$. For questions about automorphic forms which only depend on the manifold, not on the Shimura variety, shtukas with no legs can be used.
Apr 3, 2020 at 4:08 comment added GTA Shimura varieties are locally symmetric spaces. How are the two analogies (Shimura variety = one leg, locally symmetric space = no leg) consistent with each other?
Apr 3, 2020 at 1:21 history edited David Ben-Zvi CC BY-SA 4.0
Added section on physics perspective
Apr 2, 2020 at 21:08 history bounty ended Mr. Palomar
Apr 2, 2020 at 19:36 history answered David Ben-Zvi CC BY-SA 4.0