|  Self-Dual Solutions of Einstein's Equation | 
In General > s.a. actions for general relativity;
  self-dual gauge fields; solutions of Einstein's equation.
  $ Riemannian metrics: (Anti-)self-dual
    Riemannian metrics are those whose Riemann tensor is (anti-)self-dual,
*Rabcd:= \(1\over2\)εabmn Rmncd = ± Rabcd ;
    This implies the same condition on the Weyl tensor and that Rab
    = 0, so all such metrics are solutions of the (Euclideanized) Einstein equation.
  $ Lorentzian metrics:
    (Anti-)self-dual Lorentzian metrics are those whose Riemann tensor
    is (anti-)self-dual up to multiplication by i,
*Rabcd:= \(1\over2\)εabmn Rmncd = ± i Rabcd ;
    Notice that the metrics must be complex; As in the Riemannian case,
    they are solutions of the (complex) Einstein equation.
  $ Conformally (anti-)self-dual
    solutions: Those whose Weyl tensor is (anti-)self-dual,
*Cabcd:= \(1\over2\)εabmn Cmncd = ± Cabcd ±i Cabcd (complex Lorentzian case).
  * History: The most general
    were found independently by Penrose's, Newman's and Plebański's groups.
  * Properties: If one gives data on
    \(\cal I\)− for (anti-)self-dual
    solutions, and one evolves them, one finds at \(\cal I\)+
    the same data, i.e., the classical S-matrix is trivial.
  * Complex Lorentzian: We cannot
    distinguish self-dual from anti-self-dual solutions, because we cannot resolve the
    sign ambiguity in εabcd by
    comparing it with a reference one; So we call all these solutions half-flat.
  @ Solutions: Devchand & Ogievetsky CQG(96)ht/94;
    García-Compeán & Matos PRD(95)ht/94;
    Koshti & Dadhich gq/94 [general solution];
    Rosly & Selivanov ht/97 [perturbiner];
    Calderbank & Pedersen JDG(02)m.DG/01 [2 Killing vector fields];
    Chacón & García-Compeán a1812 [via Hitchin's equations];
    > s.a. black holes in modified theories; wormhole solutions.
Alternative Characterizations, Relationships
  * Relationships: Self-dual
    gravity can be expressed in terms of the Moyal bracket [@ Strachan].
  * Triad: An equivalent formulation
    is obtained by giving, on a 3-slice Σ with π2(Σ) = 0, three linearly
    independent vectors eai,
    divergenceless with respect to some reference metric \(^0q\)ab, and
    evolving them by \(\dot e\)ai:=
    2−1/2 εijk
    [ej,
    ek]a;
    Then the metric gab:= ∑i
    eai
    ebi
    + ta tb
    is a solution of Einstein's equation (all self-dual metrics are locally like this);
    If the es become linearly dependent after a while, g  becomes
    degenerate, and we could have a model for topology change.
  * Connection variables:
    In Ashtekar variables, a self-dual metric gab is equivalent to
    the vanishing, FabAB = 0,
    of the curvature defined by
2 \(\cal D\)[a \(\cal D\)b] λA = FabAB λB, where the covariant derivative \(\cal D\) is defined by \(\cal D\)a εAB = 0 .
  @ Connection variables: Bengtsson CQG(90) [and Yang-Mills Hamiltonian].
  @ And integrable theories: Strachan JPA(96)ht [deformation and Toda lattice];
    Ueno MPLA(96)ht/95.
References
  > s.a. 3D gravity; bianchi IX models;
  born-infeld theory; conformal gravity;
  differential forms; perturbations in general relativity.
  @ General: Ashtekar JMP(86),
    in(86),
     in(88),
    et al CMP(88);
    Mason & Newman CMP(89)
      [self-dual Einstein and Yang-Mills theory];
    Koshti & Dadhich CQG(90);
    Kalitzin & Sokatchev PLB(91);
    Grant PRD(93)gq;
    Husain CQG(93),
    PRL(94)gq;
    Strachan CQG(93);
    Abe MPLA(95) [moduli spaces];
    Devchand & Ogievetsky CQG(96)ht/94;
    García-Compeán et al RMF(96)ht/94 [Hopf algebra structure];
    Tafel gq/06 [description];
    Jakimowicz & Tafel CQG(06)gq [Husain and Plebański equations];
    Mansi et al CQG(09)-a0808 [and 3+1 split];
    Malykh & Sheftel JPA(11)-a1011 [and the general heavenly equation].
  @ Symmetries and conservation laws:
    Boyer & Plebański JMP(85);
    Boyer & Winternitz JMP(89);
    Husain JMP(95);
    Strachan JMP(95)ht/94;
    Popov et al PLB(96)ht.
  @ Lagrangian: Plebański & Przanowski PLA(96)ht/95,
    García-Compeán et al PLA(96) [chiral approach, WZW-like action].
  @ Integrability of self-dual Einstein equation:
    in Penrose GRG(76);
    Nutku Sigma(07)n.SI [completely integrable];
    Nutku et al JPA(08)-a0802 [multi-Hamiltonian structure];
    Krasil'shchik & Sergyeyev a1901 [with non-zero cosmological constant].
  @ Deformations: Takasaki PLB(92),
    JGP(94);
    García-Compeán et al APPB(98)ht/97,
    ht/97-MG8.
  @ Of Einstein-Yang-Mills theory: Selivanov PLB(98)ht/97 [perturbiner];
    > s.a. solitons.
  @ Riemannian metrics:
    Torre JMP(90) [linearization stability];
    Malykh et al CQG(03)gq [on K3, anti-self-dual].
  @  Generalizations: Dunajski PRS(02)m.DG/01 [(+,+,–,–) metrics, with extra structure];
    Fino & Nurowski a1109
      [in 9D, based on an irreducible representation of SO(3) × SO(3)].
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