Action of the mapping class group on character varieties and Higgs bundles

Oscar Garcia-Prada, Graeme Peter Desmond Wilkin

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We consider the action of a finite subgroup of the mapping class group $\Mod(S)$ of an oriented compact surface $S$ of genus $g \geq 2$ on the moduli space $\calR(S,G)$ of representations of $\pi_1(S)$ in a connected semisimple real Lie group $G$. Kerckhoff's solution of the Nielsen realization problem ensures the existence of an element $J$ in the Teichm\"uller space of $S$ for which $\Gamma$ can be realised as a subgroup of the group of automorphisms of $X=(S,J)$ which are holomorphic or antiholomorphic. We identify the fixed points of the action of $\Gamma$ on $\calR(S,G)$ in terms of $G$-Higgs bundles on $X$ equipped with a certain twisted $\Gamma$-equivariant structure, where the twisting involves abelian and non-abelian group cohomology simultaneously. These, in turn, correspond to certain representations of the orbifold fundamental group. When the kernel of the isotropy representation of the maximal compact subgroup of $G$ is trivial, the fixed points can be described in terms of familiar objects on $Y=X/\Gamma^+$, where $\Gamma^+ \subset \Gamma$ is the maximal subgroup of $\Gamma$ consisting of holomorphic automorphisms of $X$. If $\Gamma=\Gamma^+$ one obtains actual $\Gamma$-equivariant $G$-Higgs bundles on $X$, which in turn correspond with parabolic Higgs bundles on $Y=X/\Gamma$ (this generalizes work of Nasatyr \& Steer for $G=\SL(2,\R)$ and Boden, Andersen \& Grove and Furuta \& Steer for $G=\SU(n)$). If on the other hand $\Gamma$ has antiholomorphic automorphisms, the objects on $Y=X/\Gamma^+$ correspond with pseudoreal parabolic Higgs bundles. This is a generalization in the parabolic setup of the pseudoreal Higgs bundles studied by the first author in collaboration with Biswas \& Hurtubise.
Original languageEnglish
Number of pages28
JournalDocumenta Mathematica
Publication statusAccepted/In press - 22 Jun 2020

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