# Riemannian Curvature Functionals: Lecture III

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1 Riemannian Curvature Functionals: Lecture III Jeff Viaclovsky Park City Mathematics Institute July 18, 2013

2 Lecture Outline Today we will discuss the following: Complete the local description of the moduli space of Einstein metrics. Rigidity of Einstein metrics and discussion of some basic examples. Quadratic functionals Hitchin-Thorpe and examples of Einstein metrics in dimension 4. Self-dual metrics and their deformation theory.

3 Moduli space of Einstein metrics Recall from the previous lecture that our goal is to prove: Theorem Assume g is Einstein with Ric(g) = λ g and λ < 0. Then the space of Einstein metrics near g modulo diffeomorphism is locally isomorphic to the zero set of a map where Ψ : H 1 H 1, H 1 = {h S 2 (T M) : δ g h = 0, tr g h = 0, h + 2Rm h = 0}

4 The nonlinear map Given θ C 2,α (S 2 T M), consider the map by P : C 2,α (S 2 (T M)) C 0,α (S 2 (T M)) P g (θ) = Ric(g + θ) λ (g + θ) 1 2 L g+θβ g θ. Last time we showed the linearization is elliptic. Next, we will examine the zero set of P.

5 The zero set of P Proposition Assume that λ < 0. If θ C 3,α is sufficiently near g and satisfies P (g + θ) = 0, then Ric(g + θ) = λ(g + θ), and θ C. To prove this, apply the operator β g+θ to the equation P (g + θ) = 0 to obtain β g+θ L g+θ β g θ = 0 A computation shows that this is equivalently (exercise): ( g+θ + Ric(g + θ))(β g θ) = 0. Since θ is sufficiently small in C 2,α norm, and Ric(g) is strictly negative definite, then Ric g+θ is also strictly negative definite. Pairing with β g θ and integrating by parts then shows that β g θ = 0.

6 An exercise Exercise Prove the regularity statement. Hint: Letting g = g + θ, in harmonic coordinates, the Ricci tensor can be written in the form Ric kl ( g) = 1 2 gij 2 ij g kl + Q kl ( g, g) where Q( g, g) is an expression that is quadratic in g, polynomial in g and has g in its denominator. Use a bootstrap argument in these coordinates. Show that we only need to assume that θ C 2,α. Hint: instead of differentiating in the first step, integrate by parts.

7 A converse We have already seen that zeroes of P are Einstein metrics. Now we have a converse: Proposition If g is an Einstein metric near g with Einstein constant λ, then there exists a diffeomorphism ϕ : M M such that θ = ϕ g g satisfies P g ( θ) = 0.

8 we obviously obtain a zero of P g. Proof of proposition The proof uses a modified Ebin-Palais slice theorem: Lemma For each metric g 1 in a sufficiently small C l+1,α -neighborhood of g (l 1), there is a C l+2,α -diffeomorphism ϕ : M M such that satisfies θ ϕ g 1 g β g ( θ) = 0 The proof of the lemma is almost identical to the previous one, and is omitted. To prove the proposition, if g is Einstein then ϕ g is also Einstein. Since β g ( θ) = β g (ϕ g g) = 0,

9 Structure of nonlinear terms Let us expand P g (θ) = P g (0) + P g(θ) + Q g (θ) Proposition For θ 1, θ 2 sufficiently small, we have Q(θ 1 ) Q(θ 2 ) C 0,α C( θ 1 C 2,α + θ 2 C 2,α) θ 1 θ 2 C 2,α. The proof is left as an exercise, with the following hint: Show that Rm(g + h) = Rm(g) + (g + h) 1 2 h + (g + h) 2 h h. Contract with (g + h) 1 to get Ric(g + h) and then use (g + h) 1 = g 1 g 1 (g + h) 1 h.

10 Implicit function theorem Lemma Let H : E F be a smooth map between Banach spaces. Define Q = H H(0) H (0). Assume that there are positive constants C 1, s 0, C 2 so that the following are satisfied: (1) The nonlinear term Q satisfies Q(x) Q(y) F C 1 ( x E + y E ) x y E for every x, y B E (0, s 0 ). (2) The linearized operator at 0, H (0) : E F is an isomorphism with inverse bounded by C 2. If s and H(0) F are sufficiently small (depending upon C 1, s 0, C 2 ), then there is a unique solution x B E (0, s) of the equation H(x) = 0.

11 With the assumptions in the lemma, it follows that T is a contraction mapping, so a fixed point exists by the standard fixed point theorem (T n y 0 converges to a unique fixed point for any y 0 sufficiently small). Proof of implicit function theorem The equation H(x) = 0 expands to H(0) + H (0)(x) + Q(x) = 0. If we let x = Gy, where G is the inverse of H (0), then we have H(0) + y + Q(Gy) = 0, or y = H(0) Q(Gy). In other words, y is a fixed point of the mapping T : y H(0) Q(Gy).

12 Kuranishi map We next construct the map Ψ : H 1 H 1, whose zero set is locally isomorphic to the zero set of P : Consider H = Π P, where Π is projection to the orthogonal complement of H 1. This map now has surjective differential. Let G be a right inverse, i.e., H (0)G = Id. Given a kernel element x 0 H 1, the equation H(x 0 + Gy) = 0 expands to H(0) + H (0)(x 0 + Gy) + Q(x 0 + Gy) = 0. We therefore need to find a fixed point of the map T x0 : y H(0) Q(x 0 + Gy), and the proof is the same as before.

13 Kernel of the linearized operator To finish the proof of the theorem, we need to identify the kernel of the linearized operator: Proposition If λ < 0, then Ker(P ) consists exactly of transverse-traceless tensors satisfying h + 2Rm h = 0. If P (h) = 0, then h is smooth by elliptic regularity, Also, P h = Ric (h) λh 1 2 L gβ g h. Applying β g to this equation, yields β g L g β g h = 0, so β g h = 0 by the above argument. Taking a trace, we find that so tr g (h) = 0 since λ < 0. (tr g (h)) + 2λ tr g (h) = 0,

14 Rigidity of Einstein metrics Elements in the space H 1 are called infinitesimal Einstein deformations. In general it is quite difficult to construct the map Ψ explicitly, but one of the easiest consequences of the above discussion is the following: Corollary If Ric(g) = λ g with λ < 0, and H 1 = {0} then g is rigid (isolated as an Einstein metric). That is, if g t is a path of Einstein metrics passing through g, all with Einstein constant λ < 0, then there exist a path of diffeomorphisms ϕ t : M M such that g t = ϕ t g.

15 The negative case In the hyperbolic case, in Lecture I we proved that H 1 = {0}, so hyperbolic manifolds are locally rigid as Einstein metrics. In fact, something much stronger is true: Theorem (Besson-Courtois-Gallot) If (M 4, g) is compact and hyperbolic, then g is the unique Einstein metric on M, up to scaling. This is proved using completely different methods than we have discussed here, which we will not have time to go into. Exercise Show that any Einstein metric with negative sectional curvature is rigid, that is, H 1 = {0}.

16 The positive case It is possible to modify the above construction so that it works also in the positive Einstein case, but we will omit the details due to time considerations. We will just mention two problems that come up: A positive Einstein metric can admit a nontrivial group of isometries (identity component). However, this is not a serious problem; the end result is that the map Ψ is equivariant with respect to the isometry group, and the actual moduli space is described by Ψ 1 (0)/Isom(g), rather than just Ψ 1 (0). In the case of the sphere, we run into the problem of first nontrivial eigenfunctions yielding pure trace kernel. However, these can also be gauged away since they arise as tangents to conformal diffeomorphisms. Details will be omitted.

17 Some rigid examples in the positive case Some rigid examples: Any spherical space form S 4 /Γ with the round metric g S. In this case, we saw in Lecture I that 1 h h, 2 which obviously has trivial kernel. S 2 S 2 with the product metric g S 2 + g S 2. In this case, we saw in Lecture I that the first two eigenvalues of 1 2 h + Rm h are 1 and 1, thus 0 does not occur as an eigenvalue (CP 2, g F S ), where g F S is the Fubini-Study metric. We will not have time to prove this case in these lectures, since the nicest proof involes the theory of deformations of Kähler-Einstein metrics, and needs a considerable amount of background in complex geometry.

18 The zero case We saw that, in the case of a flat metric, H 1 consists of parallel sections. The Kuranishi map turns out to be identically zero in this case, since all of these are integrable, corresponding to deformations of the flat structure. Exercise Determine the moduli space of flat structures on a 2-torus. Another special class of Ricci-flat metrics are Calabi-Yau metrics. In this case, the following is known (we will not discuss the proof): Theorem (Tian-Todorov) For a Calabi-Yau metric (X, g), the Kuranishi map Ψ 0. That is, every infintesimal Einstein deformation integrates to an actual deformation.

19 Quadratic curvature functionals A basis for the space of quadratic curvature functionals is W = W 2 dv, ρ = Ric 2 dv, S = R 2 dv. In dimension four, the Chern-Gauss-Bonnet formula 32π 2 χ(m) = W 2 dv 2 Ric 2 dv R 2 dv implies that any one of these can be written as a linear combination of the other two (plus a topological term). Remark We are using the tensor norm on W 2, that is W 2 = W ijkl W ijkl. This differs from the norm of W as a mapping on 2-forms by a factor of 4.

20 Curvature in dimension 4 If (M 4, g) is oriented, then Λ 2 = Λ 2 + Λ 2, and the curvature operator has the corresponding decomposition: R = W + + R 12 I E E W + R 12 I, where E = Ric (R/4)g is the traceless Ricci tensor. Theorem (Hirzebruch) 48π 2 τ(m) = M W + 2 dv W 2 dv, M where τ = b + 2 b 2 is the signature of M.

21 Hitchin-Thorpe Inequality One of the only known obstructions to the existence of Einstein metrics is the following: Theorem (Hitchin-Thorpe) If (M 4, g) is Einstein and oriented, then 2χ(M) 3 τ(m), with equality if and only if g is flat or finitely covered by K3 with a Ricci-flat metric. Exercise Show that this inequality follows from the Chern-Gauss-Bonnet formula and Hirzebruch signature theorem.

22 Some examples of Einstein metrics in dimension 4 The only known compact examples with non-negative Einstein constant: S 4 or RP 4 with the round metric. S 2 S 2 with the product metric, its orientable Z/2Z quotient, and RP 2 RP 2 with the product metric. CP 2 with the Fubini-Study metric. CP 2 #CP 2 with the Page metric (an explicit U(2) invariant Einstein metric). Non-orientable quotient CP 2 #RP 4. CP 2 #2CP 2 with the Chen-LeBrun-Weber metric. Conformal to an extremal Kähler metric. CP 2 #kcp 2, k = 3,..., 8, admits positive Kähler-Einstein metrics (Tian-Yau, Tian). K3 with Calabi-Yau Ricci-flat metric (Yau), and its quotients. T 4 with a flat metric, and quotients. Remark Find a new compact positive Einstein or Ricci-flat metric in dimension 4, and you will be famous.

23 Anti-self-dual or self-dual metrics In dimension 4, the curvature condition W + = 0 is called anti-self-dual (ASD), W = 0 is called self-dual (SD). Either condition is conformally invariant. Proposition If (M 4, g 0 ) compact and self-dual, then g 0 is a global minimizer of the functional W, so is necessarily a critical metric for W. Proof. For any metric g on M, using the signature theorem, we have W(g) = W 2 dv g = W + 2 dv g + W 2 dv g = 48π 2 τ(m) + 2 W 2 dv g 48π 2 τ(m), with equality if and only if g is self-dual. M

24 Topological obstruction The only obvious topological obstruction to existence of a self-dual or anti-self-dual metric comes from the signature theorem: Proposition If (M, g) is self-dual (anti-self-dual) then τ 0 (τ 0) with equality if and only if g is locally conformally flat. One can get a stronger restriction if one assumes the scalar curvature is positive: Proposition If (M, g) is self-dual (anti-self-dual) and R > 0, then b 2 = 0 (b + 2 = 0). Proof: On 2-forms in dimension 4, the Weitzenböck formula is ( H ω ± ) ij = ( ω ± ) ij l,m W ± lmij ω± lm + R 3 ω± ij.

25 Deformation theory of self-dual metrics We first define Hc 0 (M, g) = {κ T M K g κ = 0}. If g is self-dual, we let D g denote (W ) g. We then define Hc 1 (M, g) = {h S0(T 2 M) D g h = 0, δ g h = 0}. We also define Hc 2 (M, g) = {Z S0(Λ 2 2 ) DgZ = 0}. Theorem If g is self-dual, then the moduli space of self-dual metrics near g (modulo diffeomorphism and conformal equivalence), is locally isomorphic to the zero set of a map Ψ : H 1 c (M, g)/h 0 c (M, g) H 2 c (M, g)/h 0 c (M, g).

26 The Index Theorem If (M, g) is compact and self-dual, then dim(hc 0 ) dim(hc 1 ) + dim(hc 2 ) = 1 (15χ(M) 29τ(M)). 2 This is proved using the Atiyah-Singer Index Theorem, applied to the elliptic complex Γ(T M) Kg Γ(S 2 0(T M)) Dg Γ(S 2 0(Λ 2 +)). Remark The H i c(m, g), for i = 0, 1, 2, defined above are simply the cohomology groups of this complex.

27 Weitzenbock formulas For (M, g) Einstein, with Ric = λ g, define the Lichnerowicz Laplacian by A computation shows that Theorem (Itoh) L h ij = h ij + 2R ipjq h pq 2λ h ij. If (M, g) is compact and self-dual Einstein with Ric = λ g, then Hint: D Dh = 1 2 ( L + 2λ )( L + 4 ) 3 λ h DD Z = 1 (3 8λ)( 2λ)Z. 12 (D Z) ij = 2 k l Z ikjl.

28 Exercises Compute H i c(m, g) for the following examples: S 4 with the round metric g S. CP 2 with the Fubini-Study metric g F S (this is self-dual with respect to the complex orientation). S 1 S 3 with the product metric g (this is not Einstein, so you cannot use the Weitzenbock formulas. But it is locally conformally flat). What is the dimension of the moduli space near g? It is known that n#cp 2 admits self-dual metrics for any n 1 (Donaldson-Friedman, Floer, LeBrun, Poon). What is the index for any self-dual metric on n#cp 2?

29 A conjecture We mention here the following: Conjecture (Singer 1978) If (M, g) is self-dual and R > 0, then H 2 c (M, g) = 0. A wealth of examples of self-dual metrics have been found since this conjecture was made, and all of the ones with positive scalar curvature have turned out to have H 2 c (M, g) = 0. But despite all of the evidence, a proof of this conjecture remains elusive.

30 where ω is the Kähler form. (Note: this implies that W + 2 = R 2 /6 for any Kähler metric). K3 metric The K3 surface is defined to be a nondegenerate quartic surface in CP 3, that is K3 = {[z 0, z 1, z 2, z 3 ] CP 3 z z z z 4 3 = 0}. Topology: π 1 (K3) = {e}, b 2 = 22, b + 2 = 3, b 2 = 19. Since c 1 (K3) = 0, by Yau s solution of the Calabi conjecture, K3 admits a Ricci-flat Kähler metric g Y. Proposition (K3, g Y ) is anti-self-dual with respect to the complex orientation. To see this we use that fact that for any Kähler metric, W + is entirely determined by the scalar curvature. In fact, W + = R (3ω ω I), 12

31 K3 exercise Using the Index Theorem, and the Weitzenbock formulas from above, show that: dim(h 0 c (K3, g Y )) = 0. dim(h 1 c (K3, g Y )) = 57. dim(h 2 c (K3, g Y )) = 5. In fact, using the isomorphism S 2 0 (T M) = Λ 2 + Λ 2, it can be shown that H 1 c (K3, g Y ) has a basis {ω I ω j, ω J ω j, ω K ω j }, where ω I, ω J, ω K are a basis of the space of self-dual harmonic 2-forms (these are actually Kähler forms for complex structures I, J, K), and {ωj, j = 1,..., 19} is a basis of the space of anti-self-dual harmonic 2-forms. Furthermore, by the Weitzenbock formula, Hc 1 = HE 1 (infinitesimal Einstein deformations) and the moduli space is exactly 57 dimensional; the Kuranishi map Ψ 0 by Tian-Todorov.

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