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Spinor Structures on Compact Homogeneous Manifolds

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Spinor Structures on Compact Homogeneous Manifolds Spinor structures on compact homogeneous manifolds Dmitri Alekseevsky Institute for Information Transmission Problems RAS Differential Geometry in the Large Florence, July 11-15, 2016 This talk is based on a joint work with Ioannis Chrysikos Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Abstract We study spin structures on compact simply-connected homogeneous pseudo-Riemannian manifolds (M = G=H; g) of a compact semisimple Lie group G. We classify flag manifolds F = G=H of a compact simple Lie group which are spin. This yields also the classification of all flag manifolds carrying an invariant metaplectic structure. Then we give a description of spin structures on C-spaces, i.e. homogeneous principal torus bundles over flag manifolds. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Outline I. Basic facts on spin structures on a pseudo-Riemannian manifold II. Basic facts on flag manifolds II.1. Invariant complex structures and painted Dynkin diagrams. II.2. T -roots, isotropy decomposition and invariant metrics. II.3. Line bundles and characters. II.4. Chern form and Koszul numbers. III. Spin structure on flag manifolds III.1. Criterion for existence of spin structure on a flag manifold. III.2. Classification of spin flag manifolds of classical groups. III.3 Classification of spin flag manifolds of exceptional groups. IV. Spin structures on C-spaces Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Spinor group p;q Let V := R (n = p + q) be the pseudo-Euclidean vector space with the pseudo-Euclidean metric g = h ; i of signature (p; q) = (− · · · −; + ··· +). We denote by SO(V ) = SOp;q the pseudo-orthogonal group and by Spin(V ) = Spinp;q the spinor group. Recall that Spin(V ) = Spinp;q is a Z2 covering of SOp;q. It is defined as the subgroup of the group Cl(V )∗ of invertible even elements a of the Clifford algebra Cl(V ) which normalizes V ⊂ Cl(V ) ; aVa−1 = V . The natural representation −1 Spin(V ) 3 a 7! Ada; Adav := ava is a Z2-covering Ad : Spin(V ) ! SO(V ) = Spin(V )={±1g. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Spinor structure on a pseudo-Riemannian manifold (M; g) Let π : SO(M) ! M be the SO(V )-principal bundle of orthonormal frames of a pseudo-Roemannian manifold M. A Spinp;q-structure (shortly spin structure) on M is a Spinp;q-principal bundle π~ : Q = Spin(M) ! M, which is a Z2 covering Λ: Spin(M) ! SO(M) such that the following diagram is commutative: Spin(M) × Spinp;q / Spin(M) FF FF π~ Λ×Ad Λ FF FF π F# SO(M) × SOp;q / SO(M) / M Two spin structures (Q1; Λ1) and (Q2; Λ2) are said equivalent if there is an (Spinp;q-equivariant) isomorphism U : Q1 ! Q2 which commutes with the projections on SO(M). Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Existence of spin structure The tangent bundle of an oriented n-dimensional pseudo-Riemannian manifold (M; g) admits an orthogonal decomposition TM = η− ⊕ η+ into timelike subbundle η− s.t. gjη− < 0 and spacelike subbundle η+ s.t. gjη+ > 0: Conversely, any decomposition TM = η− ⊕ η+ is an orthogonal decomposition into timelike and spacelike subbundles for some metric. (Mn; g) is time-oriented (resp. space-oriented, oriented) if w1(η−) = 0 (resp. w1(η+) = 0; w1(M) = w1(η−) + w1(η+) = 0). Proposition An oriented pseudo-Riemannian manifold (M; g) admits a spin structure if and only is w2(η−) + w2(η+) = 0 or equivalently, w2(M) = w1(η−) ^ w1(η+), where η−; η+ are timelike and spacelike subbundles. If M is timelike or spacelike orientable, then this is equivalent to w2(M) = 0. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Case of almost complex manifold Assume that M is a compact manifold with a complex structure J. Then M is oriented and w2(M) = c1(M; J)(mod2). Moreover , the following result holds Theorem (Lawson-Michelsohn; Atiyah) Let M2n be a compact complex manifold with complex structure J. Then M admits a spin 2 structure if and only if the first Chern class c1(M) 2 H (M; Z) is 2 even, i.e. it is divisible by 2 in H (M; Z). Moreover, spin structures are in 1-1 correspondence with isomorphism classes of holomorphic ⊗2 line bundles L such that L = KM where KM is the canonical line bundle. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Invariant spin structures A spin structure π~ : Q ! M on a oriented homogeneous pseudo-Riemannian manifold (M = G=L; g) is called G-invariant if the natural action of G on the bundle SO(M) of oriented orthonormal frames of M, can be extended to an action on the Spinp;q-principal bundle π~ : Q = Spin(M) ! M. Invariant spin structures on reductive homogeneous spaces can be described in terms of lifts of the isotropy representation into the spin group. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Theorem(Cahen) Let (M = G=L; g) be an oriented homogeneous pseudo-Riemannian manifold with a reductive decomposition g = l + m. A lift #e : L ! Spin(q) of the isotropy representation # : L ! SO(m) onto the spinor group Spin(m) defines a G-invariant spin structure Q = G × Spin(q): #e Conversely, if G is simply-connected then any spin structure on (M = G=L; g) is invariant and is defined by a such lift. A spin structure (if it exists) is unique if M is simply connected. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Metaplectic structure 2n Let (V = R ;!) be the symplectic vector space and Sp(V ) = Spn(R) := Aut(V ;!) the symplectic group. The metaplectic group Mpn(R) is the unique connected (double) cover 2n of Spn(R) . For a symplectic manifold (M ;!), we denote by Sp(M) the Spn(R)-principal bundle of symplectic frames, i.e. frames e1; ··· ; en; f1; ··· ; fn such that !(ei ; ej ) = !(fi ; fj ) = 0;!(ei ; fj ) = δij . A metaplectic structure on a symplectic manifold (M2n;!) is a Mpn(R)-equivariant lift of the symplectic frame bundle Sp(M) ! M to a principal (Mp)n(R)-bundle Mp(M) ! M ( with respect to the double covering ρ : Mpn(R) ! SpnR). Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Existence of metaplectic structure The first Chern class of (M2n;!) is defined as the first Chern class of (TM; J), where J is a !-compatible almost complex structure. Since the space of !-compatible almost complex structures is contractible, c1(M;!) := c1(TM; J) is independent of J. Theorem(K. Habermann) A symplectic manifold (M2n;!) admits a metaplectic structure if and only if w2(M) = 0 or equivalently, the first Chern class c1(M;!) is even. In this case, the set of metaplectic structures on (M2n;!) stands in a bijective 1 correspondence with H (M; Z2). Invariant metaplectic structures A simply connected compact homogeneous symplectic manifold (M = G=H;!) (flag manifold) admits a metaplectic structure if and only if w2(M) = 0. Such structure is unique and invariant. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Basic facts about flag manifolds A flag manifold is an adjoint orbit F = G=H = AdG w ⊂ g = Te G of a compact connected semisimple Lie group G. A flag manifold F admits invariant complex, symplectic and K¨ahler structures. Infinitesimal description: The B-orthogonal reductive decomposition of F can be written as g = h + m = (Z(h) + h0) + m 0 We fix a Cartan subalgebra a = Z(h) + a ⊂ h and denote by RH , ;aC ;aC (resp., R = RH [ RF ) the root system of (hC ) ( resp., (gC )). Recall that roots take real values on the real form 0 0 0 a0 := ia = t + a0; t := iZ(h); a0 = ia of the Cartan subalgebra. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Reductive decomposition of gC C C C C g = h + m = (a + g(RH )) + g(RF ) P where for Q ⊂ R, we denote g(Q) = α2Q gα. Note that RH jt = 0 where t := iZ(h) ⊂ a0 is the real form of the center Z(h). The isotropy representation # : H ! Aut(m) in m = ToF is the restriction AdHjm of the adjoint representation. Dmitri Alekseevsky Institute for Information Transmission ProblemsSpinor RAS structures Differential on compact Geometry homogeneous in the Large manifolds Florence, July 11-15, 2016 Black and white simple roots and weights and painted Dynkin diagram (PDD) Let ΠW = fβ1; ··· ; βv g be the fundamental system of RH and Π = ΠW [ ΠB its extension to the fundamental system of R.
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