Linear connections on Lie groups The affine space of linear connections on a compact Lie group G contains a distinguished line segment with endpoints the connections ∇ L and ∇ R which make left (resp. right) invariant vector fields parallel. The midpoint is the Levi-Civita connection of a bi-invariant Riemannian metric. Problem (Dan Freed).– Give a differential-geometric interpretation of the connection 2 3 ∇ L + 1 3 ∇ R . [This connection arises in the “cubic Dirac equation” introduced by Slabarski and rediscovered by B. Kostant and also in the non-commutative Weil algebra of A. Alexeev and Meinnenken.] 1
Hitchin representations of fundamental groups of surfaces Let S be an orientable compact 2-dimensional surface. A Fuchsian rep- resentation of π 1 ( S ) in PSL( n, R ) is a representation which factors through a cocompact representation of π 1 ( S ) in PSL(2 , R ) and the irreducible repre- sentation of PSL(2 , R ) in PSL( n, R ). A Hitchin representation is a representation which may be deformed into a Fuchsian representation. We denote by Rep H ( π 1 ( S ) , SL( n, R )) the moduli space of Hitchin repre- sentations, which is by definition a connected component of the space of all representations. It can be shown that a Hitchin representation is discrete and faithful and that the Mapping Class Group M ( S ) acts properly on the Hitchin component. 2
In 1990, N. Hitchin gave explicit parametrisations of Hitchin components: If J is a complex structure J over S , he produced a homeomorphism H J : H 0 ( K 2 J ) ⊕ . . . ⊕ H 0 ( K n J ) → Rep H ( π 1 ( S ) , SL( n, R )) . He uses the identification of representations with harmonic mappings as in K. Corlette’s seminal paper and the fact that a harmonic mapping taking values in a symmetric space gives rise to holomorphic differentials in a manner similar to that in which a connection gives rise to differential forms in Chern- Weil theory. In particular, this construction breaks the invariance under the Mapping Class Group. 3
Here is a more equivariant construction (with respect to the action of the Mapping Class Group): Let E ( n ) be the vector bundle over Teichm¨ uller space whose fibre above the complex structure J is E ( n ) = H 0 ( K 3 J ) ⊕ . . . ⊕ H 0 ( K n J ) . J We observe that the dimension of the total space of E ( n ) is the same as that of Rep H ( π 1 ( S ) , SL( n, R )) since the dimension of the “missing” quadratic differentials in E ( n ) accounts for the dimension of Teichm¨ uller space. J We now define the Hitchin map � E ( n ) → Rep H ( π 1 ( S ) , SL( n, R )) H ( J, ω ) H J (0 , ω ) . �→ (This terminology is awkward since this Hitchin map is some kind of an inverse of what is usually called the Hitchin fibration ). 4
Conjecture (Fran¸ cois Labourie).– If ρ is a Hitchin representation, then there exists a unique ρ -equivariant minimal surface in SL( n, R )/SO( n, R ) . Hence the space Rep H ( π 1 ( S ) , PSL( n, R )) / M ( S ) is homeomorphic to the vector bundle over the Riemann moduli space whose fibre at a point J is H 0 ( K 3 J ) ⊕ . . . ⊕ H 0 ( K n J ) . [This conjecture is known to be true for n = 2 where it reduces to the Riemann uniformisation theorem. F. Labourie and J. Loftin proved it for n = 3. Moreover, F. Labourie also proved that the Hitchin map is surjective: it amounts to proving the existence of the above mentioned minimal surface.] 5
Problem (William Goldman).– Which are the surface group representa- tions ρ : π − → PSL(2 , R ) that correspond to branched hyperbolic structures? For each Riemann surface Σ of genus g with fundamental group π , con- sider rank 2 stable Higgs pairs ( V, Φ) where the Higgs field Φ has no com- ponent in Ω 1 (Σ , K 2 ⊗ D ) with D is an effective divisor satisfying the degree condition deg( D ) < 2 g − 2. (It says that the harmonic metric is holomorphic.) Taking the union over all surfaces, this gives a universal symmetric power which maps into Hom( π, PSL(2 , R )) / PSL(2 , R ) by a (non-surjective) homo- topy equivalence. Problem (William Goldman).– What is the image of this symmetric power in Hom ( π, PSL(2 , R )) / PSL(2 , R ) ? Does it contain all [ ρ ] with dense image? 6
Let G be an R -split semi-simple Lie group. Let H be the Hitchin com- ponent of Hom( π 1 ( S ) , G )) /G . Problem (William Goldman).– Interpret H as locally homogeneous geo- metric structures (in the sense of Ehresmann and Thurston) on fiber bundles over S . [For example, when G = PGL(2 , R ), H is in one-to-one correspondence with hyperbolic structures on S . When G = PGL(3 , R ), H is in one-to-one correspondence with convex R P 2 -structures on S . When G = PGL(4 , R ), O. Guichard and A. Weinhard have identified a class of R P 3 -structures on the unit tangent bundle T 1 ( S ) corresponding to H .] 7
Harmonic maps of higher genus There is a well-developed theory of integrable systems for harmonic maps from a 2-torus to a Lie group G . Formally the equations for harmonic maps of a surface look like the Higgs bundle equations but with a change of sign. Problem (Nigel Hitchin).– Is there a Nahm transform, in the same con- text of the previous problem, for maps of surfaces of higher genus? 8
Metrics with special holonomy Problem (Nigel Hitchin).– Find explicit descriptions of a Calabi-Yau met- ric on a K3 surface. [Twistor theory tells us that, if we do that, then we can describe explicitly complex structures which are far from algebraic ones which does not sound too hopeful. However, here is a possible scenario. Kronheimer’s ALE construction takes a finite subgroup Γ ⊂ SU(2) and considers the vector space R of functions on Γ, then does a hyperk¨ ahler quotient of the group U ( R ) Γ (Γ-invariant unitary transformations) acting on the quaternionic vector space ( R ⊗ H ) Γ . Replace Γ by a discrete subgroup ahler isometries of C 2 and do the same thing. of hyperk¨ 9
In particular consider Γ to be the extension of a finite group by transla- tions Z n → Γ → Z 2 (with n ≤ 4) instead of just Z 2 , which gives the Eguchi-Hanson metric. With n = 4, the quotient can be interpreted as the moduli space of Z 2 - invariant SU(2) instantons on a flat torus with a certain type of singularity at the 16 fixed points. The thorny issue is the nature of that singularity, but formally there should be a hyperk¨ ahler moment map for the gauge group which requires 3 parameters for each singular point. Together with the 10 parameters for the lattice Z 4 this gives 48 + 10 = 58 parameters. The construction would be explicit in the sense that in principle we know how to solve the ASD equations on a flat torus by twistor theory.] 10
The Clemens-Friedmann construction of non-K¨ ahler 3-folds yields com- plex structures with non-vanishing holomorphic 3-forms on connected sums of S 3 × S 3 . This is an analogue of the study of complex structures on con- nected sums of S 1 × S 1 – Teichm¨ uller theory. The local geometry of the complex structure moduli space is known and is like that of an ordinary Calabi-Yau. Problem (Nigel Hitchin).– What about the global structure and its bound- ary or the analogue of the mapping class group? Is there a natural metric with skew torsion on such a 3-fold? 11
Problem (Nigel Hitchin).– Find bounds on the topology of Calabi-Yau 3-folds. [It is conjectured that the Euler characteristic of such manifolds is bounded by 960. In the case of hyperk¨ ahler 4-folds there are bounds due to Guan.] 12
Problem (Robert Bryant).– In a Calabi-Yau 3-manifold is the singular locus of a special Lagrangian 3-cycle a semi-analytic set? Is there a way to resolve singularities of such objects? [By Almgren’s regularity theorem, it is known to have Hausdorff dimension at most 1, but nothing else appears to be known about it.] 13
Problem (Simon Salamon).– Classify metrics with holonomy equal to G 2 admitting a 2-torus of isometries [extending work of V. Apostolov, S. Salamon et al.]. Problem (Simon Salamon).– Are compact manifolds with exceptional holonomy G 2 or Spin(7) necessarily formal? [Partial results by G. Cavalcanti, M. Fernandez, M. Verbitsky.] 14
Problem (Simon Salamon).– Are there metrics with holonomy G 2 as- sociated in some way to the (twistor spaces of) self-dual structures on the connected sum of n ≥ 2 copies of C P 2 ? [Question of M.F. Atiyah and E. Witten.] Problem (Simon Salamon).– Is there a compact hyperk¨ ahler 8-manifold other than the two spaces found by A. Beauville? [cf. O’Grady examples in dimensions 12 and 20]. 15
Special geometries Problem (Joel Fine),– Do there exist two homeomorphic 4-manifolds, only one of which admits an anti-self-dual metric? [Claude LeBrun has shown this is true if one replaces “anti-self-dual” by “scalar-flat and anti-self-dual”.] Problem (Simon Salamon).– Is every compact nearly-K¨ ahler 6-manifold homogeneous? 16
We now know that not every co-closed G 2 -structure on a 7-manifold can be induced by an immersion into a Spin(7)-manifold. Analyticity is sufficient but not necessary. Problem (Robert Bryant).– Can one give necessary and sufficient condi- tions? Is every (local) co-closed G 2 -structure on a 7-manifold the boundary of a smooth Spin(7)-manifold? (This is the “one-sided” version of the em- bedding problem.) What are the conditions on a co-closed G 2 -structure on the 7-sphere that determine that it is the boundary of a smooth Spin(7)-holonomy Riemannian 8-ball? 17
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