# Infinitesimal Einstein Deformations. Kähler Manifolds

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1 on Nearly Kähler Manifolds (joint work with P.-A. Nagy and U. Semmelmann) Gemeinsame Jahrestagung DMV GDM Berlin, March 30, 2007

2 Nearly Kähler manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Part I Basics on nearly Kähler manifolds

3 Nearly Kähler manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Definition. An almost Hermitian manifold (M, g, J) with fundamental form ω := g(j.,.) is nearly Kähler (or NK) if ( X J)X = 0, X TM. M is strictly nearly Kähler (SNK) if X J 0, X TM. The projection of the Levi-Civita connection on the space of Hermitian connections canonical Hermitian connection : X := X 1 2 J X J. Its torsion T = J( J) is totally skew-symmetric. A SNK manifold is neither symplectic nor Hermitian. The obstruction is given by J : dω = 3 J, N J = J( J).

4 Nearly Kähler manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Properties : J (and hence T ) is parallel wrt. in dimension 4 : NK = Kähler in dimension 6 : a NK manifold is either Kähler or SNK and : carries a -parallel SU 3 -structure : (g, J, dω). carries Killing spinors Einstein with positive scalar curvature.

5 Nearly Kähler manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Theorem. An almost Hermitian manifold (M 6, g, J) is SNK iff the Riemannian cone M := (M R +, t 2 g + dt 2 ) satisfies Hol( M) G 2. Idea of the proof : define a generic 3-form (in the sense of Hitchin) : ϕ := t 2 dt ω t3 dω and check that ω is parallel (M 6, g, J) is NK. Corollary. The sphere (S 6, can) is SNK, wrt the almost complex structure induced by ω = e e e e e e e 356.

6 Examples of NK manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Kähler manifolds. Twistor spaces over QK manifolds with Scal > 0, endowed with the non-integrable almost complex structure J. Naturally reductive 3-symmetric spaces : Homogeneous spaces G/H, with g = h m, where H is the fix point set of an automorphism σ of G of order 3 which defines an almost complex structure on m by the relation σ = 1 2 Id J ; Example : G = K K K, H = K (diagonal). In dimension 6 : S 6 = G2/SU 3 CP 3 = Sp 2 /S 1 Sp 1, F (1, 2) = SU 3 /S 1 S 1 S 3 S 3 = SU 2 SU 2 SU 2 /

7 Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Canonical connection of a G-structure Let (M n, g) be an oriented Riemannian manifold and G SO n compact. A G-structure on M reduction to G of the ON frame bundle. If G is the stabilizer of some element of a SO n representation, G-structure section of the associated vector bundle. Example : G = U m SO 2m. The Levi-Civita connection is not a G-connection in general. Its projection on the space of G-connections : canonical connection. Cleyton and Swann studied special G-structures, i.e. satisfying : The holonomy of the canonical connection acts irreducibly on R n. The torsion of is totally skew-symmetric and -parallel.

8 The Cleyton-Swann theorem Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Recall the following : Theorem. (Berger, Simons) Simply connected Riemannian manifolds with irreducible holonomy : manifolds with holonomy U m, SU m, Sp k, Sp k Sp 1, G 2, Spin 7. Symmetric spaces with irreducible isotropy. Theorem. (Cleyton, Swann) The special G-structures are : manifolds with weak holonomy SU 3 or G 2. homogeneous spaces with irreducible isotropy.

9 Decomposition of NK manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification A NK manifold is a U m -structure which automatically satisfies the second condition of special G-structures. The first condition (irreducibility) is essential in the Cleyton-Swann theorem. Theorem. (Nagy) Every NK manifold is locally a Riemannian product of Kähler manifolds. Twistor spaces over positive QK manifolds. Naturally reductive 3-symmetric spaces. 6-dimensional NK manifolds.

10 Homogeneous NK manifolds Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Corollary. The classification of homogeneous NK manifolds reduces to the dimension 6. Theorem. (Butruille) Every homogeneous SNK manifold is a naturally reductive 3-symmetric space. Algebraic problem. Difficulty : when the isotropy is small, the system becomes very complex. Most difficult case : S 3 S 3 (quadratic system involving 72 unknowns)... Recall the analogous result in QK setting Theorem. (Wolf, Alekseevski) Every homogeneous QK manifold is symmetric.

11 Further results Basic properties Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Theorem. (Belgun-M., Nagy) A compact SNK manifold whose canonical Hermitian connection has complex reducible holonomy is isomorphic to CP 3, F (1, 2) (or to a twistor space over a positive QK manifold in dimension greater than 6). Theorem. (M.-Nagy-Semmelmann) A compact 6-dimensional SNK manifold carrying unit Killing vector fields is isomorphic to S 3 S 3.

12 NK versus QK Basic properties Definition and first properties Examples of NK manifolds G-structures Decomposition of NK manifolds Attempts of classification Similarities between NK and QK geometries : nearly Kähler M 6 quaternion Kähler M 4k curvature R = R CY + R S6 R = R hyper + R HPk Einstein yes yes homogeneous classification 3-symmetric (Butruille) symmetric (Wolf, Alekseevski) other examples no no rigidity... (M.-N.-S.) yes (LeBrun-Salamon)

13 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem Part II

14 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem SU 3 structures Definition. Let M be a smooth 6-dimensional manifold. A SU 3 structure on M is a reduction of the frame bundle of M to SU 3. It consists of a 5-tuple s := (g, J, ω, ψ +, ψ ) where g is a Riemannian metric, J is a compatible almost complex structure, ω is the corresponding fundamental 2-form ω(, ) = g(j, ) and ψ + + iψ is a complex volume form of type (3, 0). Definition. Infinitesimal SU 3 deformation (ġ, J, ω, ψ +, ψ ) of s : tangent vector at 0 to a path of SU 3 structures s t with s 0 = s. Lemma. Let Sym M denote the bundle of g-symmetric endomorphisms of M anti-commuting with J. Natural isomorphism Sym M Λ (2,1)+(1,2) 0 M : S S ψ + where denotes the Lie algebra extension of an endomorphism of TM to the tensor bundle.

15 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem SU 3 structures Proposition. 1-1 correspondence between infinitesimal SU 3 deformations and sections of TM Sym M Λ (1,1) M R, given by (ξ, S, ϕ, µ) (ġ, J, ω, ψ +, ψ ) : (1) ġ = g((h + S), ) J = JS + ψ + ξ ω = ϕ + ξ ψ + ψ + = ξ ω + λψ + + µψ 1 2 S ψ + ψ = Jξ ω µψ + + λψ 1 2 S ψ with g(h, ) = ϕ(, J ), λ = 1 4 tr(h) and g(ψ+ ξ, ); = ψ+ (ξ,, ).

16 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem Gray structures A Gray structure on a 6-dimensional manifold M is a SNK structure with scal = 30. Proposition. (Hitchin, Reyes-Carrión) A Gray structure is a SU 3 structure (g, J, ω, ψ +, ψ ) which satisfies the exterior differential system { dω = 3ψ + (2) dψ = 2ω ω At the infinitesimal level, the tangent vector at t = 0 to a curve of Gray structures (g t, J t, ω t, ψ t +, ψt ) is an infinitesimal SU 3 deformation (ġ, J, ω, ψ +, ψ ) which satisfies the linearized system : { d ω = 3 ψ +, (3) dψ = 4 ω ω.

17 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem Infinitesimal Gray deformations Definition. The set of (ġ, J, ω, ψ +, ψ ) satisfying (1) and (3) is called virtual tangent space of Gray structures. Infinitesimal Gray deformations : the quotient of the virtual tangent space by the action (by Lie derivative) of vector fields. Lemma. The Laplace operator leaves invariant the space of co-closed primitive (1, 1)-forms Λ (1,1) 0 M. Denote by E(λ) the λ-eigenspace of its restriction. Theorem. Assume that (M, g) is a Gray manifold not isometric to the round sphere (S 6, can). Then the space of infinitesimal Gray deformations is isomorphic to E(12).

18 Deformations of SU 3 structures Deformations of NK structures Proof of the theorem Idea of the proof Since M is not the sphere, the full Gray structure is determined by the metric. By the Ebin slice theorem, infinitesimal Gray deformations correspond to solutions of (1) and (3) with δġ = 0 and tr(ġ) = 0. Show that µ = 0 and ξ = 0. The system (1), (3) becomes { dϕ = 3 2 S ψ +, δ(s ψ + ) = d(s ψ ) = 2 d ψ =8 ( ω ω)= 8ϕ Thus ϕ E(12). The converse follows from the fact that if ϕ is a co-closed form in Ω (1,1) 0 M then dϕ Ω (2,1)+(1,2) 0 M.

19 The curvature operator The Lichnerowicz Laplacian The main result What next Part III

20 The curvature operator The curvature operator The Lichnerowicz Laplacian The main result What next Let be a connection (not necessarily torsion-free) on some manifold M. For every 2-form α, the curvature tensor of maps α to an endomorphism R(α) of TM, which induces an endomorphism R(α) of the tensor bundles of M as before. On every tensor bundle one defines the curvature endomorphism q(r) := k α k R(α k ), where α k is an ON basis of Λ 2 M. If for some G SO n happens to be a G-connection whose curvature tensor satisfies the symmetry by pairs, the curvature endomorphism commutes with all G-equivariant endomorphisms between tensor bundles, so in particular it leaves invariant all -parallel sub-bundles.

21 The Lichnerowicz Laplacian The curvature operator The Lichnerowicz Laplacian The main result What next Example : If is the Levi-Civita connection, q(r) = Ric on Λ 1. Let (M 6, g, J) be a Gray manifold. Aim : Characterize the infinitesimal Einstein deformations of the metric g. Theorem. (Lichnerowicz) Let (M n, g) be a compact Einstein manifold with scalar curvature scal. The moduli space of infinitesimal Einstein deformations of g is isomorphic to the set of symmetric endomorphisms H of TM such that tr(h) = 0, δh = 0 and L H = 2scal/nH, where L is the Lichnerowicz Laplacian, related to the curvature operator q(r) by : L = + q(r).

22 The statement Basic properties The curvature operator The Lichnerowicz Laplacian The main result What next Theorem. Let (M 6, g, J) be a compact Gray manifold. Then the moduli space of infinitesimal Einstein deformations of g is isomorphic to the direct sum E(2) E(6) E(12). Recall that E(λ) denotes the space of primitive co-closed (1, 1)-eigenforms of the Laplace operator for the eigenvalue λ. Outline of the proof. Main tool : the canonical Hermitian connection. Step 1. Lichnerowicz theorem the moduli space of Einstein deformations of g is isomorphic to the set of H SymM with δh = 0 = trh such that ( + q(r))h = 10H. ( )

23 Main steps of the proof The curvature operator The Lichnerowicz Laplacian The main result What next Step 2. Let h := pr + H and S := pr H denote the projections of H onto Sym ± M. Define the primitive (1, 1)-form ϕ(.,.) := g(jh.,.) and the 3-form σ := S ψ +. Express the equation ( ) in terms of (need to compute the difference of the rough Laplacians and the difference q(r) q( R)). Step 3. Using the naturality of q( R), transform the equation into a system involving ϕ and σ : ( + q( R))ϕ = 4ϕ δσ, ( + q( R))σ = 6σ 4dϕ, δϕ = 0.

24 Main steps of the proof The curvature operator The Lichnerowicz Laplacian The main result What next Step 4. Transform this back into an exterior differential system : ϕ = 4ϕ δσ, σ = 6σ 4dϕ, δϕ = 0. Step 5. Define an explicit isomorphism from the space of solutions of this system to E(2) E(6) E(12). (ϕ, σ) Ψ (8ϕ + δσ, dσ, 2ϕ δσ) E(2) E(6) E(12) with inverse (α, β, γ) E(2) E(6) E(12) Φ ( α + γ 10 ) 3dα 5 dβ 2dγ,. 30

25 Possible developments The curvature operator The Lichnerowicz Laplacian The main result What next In order to apply this result, one should try to compute the spectrum of the Laplacian on 2-forms on some explicit compact nearly Kähler 6-dimensional manifolds. Besides the sphere S 6 which has no infinitesimal Einstein deformations the only known examples are the two twistor spaces CP 3 and F (1, 2) and the homogeneous space S 3 S 3 S 3 /S 3. Preliminary computations on this last manifold seem to indicate that 12 belongs to the spectrum of the Laplacian. There is therefore some hope for obtaining nearly Kähler deformations of this homogeneous metric, at least at the infinitesimal level.

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