# Geometric Structures in Mathematical Physics Non-existence of almost complex structures on quaternion-kähler manifolds of positive type

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1 Geometric Structures in Mathematical Physics Non-existence of almost complex structures on quaternion-kähler manifolds of positive type Paul Gauduchon Golden Sands, Bulgaria September, 19 26,

2 Joint work with Andrei Moroianu and Uwe Semmelmann Main references: A. Moroianu P. Gauduchon U. Semmelmann, Almost complex structures on quaternion-kähler manifolds and inner symmetric spaces, Invent. math. 184 (2011), C. LeBrun S. Salamon, Strong rigidity of positive quaternion-kähler manifolds, Invent. math. 118 (1994),

3 Definition 1 A Quaternion-Kähler manifold qk manifold for short is a (complete) oriented Riemannian manifold (M, g) of dimension 4k 8, such that the vector bundle A(M) of skew-symmetric endomorphisms of the tangent bundle TM admits a vector subbundle Q A(M) of rank 3, satisfying the two properties: (i) Q is preserved by the Levi-Civita connection of g, and (ii) Q is locally generated by positively oriented almost complex structures J 1, J 2, J 3 with J 1 J 2 J 3 = 1. 3

4 Equivalently, the holonomy group of (M, g) is contained in the subgroup Sp(1)Sp(k) of SO(4k): Sp(k) = Hermitian symplectic group = preserves the Hermitian quaternionic structure of H k = (C 2k, j) Sp(1) = group of quaternions of norm 1. The inclusion Sp(k)Sp(1) = Sp(k) Sp(1)/±1 SO(4k) is realized via the map: Sp(k) Sp(1) SO(R 4k ) (A, p) {u Aup 1 } u H k = R 4k. 4

5 Model: The quaternionic projective space. HP k = H k+1 /H quaternions) (right action by non-zero For any x in HP k, T x HP k = Hom H (x, x ) Fix any H-basis u 1 0 in x, x = H via the map p u = u 1 p. We then set J 1 X = X 1 i, J 2 X = X 1 j, J 3 X = X 1 k, i.e. (J 1 X)(u) = X(u 1 ip) i.e. (J 2 X)(u) = X(u 1 jp) i.e. (J 3 X)(u) = X(u 1 kp) If u 2 = u 1 r is any other H-basis of x, we get J 1 X = X 2 i = X 1 (rir 1 ) J 2 X = X 2 j = X 1 (rjr 1 ) J 3 X = X 2 k = X 1 (rkr 1 ) These are linear combinations of J 1, J 2, J 3, hence generate the same rank 3 subbundle of A(HP k ). 5

6 Other classical examples: The real Grassmannians Gr 4 (R 4+k ) of oriented 4-dimensional real vector subspaces of R 4+k. The complex Grassmannians Gr 2 (C 2+k ) of 2-dimensional complex vector subspaces of C 2+k. Exercise: 1. Construct a quaternion-kähler structure Q on these spaces in a similar way, by using the natural identifications: T x Gr4 (R 4+k ) = Hom R (x, x ) T x Gr 2 (C 2+k ) = Hom C (x, x ) 2. Show that the natural complex structure of Gr 2 (C 2+k ) is not a section of Q. 6

7 Basic property : Quaternion-Kähler manifolds are Einstein. (D. Alekseevsky 1967, S. Ishihara 1974, S. Salamon 1982) Three types: Positive type. Scal > 0 compact). (M is then Type zero. Scal = 0 : hyperkähler holonomy in Sp(k) SO(4k). Negative type. Scal < 0. In the sequel of the talk, we only consider qk manifolds of positive type. 7

8 Definition 2 The twistor space of a qk manifold is the sphere bundle Z = S(Q) of Q, for a suitable norm. Equivalently, Z is the 2-sphere bundle over M, whose fiber at each point x of M is the set of those complex structures of the tangent space T x M which belong to Q x. Basic properties: (S. Salamon 1982) 1. Z admits a canonical integrable almost complex structure J. 2. The horizontal distribution, H, on Z induced by the Levi-Civita connection determines a holomorphic contact structure θ on Z, with values in the holomorphic line bundle L = TM/H : θ (dθ) k is a nowhere vanishing holomorphic section of K Z L k+1, hence determines an isomorphism: L k+1 = K 1 Z. 3. Z admits a Kähler-Eistein metric, of positive scalar curvature. 8

9 It follows that Z is a Fano contact manifold, of (complex) dimension 2k + 1, admitting a Kähler-Eintein metric. Conversely: Theorem 1 (C. LeBrun 1995) Any Fano contact manifold admitting a Kähler-Einstein metric necessarily of positive scalar curvature is the twistor space of a qk manifold. Alternative proof by A. Morioianu in 1998 in the case when Z is spin (in particular when k is odd. ) 9

10 Dimension 4: Definition 3 An oriented (complete) four-dimensional Riemannian manifold (M, g) is called quaternion-kähler if: 1. g is Einstein, and 2. the conformal class [g] is anti-self-dual, i.e. W + = 0. We then have Q = A + M the bundle of self-dual skew-symmetric endomorphisms, and the twistor space Z coincides with the Atiyah Hitchin Singer twistor space. Examples: 1. The standard sphere S 4 2. The complex projective space CP 2, equipped with the Fubini-Study metric and the reverse standard orientation. 10

11 The Wolf spaces (J. A. Wolf 1965) 1. Classical Wolf spaces: HP k = Sp(k + 1)/Sp(k) Sp(1), Gr 4 (R 4+k ) = SO(4 + k)/so(k) SO(4), Gr 2 (C 2+k ) = SU(2 + k)/s(u(k) U(1)) 2. Exceptional Wolf spaces: G 2 /SO(4), F 4 /Sp(3)Sp(1), E 6 /SU(6)Sp(1), E 7 /Spin(12)Sp(1), E 8 /E 7 Sp(1) All are symmetric spaces of compact type, one for each compact simple Lie group G. For each G, the corresponding twistor space has a two-fold description: 1. The G-adjoint orbit of the highest root iθ in g (J. A. Wolf 1965), 2. the projectivization P(O min ) of the minimal nilpotent orbit in g C = the unique closed orbit in P(g C ) (A. Beauville 1998) 11

12 So far, Wolf spaces are the only known examples of qk manifolds of positive type. Current conjectures: C 1. All qk manifolds of positive type are Wolf spaces (C. LeBrun S. Salamon) C 2. All Fano contact manifolds are of the form P(O min ) (A. Beauville) Notice: C 2 contains the additional conjecture: C 3 : All Fano contact manifolds admit a Kähler-Einstein metric. Known results (so far): 1. The conjecture C 1 has been proved if k = 1 (N. Hitchin 1981, Friedrich Kurke 1982) and k = 2 (Poon Salamon 1991, LeBrun Salamon 1994). As far as I know, C 1 has remained open for k > 2, and conjectures C 2, C 3 for k For any k, there are only finitely many qk manifolds of positive type (LeBrun-Salamon, op. cit., relying on works of Mori, Vi snevski etc... on Fano manifolds). 12

13 Compatible almost complex structures on qk manifolds of positive type An almost complex structure J on a qk manifold (M, g, Q) is called compatible if J is a section of Q. The following theorem was established in 1998 by D. Alekseevsky, S. Marchiafava and M. Pontecorvo (with a participation of P. Piccinni): Theorem 2 Quaternion-Kähler manifolds of positive type have no globally defined compatible almost complex structure. Equivalently, the twistor fibration Z π M has no global section. Recall that the natural complex structure of the complex Grassmannians Gr 2 (C 2+k ) is not compatible with the quaternion-kähler structure. 13

14 The main goal of this talk is to provide a proof of the following theorem: Theorem 3 Quaternion-Kähler manifolds of positive type have no globally defined almost complex structure, except for the complex Grassmannians Gr 2 (C 2+k ). Previously known results: 1. Quaternionic projective spaces HP k have no almost complex structures (F. Hirzebruch 1953, k 2, 3, W. S. Massey 1962) 2. Grassmannians Gr 4 (R 4+k ) have no almost complex structure, except, possibly, for Gr 4 (R 8 ) and Gr 4 (R 10 ) (P. Sankaran 1991, Z.-Z. Tang 1994). 3. S 4 and CP 2 have no almost complex structure (e.g. use the criterion χ + τ 0 mod 4 for 4-dimensional compact almost complex manifolds). 14

15 Proof of Theorem 3. By the above, we can assume that k 2. The proof then relies on the Atiyah Singer index theorem and on the following facts: Fact 1. Any qk manifold M of positive type has b 2 (M) = 0, except for the complex Grassmannians Gr 2 (C 2+k ) (C. LeBrun S. Salamon, op. cit.) We can then assume b 2 (M) = 0. Fact 2. For any qk manifold M, the complexified tangent bundle T C M is locally of the form: where: T C M = H C E H = rank 2 complex vector bundle induced by the standard representation of Sp(1) on H = C 2, E = rank 2k complex vector bundle induced by the standard representation of Sp(k) on H k = C 2k. 15

16 Beware however that, except for HP k, neither H nor E are defined globally, only the quotients H/±1 and E/±1 are (S. Salamon 1982). Recall that the holonomy group is Sp(k)Sp(1) = Sp(k) Sp(1)/±1, not Sp(k) Sp(1). Fact 3. A qk manifold M of positive type and of dimension n = 4k is spin iff either M = HP k or k is even (S. Salamon 1982) If so, the spinor bundle of M is of the form by setting ΣM = p+q=k Rp,q, R p,q = Sym p H Λ q 0 E where Sym p H denotes the p-th symmetric complex tensor power of H and Λ q 0 E denotes the primitive part of Λ q E wrt the (complex) symplectic structure of E (cf. e. g. Kramer Semmelmann Weingart 1999). 16

17 The twisted spinor bundle ΣM R p,q is then globally defined even if M is not spin iff p + q + k is even. The corresponding (twisted) Dirac operator is then denoted by D p,q. Fact 4. Assume that p + q + k is even, so that ΣM R p,q and D p,q are globally defined on M. Then, the index of D p,q is given by 1. indd p,q = 0 if p + q < k 2. indd p,q = ( 1) q (b 2q 2 (M) + b 2q (M)) (LeBrun Salamon op. cit., Semmelmann Weingart 1982). if p + q = k. 17

18 Fact 5: Atiyah-Singer Index Theorem. In general, for any (oriented, even-dinensional) manifold M and for any complex vector bundle V over M, the index of the twisted Dirac operator D V : Γ(Σ + M V ) Γ(Σ M V ) is given by the following index formula: ) indd V = (ch(v ) Â(TM) [M] where Σ ± M denote the spinor bundles of M. This still makes sense and holds true whenever ΣM V is globally defined, even if M is not spin and V is only defined up to ±1. 18

19 End of the proof of Theorem 3. Assume that M is a qk manifold of positive type and apply the index formula to V = T C M Sym k 2 H = E H Sym k 2 H (recall, we assumed k 2). By using the Clebsch-Gordan decomposition of H Sym k 2 H, we get V = R k 1,1 R k 3,1 Notice that ΣM R k 1,1 and ΣM R k 3,1 are both globally defined hence also ΣM V since they both satisfy the rule p + q + k is even (equal to 2k and 2k 2 respectively). We then have indd V = indd k 1,1 + indd k 3,1 = 1 by using Fact 4 with b 2 (M) = 0, ( ) = ch(t C M) ch(sym k 2 H) Â(TM) [M] by using Fact 5 (index formula). 19

20 Notice that ch(sym k 2 H) is well-defined, even if Sym k 2 H is not (if so, define ch(sym k 2 H) )1 as (ch(sym k 2 H) 2 2 ). Now, assume, for a contradiction, that M admits an almost complex structure, i.e. that T M = T is a complex vector bundle. Then T C M = T T = T T. Since H is (complex) symplectic, hence self-dual, the term ch(sym k 2 H) Â(TM) in the rhs of the index formula only involves terms of degree 4l. Since ch j (T ) = ( 1) j ch j (T) components of degree 2j and M is of dimension 4k, in the above index formula we may replace T C by T T, hence ch(t C ) by 2 ch(t). 20

21 It follows that indd V = 2 ) (ch(sym k 2 H) ch(t) Â(TM) [M]. On the other hand, ( ) ch(sym k 2 H) ch(t) Â(TM) [M] is the index of the twisted Dirac operator acting on sections of Σ + M Sym k 2 H T, which is globally defined on M (as k 2 + k = 2k 2 is even). It is then an integer, so that indd V must be even. This contradicts indd V = 1, hence completes the proof of Theorem 3. 21

22 Weakly complex qk manifolds: Definition 4 A manifold M is weakly complex or stably complex if TM R l can be given a structure of complex vector bundle, where R l here stands for the trivial vector bundle of rank l. By using an easy variant of the above argument, we get Theorem 4 Quaternion-Kähler manifolds of positive type of dimension 4k, k 2, are not weakly complex. Proof: In the last step of the above argument, assume, for a contradiction, that T R l is a complex vector budle, for some integer l, and use as a the twisting bundle V = (TM R l ) C Sym k 2 H instead of V = T C M Sym k 2 H. Then, observe that the additional term in the index formula is l indd k 2,0, which is 0 by Fact 4. Notice: S 4 is weakly complex. 22

23 Inner symmetric spaces of compact type Wolf spaces are all irreducible inner symmetric spaces of compact type, i.e. are of the form G/H, where G is a compact simple Lie group and rkh = rkg, i.e. a maximal torus of H is a maximal torus of G. Inner symmetric spaces of compact type are even-dimensional. The irreducible, simply-connected ones are: 1. The Wolf spaces 2. The irreducible Hermitian symmetric spaces = the adjoint orbits of compact simple Lie groups 3. The even-dimensional spheres S 2l 4. The real Grassmannians Gr 2p (R 2p+n ) 5. The quaternionic Grassmannians Gr k (H k+l ) 6. The Cayley projective space F 4 /Spin(9) 7. The two exceptional inner symmetric spaces E 7 /(SU(8)/Z 2 ) and E 8 /(Spin(16)/Z 2 ). 23

24 The techniques used above can be extended to a large class of inner symmetric spaces, in addition to Wolf spaces covered by Theorem 3, to get: Theorem 5 An irreducible, simply-connected, inner symmetric space of compact type of dimension 4k is weakly complex if and only if it is a sphere S 4k or a Hermitian symmetric space (of even complex dimension). Remark 1. Spheres of all dimensions are weakly complex, whereas Hermitian symmetric spaces of all (complex) dimensions are complex manifolds. Remark 2. Our approach is ineffective for non-inner symmetric spaces of compact type, as the index of any homogeneous Dirac operator defined on such spaces is zero (R. Bott 1965). 24

25 Theorem 5 covers all inner symmetric spaces of compact type, except for: 1. The real Grassmannians of the form Gr 2p (R 2p+q ), with p and q both odd 2. The exceptional inner symmetric space E 7 /(SU(8)/Z 2 ), which is of dimension 70 Since the case of all even-dimensional real Grassmannians, except for Gr 4 (R 8 ), Gr 6 (R 12 ) and Gr 4 (R 10 ), was covered by different methods by P. Sankaran and Z.-Z. Tang op. cit., the only remaining open question about the existence issue of a (weak) complex structure on inner symmetric spaces of compact type is: Question. Does there exists a (weak) complex structure on the exceptional inner symmetric space E 7 /(SU(8)/Z 2 )? 25

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