The hyperk hler geometry of the deformation space of ...

The hyperkählergeometry of CP(S)

Brice Loustau

Complex projectivestructures

The character variety

The Schwarzianparametrization

The minimal surfaceparametrization

The hyperkähler geometry of the deformation space ofcomplex projective structures on a surface

Brice Loustau

August 3, 2012

Brice Loustau

Outline

1 Complex projective structures

2 The character variety

3 The Schwarzian parametrization

4 The minimal surface parametrization

Brice Loustau

What is a complex projective structure?

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Let S be a closed oriented surface of genus g > 2.

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Definition

A complex projective structure on S is a (G ,X )-structure on S

where the model space is X = CP1 and the Lie group of

transformations of X is G = PSL2(C).

Brice Loustau

Definition

A complex projective structure on S is a (G ,X )-structure on S

where the model space is X = CP1 and the Lie group of

transformations of X is G = PSL2(C).

Brice Loustau

CP(S) and Teichmüller space T (S)

Brice Loustau

Definition

CP(S) is the deformation space of all complex projective structureson S :

CP(S) = {all CP1-structures on S}/Diff +

0 (S) .

A point Z ∈ CP(S) is called a marked complex projective surface.

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Definition

0 (S) .

CP(S) is a complex manifold of dimension dimC CP(S) = 6g − 6.

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Definition

0 (S) .

Note: A complex projective atlas is in particular a complex atlas onS (transition functions are holomorphic).

Brice Loustau

Definition

0 (S) .

Note: A complex projective atlas is in particular a complex atlas onS (transition functions are holomorphic).

Definition

There is a forgetful map p : CP(S) → T (S) where

T (S) = {all complex structures on S}/Diff+0 (S)

is the Teichmüller space of S .

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Fuchsian and quasifuchsian structures

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If any Kleinian group Γ (i.e. discrete subgroup of PSL2(C)) actsfreely and properly on some open subset U of CP

1, the quotientinherits a complex projective structure.

Brice Loustau

If any Kleinian group Γ (i.e. discrete subgroup of PSL2(C)) actsfreely and properly on some open subset U of CP

1, the quotientinherits a complex projective structure.

Brice Loustau

Fuchsian structures

Brice Loustau

Fuchsian structures

In particular, any Riemann surface X can be equipped with acompatible CP

1-structure by the uniformization theorem:

X = H2

where Γ ⊂ PSL2(R) is a Fuchsian group.

Brice Loustau

Fuchsian structures

In particular, any Riemann surface X can be equipped with acompatible CP

1-structure by the uniformization theorem:

X = H2

where Γ ⊂ PSL2(R) is a Fuchsian group.

Note: This defines a Fuchsian section σF : T (S) → CP(S).

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Quasifuchsian structures

Brice Loustau

By Bers’ simultaneous uniformization theorem, given two complexstructures (X+,X−) ∈ T (S)× T (S), there exists a unique Kleiniangroup Γ such that:

Brice Loustau

By Bers’ simultaneous uniformization theorem, given two complexstructures (X+,X−) ∈ T (S)× T (S), there exists a unique Kleiniangroup Γ such that:

Brice Loustau

Holonomy

Brice Loustau

Holonomy

Any complex projective structure Z ∈ CP(S) defines a holonomy

representation ρ : π1(S) → G = PSL2(C).

Brice Loustau

Holonomy

Any complex projective structure Z ∈ CP(S) defines a holonomy

representation ρ : π1(S) → G = PSL2(C).

Brice Loustau

Holonomy defines a map

hol : CP(S) → X (S ,G) ;

where X (S ,G) = Hom(π1(S),G)//G is the character variety of S .

Brice Loustau

hol : CP(S) → X (S ,G) ;

where X (S ,G) = Hom(π1(S),G)//G is the character variety of S .hol is a local biholomorphism.

Brice Loustau

hol : CP(S) → X (S ,G) ;

By a general construction of Goldman, the character variety X (S ,G)enjoys a natural complex symplectic structure ωG .

Brice Loustau

hol : CP(S) → X (S ,G) ;

By a general construction of Goldman, the character variety X (S ,G)enjoys a natural complex symplectic structure ωG .

Abusing notations, we also let ωG denote the complex symplecticstructure on CP(S) obtained by pulling back ωG by the holonomymap hol : CP(S) → X (S ,G).

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The character variety (continued)

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Theorem (Goldman)

The restriction of the complex symplectic structure on the Fuchsianslice F(S) is the Weil-Petersson Kähler form:

F (ωG ) = ωWP .

Brice Loustau

Theorem (Goldman)

The restriction of the complex symplectic structure on the Fuchsianslice F(S) is the Weil-Petersson Kähler form:

F (ωG ) = ωWP .

Theorem (Platis, L)

Complex Fenchel-Nielsen coordinates (li , τi ) associated to any pantsdecomposition are canonical coordinates for the symplectic structure:

ωG =∑

dli ∧ dτi .

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Hitchin-Kobayashi correspondence

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Theorem (Hitchin, Simpson, Corlette, Donaldson)

Fix a complex structure X on S . There is a real-analytic bijection

HX : X 0(S ,G)∼→ M0

Dol(X ,G)

where M0

Dol(X ,G) is the moduli space of topologically trivialpolystable Higgs bundles on X .

Brice Loustau

HX : X 0(S ,G)∼→ M0

Dol(X ,G)

where M0

Note: HX is not holomorphic, in fact:

Brice Loustau

HX : X 0(S ,G)∼→ M0

Dol(X ,G)

where M0

Note: HX is not holomorphic, in fact:

Theorem (Hitchin)

There is a natural hyperkähler structure (g , I , J,K) on M0

Dol(X ,G).The map HX is holomorphic with respect to J. It is also asymplectomorphism for the appropriate symplectic structures.

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The cotangent hyperkähler structure

Brice Loustau

Recall that if M is any complex manifold, its holomorphic cotangentbundle T ∗M is equipped with a canonical complex symplecticstructure ωcan.

Brice Loustau

Recall that if M is any complex manifold, its holomorphic cotangentbundle T ∗M is equipped with a canonical complex symplecticstructure ωcan.

Theorem (Feix, Kaledin)

If M is a real-analytic Kähler manifold, then there exists a uniquehyperkähler structure in a neighborhood of the zero section in T ∗M

such that:

• it refines the complex symplectic structure

• it extends the Kähler structure off the zero section

• the U(1)-action in the fibers is isometric.

Brice Loustau

The Schwarzian parametrization

Brice Loustau

Recall that there is a canonical holomorphic projectionp : CP(S) → T (S).

Brice Loustau

The Schwarzian derivative is an operator on maps betweenprojective surfaces such that:

Brice Loustau

• It turns a fiber p−1(X ) into a complex affine space modeledon the vector space H0(X ,K2) = T ∗

XT (S).

Brice Loustau

XT (S).

• Globally, CP(S) ≈σ T ∗T (S) but this identification depends onthe choice of a “zero section” σ : T (S) → CP(S).

Brice Loustau

XT (S).

• Globally, CP(S) ≈σ T ∗T (S) but this identification depends onthe choice of a “zero section” σ : T (S) → CP(S).

For each choice of σ, we thus get a symplectic structure ωσ on thewhole space CP(S) (pulling back ωcan) and a hyperkähler structureon some neighborhood of the Fuchsian slice.

Brice Loustau

The Schwarzian parametrization(continued)

Brice Loustau

Theorem (L)

CP(S) ≈σ T ∗T (S) is a complex symplectomorphism iffd(σ − σF ) = ωWP (on T (S)).

Brice Loustau

Theorem (L)

Using results of McMullen (also Takhtajan-Teo, Krasnov-Schlenker):

Theorem (Kawai, L)

If σ is a (generalized) Bers section, CP(S) ≈σ T ∗T (S) is a complexsymplectomorphism.

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Theorem (L)

Using results of McMullen (also Takhtajan-Teo, Krasnov-Schlenker):

Theorem (Kawai, L)

If σ is a (generalized) Bers section, CP(S) ≈σ T ∗T (S) is a complexsymplectomorphism.

Brice Loustau

Consequences:

Brice Loustau

Consequences:

• Fibers of p and Bers slices are Lagrangian complexsubmanifolds.

Brice Loustau

Consequences:

• (generalized) Quasifuchsian reciprocity.

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Consequences:

• If σ is elected among Bers sections,

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Consequences:

• The hyperkähler stucture we get on CP(S) refines the complexsymplectic structure,

Brice Loustau

Consequences:

• but the new complex structure J depends on the choice of theBers section. In other words the bunch of hyperkähler structureswe get is parametrized by T (S).

Brice Loustau

Consequences:

• This is similar to the situation we saw with theHitchin-Kobayashi correspondence.

Brice Loustau

Consequences:

• This is similar to the situation we saw with theHitchin-Kobayashi correspondence.Quiz : what is a significant difference though?

Brice Loustau

The minimal surface parametrization

Brice Loustau

The space of almost-Fuchsian structures AF(S) ⊂ QF(S) is aneighborhood of the Fuchsian slice such that if Z ∈ AF(S), thehyperbolic 3-manifold associated to Z contains a unique minimalsurface Σ.

Brice Loustau

The Gauss-Codazzi equations satisfied by the second fundamentalform IIΣ are equivalent to the fact that IIΣ is the real part of aunique holomorphic quadratic ϕ.

Brice Loustau

This defines a mapAF(S) → T ∗T (S)

Z 7→ ([IΣ], ϕ).

Brice Loustau

Z 7→ ([IΣ], ϕ).

It is a diffeomorphism of AF(S) onto some neighborhood of thezero section of T ∗T (S).

Brice Loustau

Z 7→ ([IΣ], ϕ).

It is a diffeomorphism of AF(S) onto some neighborhood of thezero section of T ∗T (S).

Again, one can use this “minimal surface parametrization” to pullback the hyperkähler structure of T ∗T (S) on CP(S).

Brice Loustau

The minimal surface parametrization(continued)

Brice Loustau

The notion of renormalized volume of almost-Fuchsian manifoldsdefines a function W on AF(S).

Brice Loustau

Using arguments of Krasnov-Schlenker to compute the variation ofW under an infinitesimal deformation of the metric, one shows:

Brice Loustau

Using arguments of Krasnov-Schlenker to compute the variation ofW under an infinitesimal deformation of the metric, one shows:

Theorem (L)

The minimal surface parametrization AF(S)∼→ T ∗T (S) is a real

symplectomorphism (for the appropriate symplectic structures).

The hyperk hler geometry of the deformation space of ...

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