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G2 -Orbifolds with ADE-Singularities

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G2 -Orbifolds with ADE-Singularities G2-orbifolds with ADE-singularities HABILITATIONSSCHRIFT Der Fakult¨atf¨urMathematik der Technischen Universit¨atDortmund vorgelegt von Dr. Frank Reidegeld Dortmund, April 2017 Contents 1 Introduction 3 2 The ADE-classification 7 3 K3 surfaces 15 3.1 Introduction to lattices . 15 3.2 Basic facts about K3 surfaces . 17 3.3 Moduli spaces of K3 surfaces . 19 3.4 Singular K3 surfaces . 24 3.5 Non-symplectic automorphisms . 26 4 Introduction to G2-manifolds 39 4.1 Basic facts about G2-manifolds . 39 4.2 Introduction to orbifolds . 44 4.3 Joyce's construction of G2-manifolds . 47 4.4 Twisted connected sums . 50 4.5 G2-orbifolds and physics . 55 5 G2-orbifolds with ADE-singularities 59 5.1 Quotients of the product of a K3 surface and torus . 59 5.2 Twisted connected sums with ADE-singularities . 68 5.3 Torus quotients and their resolutions . 74 1 5.3.1 Basic facts . 74 5.3.2 Known examples . 76 5.3.3 Choice of the group action on T 7 ................... 79 5.3.4 Explicit examples . 85 5.3.5 More complicated examples . 103 6 Conclusion and Outlook 112 2 Chapter 1 Introduction A G2-structure is a 3-form φ on a 7-dimensional manifold M that satisfies certain algebraic conditions. On a manifold with a G2-structure there exists a natural metric that is induced by φ. If φ is parallel with respect to the Levi-Civita connection, the holonomy is a subgroup of the exceptional Lie group G2. If the holonomy acts irreducibly on the tangent space, we call (M; φ) a G2-manifold. The group G2 was one of the last groups on Berger's list of possible holonomy groups, for which it was unknown if metrics with these holonomy groups actually exist. The first local examples of metrics with holonomy G2 have been constructed by Bryant [12], the first complete but non-compact ones by Bryant and Salamon [13] and the first compact examples by Joyce [36]. The idea of Joyce for the construction of G2-manifolds was to 7 divide a torus T that carries a flat G2-structure by a finite group Γ that preserves the 7 G2-structure. The singularities of T =Γ can be resolved and it is possible to show by several steps the existence of a metric with holonomy G2 on the resolved manifold. Another idea for the construction of G2-manifolds has been proposed by Donaldson and was carried out in detail by Kovalev [43]. The starting point of this construction are two asymptotically cylindrical (ACyl) Calabi-Yau manifolds Wi with i = 1; 2 that approach 1 1 1 Di × S × (0; 1) at infinity, where the Di are K3 surfaces. W1 × S and W2 × S can be glued together after cutting off the cylindrical ends. The glueing map interchanges the circle factors of the ends and acts as an isometry with certain properties on the Di. The manifold that we obtain by this method is called a twisted connected sum and carries a metric with holonomy G2. By the twisted connected sum method several authors [17, 43, 44] constructed a large number of G2-manifolds. Most of the known examples of compact G2-manifolds are smooth. In this thesis, we study G2-orbifolds with a certain kind of singularities, namely ADE-singularities. This means that at the singular points the orbifolds are locally diffeomorphic to R3 × C2=∆, where ∆ is a finite subgroup of SU(2), which can be embedded in to G2. The finite subgroups of SU(2) have been classified by Klein [41]. For any finite subgroup ∆ of SU(2) there exists 3 an associated affine Dynkin diagram. This relation is called the McKay correspondence. By deleting one node we obtain a Dynkin diagram of a simple finite-dimensional Lie algebra. The diagrams that can be obtained this way are exactly the A-, D- and E-series from the classification of the simple finite-dimensional Lie algebras, which is the reason why one of the names for singularities of type C2=∆ is ADE-singularities. There are two motivations to study G2-orbifolds with ADE-singularities. One of them is mathematical and the other one is physical. An important object in the theory of G2-manifolds is the moduli space M of parallel G2- structures on a fixed manifold M. Joyce [36] has proven that this moduli space is a smooth manifold of dimension b3(M). Beside this fact very little is known about the geometry of M. It is conjectured that the boundary of M consists of singular G2-manifolds. More explicitly, Karigiannis [39] conjectures that the moduli space of parallel G2-structures with a conical singularity is a boundary component of the moduli space of parallel G2-structures on a desingularized manifold. Halverson and Morrison [32] conjecture that the boundary components of M correspond to G2-manifolds whose singular sets are of codimension 4, 6 and 7. This conjecture implies that for any parallel G2-structure φ on an orbifold with ADE-singularities there exists a one-parameter family of smooth G2-structures that converges in a suitable sense to φ. A proof of the conjecture from [32] and even of the weaker conjecture on the G2-structures converging to an ADE-singularity is probably very hard. For example, a resolution of an ADE-singularity in the above sense is a step in the proposed construction of G2-manifolds in the unpublished paper [40]. Although the conjectures on the moduli space of G2-structures are not yet proven, constructing examples of G2-orbifolds with ADE-singularities is an important step to better understand the shape of M. G2-manifolds do not only play a role in pure mathematics but also in theoretical physics. 1;3 7 7 In M-theory, spacetime is often modeled as R × M , where M is a G2-manifold that is too small to be observed at low energies. If M 7 is smooth, the quantum field theory on the 4-dimensional Minkowski space that we can observe does not fit to our observations. More precisely, we obtain a field theory with gauge group U(1)b2(M) in the low-energy limit, but the weak and the strong interaction are described by Yang-Mills theories with a non-abelian gauge group. Moreover, the existence of chiral fermions cannot be explained by this hypothesis. If M 7 is not smooth but has conical singularities, the existence of chiral fermions can be shown [3, 4, 9]. Moreover, M-theory compactified on a G2-orbifold with ADE-singularities yields, at least if some topological conditions on the singular locus are satisfied, a four-dimensional quantum field theory whose gauge group is determined by the Dynkin diagram that is associated to the singularity by the McKay correspondence. Up to now, there are only few examples of G2-orbifolds with ADE-singularities known. In [3], the author considers a class of non-compact G2-orbifolds that can be obtained by dividing a complete G2-manifold of cohomogeneity one by a subgroup of SU(2). Moreover, Joyce [36] constructs a torus quotient with A1-singularities along 12 three-tori which is the starting point for the construction of a smooth G2-manifold. Further examples of 4 torus quotients with An-singularities can be found in [10]. In this thesis, we construct several examples of compact G2-orbifolds of ADE-singularities. One of our priorities is to construct G2-orbifolds with as many as possible types of ADE-singularities. Our results can be summed up as follows. Let S be a K3 surface with a hyper-K¨ahlermetric and let T 3 be a flat torus. Since the 3 3 holonomy of S × T is Sp(1), which can be embedded into G2, S × T carries a parallel 3 3 G2-structure φ. If Γ is a group that acts freely on S × T and preserves φ,(S × T )=Γ carries a parallel G2-structure, too. If we allow S to have singularities or Γ to act non-freely, we obtain G2-orbifolds with ADE-singularities. We find examples of such quotients that 3 2 have E8-singularities along 2 submanifolds of type T =Z2 and A1-singularities along 3 3 2 submanifolds of type T =Z2. Let G1;:::;Gn be a set of connected Dynkin diagrams that can be obtained by deleting some nodes from the union of 2 diagrams of type E8 and 3 of 3 type A1. There exist further quotients of type (S × T )=Γ whose singular locus consists of 3 2 n submanifolds that are isometric to the flat manifold T =Z2 with singularities of type Gi along them. Another class of examples is constructed with help of twisted connected sums. Kovalev and Lee [44] find many examples of smooth G2-manifolds by applying the twisted connected sum construction to a pair of suitable ACyl Calabi-Yau manifolds W1 and W2. At least one of them is constructed by a procedure that is described in [44] from of a K3 surface S that admits a non-symplectic involution. We modify this construction such that S is a K3 surface with singularities. We obtain a series of G2-orbifolds that have singularities along one or several three-spheres. The number and type of the singularities is described by two copies of an arbitrary subdiagram of E8. Finally, we consider quotients of a torus with a flat G2-structure by a finite group with ADE-singularities. A simple algebraic criterion restricts the possible ADE-singularities of a torus quotient to A1, A2, A3, A5, D4, D5 and E6.
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