|  Constraint Equations in General Relativity | 
In General
  > s.a. canonical formulation [constraint algebra]; initial-value
  formulation; ADM and connection formulation.
  * Idea: They are the
    Einstein equation with one or more indices projected arthogonally
    to a spacelike hypersurface or, in differential geometry terms,
    just the Gauss-Codazzi equations with the Ricci tensor substituted
    for in terms of the matter stress-energy.
  * In terms of first and
    second fundamental form: For real, Lorentzian gravity
Gab qma nb = Da Kam − Dm K = 8πG Tab qma nb = 8πG jm
2 Gab na nb = 3R − Kab Kab + K2 = 16πG Tab na nb = 16πG ρ ,
    where N:= [−(∇a t)
    (∇a t)]−1/2
    is the lapse, na:= 
    −N∂a t is the unit normal to Σ,
    and K:= Kaa
    = qab Kab.
  * For complex / euclidean general relativity:
    Flip the signs of the two KK terms in the second constraint equation.
  @ General references:
    Moncrief PRD(72) [redundancy],
    & Teitelboim PRD(72) [Hamiltonian and diffeomorphism];
    Dittrich CQG(06)gq/05 [diff-invariant Hamiltonian constraints];
    Balasin & Wieland a0912
      [simple Hamiltonian constraint for specific value of the Barbero-Immirzi parameter];
    Mars GRG(13)-a1303 [for general hypersurfaces, and applications to  shells];
    Rácz CQG(16)-a1508 [as evolutionary systems].
  @ On closed / compact manifolds: Isenberg CQG(95) [constant mean curvature],
    & Moncrief CQG(96) [non-constant mean curvature];
    Choquet-Bruhat CQG(04)gq/03-in [compact nΣ];
    Maxwell JHDE(05)gq [rough/low-regularity];
    Holst et al PRL(08) [far-from-constant mean curvature],
    CMP(09)-a0712 [without near-CMC assumption];
    Dilts CQG(14)-a1310 [with boundary];
    Canepa et al AHP-a2010 [with null boundary].
Variables and Solution Methods
  * Lichnerowicz-York solution method:
    A conformal technique initiated by Lichnerowicz and perfected by York, the only
    efficient and robust method of generating consistent initial data; In the spatially
    compact case, the complete scheme consists of the ADM Hamiltonian and momentum
    constraints, the ADM Euler-Lagrange equations, York's constant-mean-curvature (CMC)
    condition, and a lapse-fixing equation (LFE) that ensures propagation of the CMC
    condition by the Euler-Lagrange equations; The variables are a conformal factor
    ψ, a spatial metric gij,
    a symmetric tensor Aij and a scalar
    τ, in terms of which the physical metric and extrinsic curvature are
hij = ψ4 gij, Kij = ψ−2 Aij + \(1\over3\)ψ4 gij τ ;
    The Hamiltonian constraint is rewritten as the Lichnerowicz-York
    equation for the conformal factor ψ of the physical metric
    ψ4 gij,
    given an initial unphysical 3-metric gij;
    The CMC condition and LFE introduce a distinguished foliation (definition of simultaneity)
    on spacetime, and separate scaling laws for the canonical momenta and their trace are used.
  @ General references: Isenberg & Marsden JGP(84) [York map];
    Maxwell CMP(05)gq/03 [constant mean curvature conformal method];
    Anderson et al CQG(05)gq/04 [physical degrees of freedom];
    Pfeiffer et al PRD(05) [stationary + gravitational wave];
    O'Murchadha APPB(05)gq [uses of Lichnerowicz-York equation];
    Tiemblo & Tresguerres GRG(06)gq/05 [and Poincaré gauge theory, single condition];
    Martín et al IJGMP(09)-a0709 [framework for solutions];
    Maxwell a0804 [freely-specified mean curvature];
    Corvino & Pollack a1102-fs;
    Maxwell CQG(14) [conformal method and conformal thin-sandwich method];
    Tafel GRG(15)-a1405 [(2+1)-decomposition approach];
    Cang AHP(15)-a1405 [application of fixed-point theorems];
    Alves a1603-wd
      [Hamiltonian constraint, reduction to a first-order equation];
    Anderson ATMP-a1812
      [Lichnerowicz-Choquet-Bruhat-York conformal method, in the far from CMC-regime];
    Canepa et al a2001 [tetrad variables].
  @ And conformal structure: Beig & Ó Murchadha CMP(96)gq/94 [and spatial conformal Killing vectors];
    Szabados CQG(02)gq/01 [Hamiltonian and Chern-Simons functional];
    Butscher CMP(07)gq/02 [conformal constraint equations];
    Pfeiffer gq/04-proc [conformal method];
    Pfeiffer & York PRL(05),
    Walsh CQG(07) [non-uniqueness of solutions];
    Gourgoulhon JPCS(07)-a0704 [and 3+1 numerical initial data];
    Holst et al a0708;
    Nguyen a1507 [non-existence and non-uniqueness results].
  @ Black holes: Brandt & Brügmann PRL(97) [multi-black-hole];
    Loustó & Price PRD(97),
    PRD(98) [data for binary collisions];
    Baker & Puzio PRD(99)gq/98 [axisymmetric];
    Dain et al PRD(05)gq/04 [multi-black-hole];
    Smith GRG(09) [horizon with prescribed geometry];
    Baumgarte PRD(12)-a1202.
  @ Binary black holes: Baumgarte PRD(00)gq;
    Marronetti et al PRD(00)gq,
    & Matzner PRL(00)gq [arbitrary P, L];
    Dain PRD(01)gq/00 [2 Kerr, head-on].
  @ Asymptotically hyperbolic: Isenberg & Park CQG(97)gq/96;
    Sakovich CQG(10)-a0910 [Einstein-scalar, constant mean curvature solutions];
    Gicquaud & Sakovich CMP(12)-a1012 [non-constant mean curvature].
  @ Rough / low-regularity solutions: Maxwell gq/04;
    Behzadan & Holst a1504 [asymptotically flat, non-CMC].
  @ Other solutions: Maxwell CMP(05)gq/03 [with apparent horizon boundaries];
    Choquet-Bruhat et al gq/05,
    CQG(07)gq/06,
    gq/06 [Einstein-scalar];
    Korzyński PRD(06)gq [on dynamical horizons];
    Huang CQG(10) [with prescribed asymptotics];
    Tafel & Jóźwikowski CQG(14)-a1312;
    > s.a. models in numerical relativity.
References
  > s.a. numerical relativity; canonical quantum gravity.
  @ Reviews: Bartnik & Isenberg gq/04-proc;
    Carlotto LRR(21).
  @ Gluing solutions: Isenberg et al CMP(02)gq/01 [and wormholes],
    AHP(03)gq/02;
    Isenberg gq/02-GR16;
    Chruściel et al CMP(05)gq/04,
    PRL(04)gq [more general];
    Isenberg et al ATMP(05)gq [with matter];
    Chruściel et al CMP(11)-a1004 [N-body initial-data sets];
    > s.a. solution methods.
  @ Space of solutions: Ó Murchadha CQG(87) [ADM energy as Morse function];
    Chruściel & Delay JGP(04)gq/03 [manifold structure];
    Dain gq/04-proc [black holes as boundaries];
    Rai & Saraykar a1605 [coupled scalar fields, Hilbert space structure];
    Holst et al a1711 [drift method, rev and applications].
  @ In other gravity theories: Jacobson CQG(11)-a1108
      [generally-covariant theories with tensor matter fields]. 
  @ Constraint propagation:
    Frittelli PRD(97) [and numerical evolution];
    York gq/98 [and canonical formalism];
    York a1512-wd [causal propagation].
  @ Related topics: Kuchař & Romano PRD(95)gq [sets that generate true Lie algebras];
    Frauendiener & Vogel CQG(05)gq/04 [instability of constraint surface];
    Gambini & Pullin GRG(05)gq-GRF [getting rid of constraints in discretization];
    Corvino & Schoen JDG(06) [vacuum, asymptotics];
    Bojowald et al PRD(06)gq [effective constraints from lqg];
    Szabados CQG(08)-a0711 [Hamiltonian constraint for Einstein-Yang-Mills theory as Poisson bracket];
    Ita a0904v5 [diffeomorphisms and Gauss' law];
    Holst & Kungurtsev PRD(11)-a1107 [bifurcation analysis of conformal formulations].
   Various theories: see formulations of general relativity;
   linearized general relativity; modified gravity.
 Various theories: see formulations of general relativity;
   linearized general relativity; modified gravity.
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