Higgs bundles and higher Teichmüller spaces

Transcription

1 Oscar García-Prada ICMAT-CSIC, Madrid Hamburg, September 2018

2 1. Moduli space of representations S oriented smooth compact surface of genus g 2 π 1 (S) fundamental group of S G connected semisimple Lie group (real or complex) A representation of π 1 (S) in G is a homomorphism ρ: π 1 (S) G Hom(π 1 (S), G) is an analytic variety, which is algebraic if G is algebraic G acts on Hom(π 1 (S), G) by conjugation: (g ρ)(γ) = gρ(γ)g 1 for g G, ρ Hom(π 1 (S), G)

3 ρ is a reductive representation if composed with the adjoint representation in the Lie algebra of G, decomposes as a sum of irreducible representations Hom + (π 1 (S), G): set of reductive representations Moduli space of representations or character variety The moduli space of representations of π 1 (S) in G is defined to be the orbit space R(S, G) = Hom + (π 1 (S), G)/G R(S, G) is an analytic variety (algebraic if G is algebraic) Interested in the topology and geometry of R(S, G) Complex algebraic geometry approach: Higgs bundles

4 2. Higgs bundles X H G compact Riemann surface maximal compact subgroup of G θ Cartan involution of g, Lie algebra of G, defining the Cartan decomposition: g = h + m where h is the Lie algebra of H We have [m, m] h, [m, h] h The Cartan decomposition is orthogonal with respect to the Killing form of g Complexification of isotropy representation ι : H C GL(m C )

5 A G-Higgs bundle on X is a pair (E, ϕ) consisting of E a holomorphic principal H C -bundle over X ϕ a holomorphic section of E(m C ) K, where E(m C ) is the associated vector bundle with fibre m C via the complexified isotropy representation and K is the canonical line bundle of X To define stability for G-Higgs bundles, consider for s ih: Parabolic subgroup P s = {g H C : e ts ge ts is bounded as t } Character χ s : p s C defined by s (p s Lie algebra of P s ) Subspace m s = {Y m C : ι(e ts )Y is bounded as t } For σ a reduction of E to P s deg(e)(σ, s) := i χ s (F ). 2π X F : curvature of a connection on the P s -bundle defined by σ

6 Stability of G-Higgs bundles (E, ϕ) is: (semi)stable if deg(e)(σ, s) > 0( 0) for any s ih and any holomorphic reduction σ Γ(E(H C /P s )) such that ϕ H 0 (X, E σ (m s ) K) polystable if (E, ϕ) can be reduced to a G -Higgs bundle, with G G reductive and (E, ϕ) stable as a G -Higgs bundle The moduli space of polystable G-Higgs bundles M(X, G) is the set of isomorphism classes of polystable G-Higgs bundles M(X, G) is as complex algebraic variety (GIT construction, Schmitt, 2008)

7 G = SL(n, C) When G is a clasical group we can formulate the theory in terms of vector bundles In this case H = SU(n), H C = SL(n, C) and m C = sl(n, C) Hence, an SL(n, C)-Higgs bundle is equivalent to a pair (V, ϕ) V rank n holomorphic vector bundle with det V = O ϕ : V V K with Tr ϕ = 0 (V, ϕ) is stable: deg(v ) < 0 for every V V such that ϕ(v ) V K (V, ϕ) is polystable: (V, ϕ) = (V i, ϕ i ) with deg V i = 0 and (V i, ϕ i ) stable We recover the original notions introduced by Hitchin (1987)

8 G = SU(p, q) In this case H = S(U(p) U(q)), H C = S(GL(p, C) GL(q, C)), and m C = Hom(C q, C p ) Hom(C p, C q ) Hence, an SU(p, q)-higgs bundle is equivalent to a tuple (V, W, β, γ) V and W are rank p and q holomorphic vector bundles, respectively, with det V det W = O β : W V K and γ : V W K (V, W, β, γ) is stable: deg(v ) + deg(w ) < 0 for every V V and W W such that β(w ) V K and γ(v ) W K (V, W, β, γ) is polystable if the associated SL(p + q, C)-Higgs bundle ( ) 0 β V W and ϕ = γ 0 is polystable

9 G = Sp(2n, R) In this case H = U(n), H C = GL(n, C), and m C = S 2 (C n ) S 2 (C n ) Hence, an Sp(2n, R)-Higgs bundle is equivalent to a triple (V, β, γ) V is rank n holomorphic vector bundles, β : V V K and γ : V V K symmetric stability of (V, β, γ) is a condition on certain two step filtrations V 1 V 2 V (not enough to check the condition for subbundles of V ) (V, β, γ) is polystable if the associated SL(p + q, C)-Higgs bundle ( ) V V 0 β and ϕ = γ 0 is polystable

10 Theorem A G-Higgs (E, ϕ) is polystable if and only if there exists a reduction h of the structure group of E from H C to H, such that F h [ϕ, τ h (ϕ)] = 0 (Hitchin equation) τ h : Ω 1,0 (E(m C )) Ω 0,1 (E(m C )) is the combination of the anti-holomorphic involution in E(m C ) defined by the compact real form at each point determined by h and the conjugation of 1-forms F h is the curvature of the unique H-connection compatible with the holomorphic structure of E Proved by: Hitchin (1987): G = SL(2, C) Simpson (1988): G complex Bradlow G Mundet (2003) & G Gothen Mundet (2009): G real

11 Non-abelian Hodge correspondence Let S be a smooth compact surface and J be a complex structure on S. Let X = (S, J). There is a homeomorphism R(S, G) = M(X, G) Let (E, ϕ) be a polystable G-Higgs bundle and h a solution to Hitchin equations = E τ h ( E ) + ϕ τ h (ϕ) is a flat G-connection and the holonomy representation ρ is reductive Converse: Existence of a harmonic metric on a reductive flat G-bundle. Proved by Donaldson (1987) for G = SL(2, C) and Corlette (1988) for real reductive G

12 Theorem Let ρ be a representation of π 1 (X ) in G with corresponding flat G-bundle E ρ. Let E ρ (G/H) be the associated G/H-bundle. Then the existence of a harmonic section of E ρ (G/H) is equivalent to the reductiveness of ρ. Recall that a section of E ρ (G/H) X is equivalent to a π 1 (X )-equivariant map f : X G/H where π 1 (X ) acts on X by Deck transformations and on G/H via ρ The harmonicity of the section is equivalent to solving Hitchin equations

13 3. Topological invariants Given ρ: π 1 (S) G, there is an associated flat G-bundle on S, defined as E ρ = S ρ G ( S: universal cover of S): Hom(π 1 (S), G)/G = H 1 (S, G) = iso. classes of flat G-bundles Let G be the universal covering group of G. We have an exact sequence 1 π 1 (G) G G 1 which gives rise to the (pointed sets) cohomology sequence H 1 (S, G) H 1 (S, G) c H 2 (S, π 1 (G)) topological invariant of ρ: c(ρ) := c(e ρ ) H 2 (X, π 1 (G)) = π 1 (G) We can define the subvariety R c (S, G) := {ρ R(S, G) : c(ρ) = c}

14 Similarly, we can define a topological invariant of a G-Higgs bundle (E, ϕ) over X as the topological class of the H C -bundle E (recall H G is a maximal compact subgroup) H 1 (X, H C ) = isomorphisms classes of H C -bundles We have H 1 (X, H C ) H 1 (X, H C ) c H 2 (X, π 1 (H C )) topological invariant of (E, ϕ): c(e, ϕ) H 2 (X, π 1 (H C )) = π 1 (H C ) We can define the subvariety M c (X, G) := {(E, ϕ) M(X, G) : c(e, ϕ) = c}

15 Recall π 1 (G) = π 1 (H) = π 1 (H C ) For c π 1 (G) = π 1 (H C ) we have de homeomorphism R c (S, G) = M c (X, G) Theorem If G is compact (Ramanathan, 1975) or complex (J. Li, 1993; G-Oliveira, 2017) π 0 (R(S, G)) = π 0 (M(X, G)) = π 1 (G) The story is very different for non-compact real Lie groups (non-complex). The map π 0 (R(S, G)) = π 0 (M(X, G)) π 1 (G) may be neither surjective nor injective!

16 4. G = SL(2, R) The topological invariant of ρ R(S, SL(2, R)) in this case is an integer (basically the Euler class) d Z = π 1 (G) R d := {ρ R(S, SL(2, R)) : with Euler class d} Theorem (Milnor, 1958) R d is empty unless d g 1 An SL(2, R)-Higgs bundle is a tuple (L, β, γ) L line bundle over X β H 0 (X, L 2 K) and γ H 0 (X, L 2 K) Equivalently it can be seen as an SL(2, C)-Higgs bundle (V, ϕ) with V = L L 1 and ( ) 0 β ϕ = γ 0 Milnor s inequality follows from the stability of (V, ϕ) (Hitchin, 1987)

17 Theorem (Goldman, 1988; Hitchin 1987) R d is connected if d < g 1 R d has 2 2g connected components if d = g 1 Let R max := R d for d = g 1 Each connected component of R max consists entirely of Fuchsian representations (discrete and faithful) and can be identified with Teichmüller space (Goldman, 1980) Question: Are there other (non compact) groups with similar features to those of SL(2, R)? Split real groups (SL(2, R) is the split real form of SL(2, C)) Groups of Hermitian type (Poincaré disc D = SL(2, R)/SO(2)) Orthogonal groups SO(p, q) (PSL(2, R) = SO 0 (1, 2))

18 5. Split real forms Split real form = in the Cartan decomposition g = h m, the space m contains a maximal abelian subalgebra of g Every complex semisimple Lie group has a split real form Examples: SL(n, R), Sp(2n, R), SO(n, n), SO(n, n + 1) Consider G = SL(n, R) A basis for the invariant polynomials on sl(n, C) is provided by the coefficients of the characteristic polynomial of a trace-free matrix, det(x A) = x n + p 1 (A)x n p n 1 (A), where deg(p i ) = i + 1.

19 Consider the Hitchin map defined by n 1 p : M(X, SL(n, C)) H 0 (K i+1 ) i=1 p(e, ϕ) = (p 1 (ϕ),..., p n 1 (ϕ)), Hitchin (1992) constructed a section of this map giving an n 1 isomorphism between the vector space H 0 (K i+1 ) and a i=1 connected component of the moduli space M(X, SL(n, R)) M(X, SL(n, C)) This is called a Hitchin component (coincides with a Teichmüller component = H 0 (X, K 2 ) when G = SL(2, R)) Hitchin (1992) gives a general construction for any split real form

20 The Hitchin component is essentially unique if G is a split form of adjoint type (i.e. without centre) Every representation in the Hitchin component can be deformed to a representation factoring as π 1 (S) SL(2, R) G, where π 1 (S) SL(2, R) is in a Teichmüller component and SL(2, R) G is a principal representation A Hitchin component consists entirely of discrete and faithful representations (Labourie 2006) For G = SL(3, R), the Hitchin component parametrizes convex projective structures on the compact surface (Choi Goldman 1997) The Hitchin component parameterizes certain type of geometric structures (Guichard Wienhard 2008)

21 6. The Hitchin map and the Hitchin Kostant Rallis section G H G semisimple real Lie group maximal compact subgroup of G θ Cartan involution of g, Lie algebra of G, defining the Cartan decomposition: g = h + m a m maximal abelian subalgebra g C = λ Λ(a C ) g λ, Λ(a C ) (a C ) = restricted roots of g C with respect to a C W (a C ): restricted Weyl group of g C associated to a C (group of automorphisms of a C generated by reflections on the hyperplanes defined by the restricted roots Λ(a C ))

22 Chevalley: C[m C ] HC = C[a C ] W (ac ) Hence we have an algebraic morphism m C m C H C = a C /W (a C ) This map is C -equivariant. In particular, it induces a morphism h : m C K a C K/W (a C ). The map is also H C -equivariant, thus defining a morphism h : M(X, G) B(X, G) := H 0 (X, a C K/W (a C )) h is called the Hitchin map B(X, G) is called the Hitchin base

23 Kostant Rallis constructed ĝ g: maximal split subalgebra of g ĝ is a θ-invariant subalgebra of g generated by a and a principal normal three dimensional subalgebra ( = sl(2, C)) of g C invariant by the conjugation defining g g C Ĝ G: maximal split subgroup (analytic subgroup corresponding to ĝ) Ĥ Ĝ maximal compact subgroup, ĝ = ĥ + m We have Hence C[m C ] HC = C[a C ] W (ac ) = C[ m C ]ĤC. B(X, G) = B(X, Ĝ) If G is complex Ĝ = G split, the split real form of G. In particular B(X, G) = B(X, G split )

24 Let G be a real form of a complex semisimple Lie group G C : G is quasi-split if c g (a) is abelian (split real forms, SU(p, p), SU(p, p + 1), SO(p, p + 2), and E 2 6 ) x m C is said to be regular if dim c m C(x) = dim a C, where c m C(x) = {y m C : [y, x] = 0}. G is quasi-split if and only if m C g C reg = m C reg. There is an exact sequence where Q is finite 1 G N G C(G) Q 1,

25 Theorem (G-Peón Ramanan 2017) The choice of a square root of K determines Q inequivalent sections of the Hitchin map Each such section s G satisfies h : M(X, G) B(X, G). 1. If G is quasi-split, s G (B(X, G)) is contained in the stable locus of M(X, G). 2. If G is not quasi-split, the image of the section is contained in the strictly polystable locus. 3. For arbitrary groups, the Higgs field is everywhere regular. 4. The section factors through M(X, Ĝ). 5. If G = G split is the split real form s G is the factorization of the Hitchin section through M(X, G split ) and s G (B(X, G split )) is a topological component of M(X, G split ) (Hitchin component).

26 7. Non-compact real forms of Hermitian type G of Hermitian type means that G/H admits a complex structure compatible with the Riemannian structure of G/H, making G/H a Kähler manifold If G is simple the centre of h is one-dimensional and the almost complex structure on G/H is defined by a generating element in J Z(h) This complex structure defines a decomposition m C = m + m, where m + and m are the (1, 0) and the (0, 1) part of m C respectively Classical connected simple groups of Hermitian type: SU(p, q), Sp(2n, R), SO (2n), SO 0 (2, n) Two exceptional real forms E6 14 and E7 25

27 Let (E, ϕ) be a G-Higgs bundle over X. The decomposition m C = m + m gives a vector bundle decomposition Hence E(m C ) = E(m + ) E(m ) ϕ = (ϕ +, ϕ ) H 0 (X, E(m + ) K) H 0 (X, E(m ) K) The torsion-free part of π 1 (H) is isomorphic to Z (most of the time π 1 (H) = Z) and hence the topological invariant of either a representation of π 1 (X ) in G, or of a G-Higgs bundle, is essentialy given by an integer d Z. This is related to the so-called Toledo invariant.

28 The Toledo character χ T : h C C is defined, for Y h C in terms of the Killing form, by χ T (Y ) = 1 ij, Y, N where N is the dual Coxeter number. The Toledo character χ T defines a symmetric Kähler form on G/H by ω(y, Z) = iχ T ([Y, Z]), for Y, Z m, with minimal holomorphic sectional curvature 1. Define o J to be the order of e 2πJ and l = Z C 0 [HC, H C ], where Z C 0 is the identity component of Z(HC ). For q Q, the character qχ T lifts to a character H C C if and only if q is an integral multiple of q T = ln o J dim m

29 Let (E, ϕ) be a G-Higgs bundle. Maybe up to an integer multiple, χ T lifts to a character χ T of H C. Let E( χ T ) be the line bundle associated to E via the character χ T. We define the Toledo invariant τ of (E, ϕ) τ = τ(e) := deg(e( χ T )). If χ T is not defined, but only χ q T, one must replace the definition by 1 q deg E( χq T ). Let ρ : π 1 (X ) G be reductive and let (E, ϕ) be the corresponding polystable G-Higgs bundle. Let f : X G/H be the corresponding harmonic metric. Then τ(e) = 1 f ω, 2π In particular, τ(e) = τ(ρ) the Toledo invariant of ρ. Let d Z be the projection on the torsion-free part of the class c(e, ϕ) = c(ρ). Then d is related to τ by τ = d q T X

30 Theorem (Milnor Wood inequality) Let (E, ϕ) be semistable. The Toledo invariant τ satisfies τ rank(g/h)(2g 2) Proved for the classical groups for representations by Domic Toledo (1987) and for Higgs bundles by Bradlow G Gothen (2001) General proof for representations by Burger Iozzi Wienhard (2010), and for Higgs bundles by Biquard G Rubio (2017) For G = SU(p, q), G-Higgs bundle: (V, W, β, γ) Toledo invariant = τ = 2 deg V (deg W = deg V ) Milnor Wood inequality: τ min{p, q}(2g 2) For G = Sp(2n, R), G-Higgs bundle: (V, β, γ) Toledo invariant = τ = 2 deg V Milnor Wood inequality: τ n(2g 2)

31 G/H can be realized as a bounded symmetric domain D in m +, say (Cartan for the classical groups and Harish-Chandra in general) D is called of tube type if it is biholomorphic to a tube T Ω over a cone Ω: T Ω = R n + iω, for Ω R n The Poincaré disc, the domain corresponding to G = SU(1, 1), is of tube type. The tube is the upper-half plane and the biholomorphism is the Cayley transform The Shilov boundary of D is defined as the smallest closed subset Š of the topological boundary D for which every function f continuous on D and holomorphic on D satisfies that f (z) max f (w) for every z D w Š D is of tube type if and only if Š is a compact symmetric space of the form H/H. In this case Ω = G /H is its non-compact dual symmetric space

32 The symmetric spaces defined by Sp(2n, R), SO 0 (2, n) are of tube type. The symmetric space defined by SU(p, q) is of tube type if and only if p = q. The symmetric space defined by SO (2n) is of tube type if and only if n is even. The E 7 Hermitian real form is of tube type The E 6 Hermitian real form is not of tube type Every bounded symmetric domain has a maximal tube subdomain

33 Want to study the Maximal Toledo invariant moduli space in the tube case (the non-tube reduces to the tube case) M max (X, G) := M τ (X, G) for τ = rank(g/h)(2g 2) Assume that G is of tube type From Chevally theorem on invariant polynomials, one can deduce the existence of a homogeneous polynomial function det : m + C, satisfying for h H C and x m + det(ad(h)x) = χ T (h) det(x) Define Then m + reg := {x m + det(x) 0} m + reg = H C /H C (recall that Š = H/H )

34 Let G /H be the non-compact dual of the Shilov boundary H/H, and g = h + m be the corresponding Cartan decomposition. Any element x m + reg defines an H C -equivariant isomorphism ad(x) : m m C Let (E, ϕ +, ϕ ) M max (X, G) with τ = rank(g/h)(2g 2). Then ϕ + (x) m + reg = H C /H C for every x X and if κ is an o J -root of K, ϕ + defines a reduction of the H C -bundle E κ to an H C -bundle E. We also have an isomorphism ad(ϕ + ) : E (m ) E (m C ) So that we can define a Higgs field ϕ := ad(ϕ + )(ϕ ) = [ϕ +, ϕ ] H 0 (X, E (m C ) K 2 ). The pair (E, ϕ ) is a K 2 -twisted G -Higgs bundle.

35 Theorem (Cayley Correspondence) Let G be a such G/H is a Hermitian symmetric space of tube type, and let Ω = G /H be the non-compact dual of the Shilov boundary Š = H/H of G/H. Assume that o J divides 2g 2. Then the correspondence (E, ϕ) (E, ϕ ) defines an isomorphism M max (X, G) = M K 2(X, G ), where M K 2(X, G ) is the moduli space of K 2 -twisted G -Higgs bundles Proved for the classical groups by Bradlow G Gothen (2006) G Gothen Mundet (2013) (G = Sp(2n, R)) General case proved by Biquard G Rubio (2017)

36 The connected components of M(X, G) are not fully distinguished by the usual topological invariants. The dual group G detects new hidden invariants (for example for G = Sp(2n, R), Š = H/H, G = GL(n, R), Stiefel Whitney classes R max (S, G) consists entirely of discrete and faithful representations (Burger Iozzi Labourie Wienhard, 2006) The mapping class group of S acts properly on R max (S, G) (Wienhard, 2006) All common features with Hitchin components G = Sp(2n, R) is both split and Hermitian and M Hitchin (X, G) M max (X, G) A higher Teichmüller component of R(S, G) or M(X, G) is defined as one that has this kind of properties Question: Are there other groups besides split and hermitian real forms for which higher Teichmüller components exist?

37 8. Higher Teichmüller components for G = SO(p, q) Joint work with M. Aparicio, S. Bradlow, B. Collier, P. Gothen and A. Oliveira (arxiv: and arxiv: ) An SO(p, q)-higgs bundle is defined by a triple (V, W, η) where V and W are respectively rank p and rank q vector bundles with orthogonal structures such that det(w ) det(v ), and η is a holomorphic bundle map η : W V K For p > 2, rank p orthogonal bundles on X are classified topologically by their first and second Stiefel Whitney classes, sw 1 H 1 (X, Z 2 ) and sw 2 H 2 (X, Z 2 ) Since det(w ) det(v ) sw 1 (V ) = sw 1 (W ) The components of the moduli space M(SO(p, q)) are thus partially labeled by triples (a, b, c) Z 2g 2 Z 2 Z 2, where a = sw 1 (V ) H 1 (X, Z 2 ), b = sw 2 (V ) H 2 (X, Z 2 ), and c = sw 2 (W ) H 2 (X, Z 2 ) M(SO(p, q)) = M a,b,c (SO(p, q)) Oscar García-Prada ICMAT-CSIC, (a,b,c) Z Madrid 2g Higgs Z 2 bundles Z 2 and higher Teichmüller spaces

38 Proposition Assume that 2 < p q. For every (a, b, c) Z 2g 2 Z 2 Z 2 the space M a,b,c (SO(p, q)) has a non-empty connected component denoted by M a,b,c top (SO(p, q)) Define M top (SO(p, q)) = a,b,c M a,b,c top (SO(p, q)) Our main result shows that the moduli space M(SO(p, q)) has additional exotic components disjoint from the components of M top (SO(p, q)) We identify these exotic components as products of moduli spaces of L-twisted Higgs bundles, where in each factor L is a positive power of the canonical bundle K

39 Theorem (Generalized Cayley Correspondence) Fix integers (p, q) such that 2 < p < q 1. For each choice of a Z 2g 2 and c Z 2, the moduli space M(SO(p, q)) has a connected component disjoint from M top (SO(p, q)). This component is isomorphic to M a,c K p (SO(1, q p+1)) M K 2(SO 0 (1, 1)) M K 2p 2(SO 0 (1, 1)) The case of SO(2, q) is special since the group is of Hermitian type The existence of exotic components for SO(p, p + 1) for p > 2 is proved by Collier (2017) Conjecture: These exotic components are higher Teichmüller components (i.e. consist entirely of discrete and faithful representations)

40 Evidence: Notion of positivity recently introduced by Guichard Wienhard (2016) This generalizes Lusztig positivity for split real forms (related to Fock Goncharov positivity) It generalizes also a notion of positivity for Hermitian groups (related to causality) Conjecture (Guichard Wienhard): The only groups for which there are higher Teichmüller components are those admitting a positive structure The only classical groups admitting positive structures are: split groups, hermitian groups of tube type and SO(p, q)! Sp(2n, R) admits two different positive structures The exceptional groups admitting positive structures are real forms of F 4, E 6, E 7 and E 8, whose restricted root system is of type F 4 F4 4 is such an example: Existence of exotic components is proved by Bradlow Collier G Gothen Oliveira (work in progress). This group has also two different positive structures

41 9. Prehomogeneous vector spaces Theory introduced by Sato (Sato Kimura, 1977) G Complex reductive Lie group A prehomogeneous vector space for G is a complex finite dimensional vector space V together with a holomorphic representation of G on V such that G has an open and dense orbit in V. It turns out that such an orbit is unique and dense. Let V be a prehomogeneous vector space for G with representation ρ. A non-constant holomorphic function f : V C is called a relative invariant for the action of G if there exists a character χ : G C such that f (ρ(g)x) = χ(g)f (x) for every g G and x V. Let Ω be the open orbit in V and S := V Ω be the singular set in V. The prehomogeneous vector space V is called regular if the isotropy subgroup G x is reductive for any x Ω.

42 A relative invariant is, up to constant multiple, uniquely determined by its corresponding character. In particular any relative invariant is a homogeneous function. The existence of a relative invariant is equivalent to S having an irreducible component of codimension one. The prehomogeneous vector space V is regular if and only if S is a hypersurface. Let S = i=l i=1 S i be the decomposition of S in irreducible components. If V is regular, then for every S i (which is a hypersurface) there is a character χ i and a corresponding relative invariant f i such that S i = {x V f i (x) = 0}. The number l coincides with the number of G-irreducible summands in V. In particular, if V is irreducible l = 1.

43 Assume that V is regular Let x Ω. Let χ be a character of G and f be the corresponding relative invariant. We define the rank of x with respect to χ as rank χ (x) = deg(f ). We can actually define the rank of any point x V. To do this, we polarize f to get an r-linear map q on V such that q(x,..., x) = f (x); then the rank of x is the maximal integer r such that the (r r )-form q(x,..., x,,..., ) is not identically zero. In fact, rank can be defined for any point of V even if V is not regular

44 Let g be a complex semisimple Lie algebra. A Z-grading of g is a decomposition g = k Z g k such that [g j, g k ] = g j+k. A Z-grading of g defines a parabolic subalgebra p g C, given by p = g i i 0 And in fact any Z-grading is defined by a parabolic subalgebra. Let G be a complex semisimple Lie group with a graded Lie algebra g = k Z g k, and let G 0 be the analytic subgroup of G with Lie algebra g 0. Then g 1 is a prehomogeneous space for the adjoint action of G 0. In fact g 1 has only a finite number of G 0 -orbits (hence one of them must be open). The prehomogeneus vector space g 1 is said to be of parabolic type.

45 Example: The Hermitian case The representation of H C on m +, where G/H is Hermitian is a prehomogeneous vector space of parabolic type: Z-grading: g C = m h C m + m + is regular if and only if G/H is of tube type If G is simple and of tube type, then m + is irreducible and det is the basic relative invariant for the action of H C on m +, corresponding of course to the Toledo character of H C The same applies to m

46 Let G be a real form of a semisimple complex Lie group G C, i.e. G = (G C ) σ, where σ is a conjugation of G C. The group G is said to be of Hodge type if σ = τ Int(g), where τ is a compact conjugation and g G C Let G be Lie group of Hodge type and H G be a maximal compact subgroup. A period domain is a G-homogeneous space G/H 0 equipped with a homogeneous complex structure, where H 0 H is the H-centralizer of a compact torus T 0 H A period domain defines a canonical Z-grading g C = k Z gc k, satifying HC 0 is the complex subgroup of G C corresponding to g C 0, and h C = k=even g C k m C = k=odd We will refer to the structure defined above as a Hodge structure on G. g C k.

47 Some examples of groups of Hodge type and Hodge structures Hermitian groups are of Hodge type. The symmetric space G/H is itself a period domain (H 0 = H) G = SO(2p, q), H = SO(2p) SO(q), H 0 = U(p) SO(q) G = Sp(p, q), H = Sp(p) Sp(q), H 0 = U(p) Sp(q) Let G C be a semisimple complex Lie groups and g C = k Z gc k be a Z-grading. Then there exists a real form G of G C of Hodge type and a Hodge structure on G such that g C 2i h C and g C 2i+1 mc Let s go back to Higgs bundles!

48 10. Hodges bundles and variations of Hodge structure G: real form of a semisimple complex Lie group G C Consider a Z-grading of g C g C = k Z g C k, gc k = hc k mc k A G-Higgs bundle (E, ϕ) in M(X, G) is called a Hodge bundle if E admits a reduction of structure group to H C 0, the Lie group corresponding to h C 0, and ϕ H0 (X, E(m C 1 ) K) Consider the natural action of C on the moduli space M(X, G) of G-Higgs bundles given for λ C by λ (E, ϕ) := (E, λϕ) A point (E, ϕ) in M(X, G) is fixed under the C -action if and only if it is a Hodge bundle (Simpson, 1992) There is a good reason to call this a Hodge bundle: if (E, ϕ) is a Hodge bundle then there exists a real form of Hodge type G G C such that the grading of g C comes from a period domain G /H 0

49 Assume that G is of Hodge type anf let (E, ϕ) in M(X, G). Let ρ : π 1 (X ) G be the corresponding representation. Then (E, ϕ) is a Hodge bundle coming from a Hodge structure on G if and only if the equivariant harmonic map f : X G/H lifts to an equivariant horizontal holomorphic map f : X G/H 0 The pair (ρ, f ) is called a variation of Hodge structure (VHS) Solving Hitchin equations for a polystable G-Higgs bundle one obtains a function f : M(X, G) R defined by f (E, ϕ) = ϕ 2 L 2, called Hitchin function A smooth point (E, ϕ) is a critical point of the Hitchin function if and only it is a Hodge bundle

50 Assume that G is of Hodge type and G is equipped with a Hodge structure: H 0 H, Z-grading of g C, etc. Then g C 1 is a prehomogeneous space for the action of H0 C Assume that g C 1 is regular Milnor Arakelov inequality (Biquard Collier G Toledo, 2018) Let (E, ϕ) be a semistable Hodge bundle. Let χ be a character of H C 0. Then τ χ rank χ (ϕ)(2g 2), where rank χ (ϕ) is the rank of ϕ(x) for a generic x X. In particular τ χ r χ (2g 2), where r χ = deg(f χ ) and f χ is the homogeneous function corresponding to χ

51 There is a particular character χ T of H0 C (coming from the complex structure of G/H 0 ) for which more is actually true Let τ := τ χt. Of course: τ r χt (2g 2) A Hodge bundle is called maximal if τ = r χt (2g 2) Cayley correspondence (Biquard Collier G Toledo, 2018) Let (E, ϕ) be a Hodge bundle with τ = r χt (2g 2). We have the following: (1) rank ϕ(x) = r χt for every x X (2) The Higgs field ϕ defines a reduction of E κ to a H 0 C-bundle E over X, where H 0 C is the isotropy subgroup of an element in the open orbit of g C 1, and every H C 0 -bundle comes from a Hodge bundle with τ = r χt (2g 2) (3) Moreover, (E, ϕ) is (poly,semi)stable if and only if E is (poly,semi)stable

52 Suppose that a maximal Hodge bundle (E H C, ϕ 1 ) is a 0 minimum of the Hitchin function, where recall ϕ 1 takes values in g C 1. Then consider a Higgs bundle of the form (E, ϕ) = (E H C 0, ϕ 1, k<0 ϕ 2k+1 ) with ϕ 2k+1 H 0 (X, (E H C 0 (g C 2k+1 ) K) There is a Cayley correspondence for these objects which sends (E, ϕ) to a pair (E, ϕ ) where E is an H 0C -bundle and ϕ = k<0 ϕ 2k+1 := [ϕ 1, k<0 ϕ 2k+1 ] (E H C, ϕ 1 ) is a minimum is equivalent (Bradlow G Gothen, ) to having (H 0 -equivariant) isomorphisms ad(ϕ 1 ): E(g C 2k ) K = E(g C 2k+1 ), k < 1

53 This implies that ϕ 2k+1 H0 (X, (E H 0 C(g C 2k ) K 1 k ) fork < 1 On the other hand ϕ 1 H0 (X, (E H 0 C(V ) K 2 where V h C 0 = gc 0 Claim (Bradlow Collier G Gothen Oliveira, work in progress (1) If the dimension of the moduli space of objects (E H C 0, ϕ 1, k<0 ϕ 2k+1) equals the dimension of M(X, G), then this moduli space is a union of higher Teichmüller components of M(X, G). The Cayley partner detects new topological invariants (2) Moreover, the parabolic subgroup P G defining positivity on G in the sense of Guichard Wienhard can be identified from the structure of the Cayley parter (the rigidity given by being a maximal minimum!). This identification will use the solution to the Hitchin equation, that is a reduction of the bundle E to an H-bundle.