A surgery formula for the smooth Yamabe invariant

A surgery formula for the smooth Yamabe invariant B. Ammann 1 M. Dahl 2 E. Humbert 3 1 Universität Regensburg Germany 2 Kungliga Tekniska Högskolan, Stockholm Sweden 3 Université Henri Poincaré, Nancy France Conference on spectral theory, Potsdam 2008

Einstein-Hilbert functional History Surgery Main results Topological conclusions Comments on the proofs The numbers L(n, k)

Einstein s equation 1915: Einstein s General Relativity. What is the equation that describes evolution of spacetime (without matter)? Answer: Einstein metrics where κ is constant. Ric = κg,

Lagrangian formulation What is the associated Lagrange functional? Let M be the set of all semi-riemannian metrics on M n. The Einstein equations are stationary points of the Einstein-Hilbert-functional E : M R, M g scal g dv g ( M dv g) (n 2)/n David Hilbert, 1915 among compactly supported perturbations.

The Setting Now: Let M be n-dimensional, n 3. Search for an Einstein metric on M. This are the stationary points of E : M 1 R, g M 1 := {metrics on M of volume 1}. [g] := {u 4/(n 2) g vol(u 4/(n 2) g) = 1}. M scal g dv g

Observation Inside a conformal class Y (M, [g]) := inf E( g) >. g [g] The conformal Yamabe invariant. Y (M, [g]) Y (S n ) where S n is the sphere with the standard structure. On the set of conformal classes σ(m) := sup Y (M, [g]) (, Y (S n )] [g] M 1 The smooth Yamabe invariant, also called Schoen s σ-constant. Remark σ(m) > 0 if and only if M caries a metric of positive scalar curvature.

Yamabe s idea To find stationary points of E: Find a minimizer in each conformal class, i.e. g [g] 1 with E( g) = Y (M, [g]). Such metrics always exist, and are called Yamabe metrics. They are unique if Y (M, [g 0 ]) < 0. Find a maximizing conformal class [g max ], i.e. Y (M, [g max ]) = σ(m). Hope: The minimaximizer g max is Einstein.

Example CP 2 The Fubini-Study g FS metric is Einstein and 53, 31... = E(g FS ) = Y (CP 2, [g FS ]) = σ(cp 2 ). Claude LeBrun 97 Similar examples σ(s n ) = n(n 1)ω 2/n n. Gromov & Lawson, Schoen & Yau 83: Tori R n /Z n. σ(r n /Z n ) = 0. LeBrun 99: All Kähler-Einstein surfaces with non-positive scalar curvature. Bray & Neves 04: RP 3. σ(rp 3 ) = 2 2/3 σ(s 3 ). Perelman, M. Anderson 06: compact quotients of 3-dimensional hyperbolic space

Dimension n = 3 Einstein 3-manifolds with κ = 0 are flat. Einstein 3-manifolds with κ > 0 are quotients of spheres. Einstein 3-manifolds with κ < 0 are quotients of hyperbolic spaces. Examples where Yamabe s idea fails There is no Einstein metric on non-prime 3-manifolds and S 2 S 1. Schoen: σ(s n 1 S 1 ) = σ(s n ). The supremum is not attained. Other problem In the case σ(m) < 0 the minimizers are unique. Hence, if a maximizing conformal class exists, then the unique minimizing metric in that class is Einstein. However, in the case σ(m) > 0, a minimizing metric in a maximizing class may be non-einstein!

Some known values of σ All examples above. Akutagawa & Neves 07: Some non-prime 3-manifolds, e.g. σ(rp 3 #(S 2 S 1 )) = σ(rp 3 ). Compact quotients of nilpotent Lie groups: σ(m) = 0. Unknown cases Nontrivial quotients of spheres, except RP 3. S k S m, with k, m 2. No example of dimension 5 known with σ(m) 0 and σ(m) σ(s n ).

Relations to similar invariants...... to the spectrum of the conformal Laplacian ( inf λ 1 4 n 1 ) { g + scal g Y (M, [g]) if Y (M, [g]) 0 = g [g] n 2 if Y (M, [g]) < 0... to the Perelman-invariant (Akutagawa, Ishida, LeBrun 06) λ(m) := sup λ 1 (4 g + scal g ) g M 1 { σ(m) if σ(m) 0 = + if σ(m) > 0... to the L n/2 -norm of scal inf scal g L g M n/2 ( g) = 1 { σ(m) if σ(m) 0 0 if σ(m) > 0 = Hence min{σ(m), 0} is determined by an infimum.

Surgery Let Φ : S k B n k M be an embedding. We define M Φ k := M \ Φ(Sk B n k ) (B k+1 S n k 1 )/ where / means gluing the boundaries via M Φ(x, y) (x, y) S k S n k 1. We say that M Φ k is obtained from M by surgery of dimension k. Example: 0-dimensional surgery on a surface.

Known surgery formulas Theorem (Gromov & Lawson 80, Schoen & Yau 79) If 0 k n 3, then σ(m) > 0 = σ(m Φ k ) > 0. Theorem (Kobayashi 87) If k = 0, then σ(m Φ 0 ) σ(m). Theorem (Petean & Yun 99) If 0 k n 3, then σ(mk Φ ) min{σ(m), 0}. The proof uses the characterization of min{σ(m), 0} as an infimum.

Main results Theorem (ADH, # 1) Let 0 k n 3. There is a positive constant L(n, k) depending only on n and k such that σ(mk Φ ) min{σ(m), L(n, k)} if Mk Φ is obtained from M by k-dimensional surgery. Furthermore L(n, 0) = Y (S n ). This theorem implies all three previously known surgery formulas. Thm # 1 follows directly from Thm # 2. Theorem (ADH, #2) Suppose that Mk Φ is obtained from M by k-dimensional surgery. Then for any metric g on M there is a sequence of metrics g i on such that M Φ k min {Y (M, [g]), L(n, k)} lim inf i Y (MΦ k, [g i]) Y (M, [g]).

Topological conclusions L(n) := min{l(n, 1), L(n, 2),..., L(n, n 3)}, σ(m) := min{σ(m), L(n)}. Let Mk Φ be obtained from M by surgery of dimension k {2, 3,..., n 3}, then σ(m) = σ(m k Φ). Goal: Find a bordism invariant!

Let Γ be a finitely presented group. Let Ω spin n (BΓ) the spin cobordism group over BΓ. Any class in Ω spin n (BΓ) has a π 1 -bijective representative, i.e. it is represented by (M, f ), where M is a connected compact spin manifold, and where f : M BΓ induces an isomorphism π 1 (M) π 1 (BΓ) = Γ. Lemma n 5. Let (M 1, f 1 ) and (M 2, f 2 ) be spin cobordant over BΓ and let (M 2, f 2 ) be π 1 -bijective. Then σ(m 2 ) σ(m 1 ). We define s Γ ([M, f ]) := max σ(n) where the maximum runs over all (N, h) with [M, f ] = [N, h] Ω spin n (BΓ).

σ := min{σ(m), L(n)} s Γ ([M, f ]) := σ(m) if (M, f ) is π 1 -bijective s Γ : Ω spin n (BΓ) (, L(n)] Corollary Let n 5 and ɛ (0, L(n)). The groups {x Ω spin n (BΓ) s Γ (x) > ɛ} and {x Ω spin n (BΓ) s Γ (x) ɛ} are subgroups.

Application Let n 5. Take M n with σ(m) (0, L(n)). Let p, q N be relatively prime. Then or σ(m# #M) = σ(m) }{{} p times σ(m# #M) = σ(m). }{{} q times Are there such manifolds M? Schoen conjectured: σ(s n /Γ) = σ(s n )/(#Γ) 2/n (0, L(n)) for #Γ large.

Proof of Theorem # 2 Theorem (ADH, #2) Suppose that Mk Φ is obtained from M by k-dimensional surgery. Then for any metric g on M there is a sequence of metrics g i on such that M Φ k min {Y (M, [g]), L(n, k)} lim inf i Y (MΦ k, [g i]) Y (M, [g]). Easy part: Difficult and important part lim inf i Y (MΦ k, [g i]) Y (M, [g]) min {Y (M, [g]), L(n, k)} lim inf i Y (MΦ k, [g i]) For this, we construct a sequence explicitly and study the behavior of the minimizers in [g i ] for i.

Construction of the metrics Let Φ : S k B n k M be an embedding. We write close to S := Φ(S k {0}), r(x) := d(x, S) g g S + dr 2 + r 2 g n k 1 round where g n k 1 round is the round metric on S n k 1. t := log r. 1 r 2 g e2t g S + dt 2 + g n k 1 round We define a metric g for r > r 1 1 g i = g for r (2ρ, r r 2 0 ) f 2 (t)g S + dt 2 + g n k 1 for r < 2ρ that extends to a metric on M Φ k. round

g i = g g i = F 2 g S n k 1 has constant length

Proof of Theorem #2, continued Any class [g i ] contains a minimizing metric written as u 4/(n 2) i g i. We obtain a PDE: u i > 0, 4 n 1 n 2 g i u i + scal g i u i = λ i ui n+2 n 2 u 2n/(n 2) i dv g i = 1, λ i = Y ([g i ]) This sequence might: Concentrate in at least one point. Then lim inf λ i Y (S n ). Concentrate on the old part M \ S. Then lim inf λ i Y ([g]). Concentrate on the new part. Then study pointed Gromov-Hausdorff limits. Limit spaces: M c := H k+1 c S n k 1 H k+1 c : simply connected, complete, K = c 2

The numbers L(n, k) L(n, k) := inf c [0,1] Y (Hk+1 c S n k 1 ) Note: H k+1 1 S n k 1 = S n \ S k. k = 0: L(n, 0) = Y (R S n 1 ) = Y (S n ) k = 1,..., n 3: L(n, k) > 0 Conjecture #1: L(n, k) = Y (R k+1 S n k 1 ) Conjecture #2: For determining Y (H k+1 c S n k 1 ) it suffices to consider O(k + 1) O(n k) invariant functions.

If both conjectures are true, then L(4, 1) = 59, 4... Y (S 4 ) = 61, 5... Y (S 2 S 2 ) = 50, 2... One obtains S 2 S 2 via 1-dimensional surgery from S 4. Hence σ(s 2 S 2 ) 59, 4 > 50, 2... = Y (S 2 S 2 ).