# How to recognise a conformally Einstein metric?

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1 How to recognise a conformally Einstein metric? Maciej Dunajski Department of Applied Mathematics and Theoretical Physics University of Cambridge MD, Paul Tod arxiv: , Comm. Math. Phys. (2014). Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

2 Conformal geometry and null geodesics Two (pseudo) Riemannian metrics g and ĝ = Ω 2 g on M (where Ω : M R + ) are conformally equivalent. Conformal rescalings preserve angles, and null geodesics: H = g ab p a p b, Ĥ = Ω 2 H, X H Hamiltonian vector field on T M, ω = dp a dx a Integral curve of the geodesic spray X H T*M Null geodesic of g. H=0 (M, g) Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

3 Conformal geometry and field equations Four dimensions ĝ ab = Ω 2 g ab, ˆɛ abcd = Ω 4 ɛ abcd, so ˆɛ cd cd ab = ɛ ab and so : Λ 2 (M) Λ 2 (M) is conformally invariant. Maxwell s equations df = 0, d F = 0 are conformally invariant. Einstein equations are NOT conformally invariant. Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

4 Conformal to Einstein (M, g) (pseudo) Riemannian n dimensional manifold with Levi Civita connection. Curvature decomposition: Riemann=Weyl+Ricci+Scalar [ a, b ]V c = R ab c d V d R abcd = C abcd + 2 n 2 (g 2 a[cr d]b g b[c R d]a ) (n 1)(n 2) Rg a[cg d]b. (M, g) is 1 Einstein, if R ab = 1 n Rg ab. 2 Ricci flat if R ab = 0. 3 Conformal to Einstein (Ricci flat) if there exists Ω : M R + such that ĝ = Ω 2 g is Einstein (Ricci flat). Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

5 Two problems in conformal geometry (M, g) Lorentzian four-manifold. Does there exist Ω : M R +, and a local coordinate system (x, y, z, t) such that ĝ = Ω 2 g = dt 2 + dx 2 + dy 2 + dz 2. Answer. Need C ab c d = 0 (Conformally invariant, as Ĉ = C). Given an n dimensional manifold (M, g), does there exist Ω : M R + such that Ω 2 g is Einstein? 1 n = 2. All metrics are Einstein. 2 n = 3. Einstein=constant curvature. Need Y abc = 0 (Cotton tensor). 3 n = 4?? Brinkman 1920s, Szekeres 1963, Kozameh Newman Tod 1985,... Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

6 Conformal gravity B bc := ( a d 1 2 Rad )C abcd, Bach E abc := C ef cb d ( C) afed ( C) ef cb d (C) afed, Eastwood-Dighton. Necessary conditions: B ab = 0, E abc = 0. Sufficient conditions (locally): B ab = 0, E abc = 0 + genericity on Weyl. Conformal gravity: C 2 vol g Euler-Lagrange B ab = 0. M Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

7 Anti-self-duality (genericity fails) An oriented 4-manifold (M, g) is anti-self-dual (ASD) if C abcd = 1 2 ɛ ab ef C cdef. B ab = 0, E abc = and yet (in the real analytic category) ASD depends on 6 functions of 3 variables. R ab = 0+ASD depends o 2 functions of 3 variables. Signature (4, 0) or (2, 2). (Lorentzian+ASD=conformal flatness). Question: Given a 4-manifold (M, g) with ASD Weyl tensor, how to determine whether g is conformal to a Ricci flat metric? Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

8 Twistor equation C T M = S S, where (S, ε), (S, ε ) are rank two complex symplectic vector bundles (spin bundles) over M. Spinor indices (love it or hate it): a = AA, where A, B, = 0, 1, and A, B, = 0, 1. Metric g = ε ε. Twistor operator. D : Γ(S ) Γ(T M S ). Conformally invariant if π A A(A π B ) g Ω 2 g, ε Ω ε, ε Ω ε, π Ω π. Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

9 Prolongation of the twistor equation Proposition. An ASD metric g is conformal to a Ricci flat metric if and only if dim(ker D) = 2. Prolongation π α ɛ = 0, α + π P = 0, where α Γ(S), and P ab = (1/12)Rg ab (1/2)R ab. Connection D on a rank-four complex vector bundle E = S S. Spinor conjugation in Riemannian signature: σ : S S. π = (p, q), σ(π) = ( q, p), so π Ker D σ(π) Ker D. Theorem 1. There is a one-to-one correspondence between parallel sections of (E, D) and Ricci flat metrics in an ASD conformal class. Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

10 Tensor Obstructions Integrability conditions for parallel sections: RΨ = 0 where R = [D, D] is the curvature (4 by 4 matrix), and Ψ = (π, α) T. If R = 0 then g is conformally flat. Otherwise differentiate: (DR)Ψ = 0, (DDR)Ψ = 0,... Necessary conditions: rank (R) = 2. Sufficient conditions: rank (R, DR) = 2. Set V a = 4 C 2 C bcd a e C bcde. Theorem 2. An ASD Riemannian metric g is conformal to a Ricci flat metric if and only if det(r) := 4 e C bcde f C bcdf V 2 C 2 = 0, and T ab := P ab + a V b + V a V b 1 2 V 2 g ab = 0. Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

11 Example Two parameter family of Riemannian metrics (LeBrun 1988) g = f 1 dr r2 (σ σ fσ 2 3), where f = 1 + A r 2 + B r 4, where σ j are 1-forms on SU(2) such that dσ 1 = σ 2 σ 3 etc. Kahler and vanishing Ricci scalar, so ASD. Conformal to Ricci flat? det(r) = 0, T ab = A(4B A2 ) (Ar 2 + 2B) 2 g ab. If A = 0 then g is Ricci flat (Eguchi Hanson). If B = A 2 /4 then ĝ = (2r 2 + A) 1 g is Ricci flat. ASD Taub NUT ( ) ĝ = U dr 2 + R 2 (σ1 2 + σ2) 2 + U 1 σ3, 2 where U = 1 R 4A. g is ASD Kahler, Ω 2 g is hyper-kahler! Are there more such examples?? (Clue: The Kahler form for g becomes a conformal Killing Yano tensor for ĝ). Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

12 Another example Left invariant metrics on a four dimensional nilpotent Lie group g = σ σ σ σ 3 2, where dσ 0 = 2σ 0 σ 3 σ 1 σ 2, dσ 1 = σ 1 σ 3, dσ 2 = σ 2 σ 3, dσ 3 = 0. Find C + = 0, but R ab 0. Conf. to. Einstein obstructions vanish. Let σ 3 = d ln(z) where z : M R +. ĝ = z 3 g Local coordinates (τ, x, y, z) s.t. ASD and Ricci-flat. σ 0 = z 2 (dτ + ydx), σ 1 = z 1 dx, σ 2 = z 1 dy, and ĝ = U(dx 2 + dy 2 + dz 2 ) + U 1 (dτ + α) 2, where (U = z, α = ydx) satisfy the monopole eq 3 du = dα on R 3. Domain Wall in (M R, g dt 2 ) (Gibbons+Rychenkova 2000, MD+Hoegner 2012). Approximate form of a regular metric on a complement of a cubic in CP 2. Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

13 Thank You! Dunajski (DAMTP, Cambridge) How to recognise a conformally Einstein metric? October / 13

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