|  Formulations of General Relativity | 
As Minkowski Space Field Theory
  > s.a. perturbations; stress-energy pseudotensors.
  * Spin-2 field in Minkowski:
    Recast general relativity as a non-linear theory of the departure of the
    metric from ηab;
    This works at a linearized level, where one gets a spin-2 field theory, but such
    theories cannot describe global features such as different spacetime topologies.
  @ References: Ogievetsky & Polubarinov AP(65);
    in Weinberg 72;
    Penrose in(80),
    in(82);
    Weinberg & Witten PLB(80);
    Castagnino & Chimento GRG(80)-a1206;
    Zel'dovich & Grishchuk SPU(86);
    Nikolić GRG(99)gq;
    Straumann ap/00-conf;
    Trenčevski IJTP(11)gq/04 [2-form field and non-linear connection]; 
    Pitts & Schieve FP(04)gq [causality],
    FP(03)gq/04 [FLRW singularity];
    Padmanabhan IJMPD(08)gq/04 [no-go results];
    Notte-Cuello & Rodrigues IJMPD(07)mp/06 [Yang-Mills type];
    Nieuwenhuizen EPL(07)-a0704;
    Pitts & Schieve TMP(07) [massive];
    in Leclerc CQG(07)gq;
    Hacyan a0712 [historical];
    Baryshev AIP(06),
    a0809-proc [and tests];
    Notte-Cuello et al JPM(10)-a0907; & Nambu, Feynman, Thirring;
    Deser GRG(10) [and self-interactions];
    Rodrigues RPMP(12)-a1109-conf [intro, and legitimate energy-momentum tensor];
    Scharf a1208;
    > s.a. gauge fixing; phenomenology
      of gravity; theories of gravity.
Metric and Other Variables > s.a. einstein's equation
  [including approximation schemes]; quasi-local formulation.
  * Possibilities: General relativity
    has three fully equivalent representations as a theory of metric curvature, and
    torsion or non-metricity of a connection.
  * Metric variables: The degrees
    of freedom are the electric and magnetic parts of the Weyl tensor.
  * Metric + connection version:
    The Palatini formulation of general relativity, or metric-affine gravity.
  @ References:
    Krasnov ht/06 [metric and connection];
    Tamanini PRD(12)-a1205 [metric + two affine connections];
    Harada a2001 [?];
    Jiménez et al a1903 [three alternative formulations].
  > Other variables: see 1st-order action
    [Palatini]; affine connections [non-metricity]; other types
    of action; teleparallel theories [torsion].
As a Theory of Null Surfaces
  > s.a. 3D general relativity.
  * Variables: Surfaces
    Z(xa; ζ,
    ζ*), where ζ and ζ* are stereographic
    coordinates on S2 and parametrize the different
    null surfaces through each point (there is an S2's
    worth, but only 9 are independent), and a conformal factor Ω.
  * And metric: One can
    construct explicitly a conformal metric and Ω from the condition
    that g'ab
    ∇a
    Z ∇b
    Z = 0 for all (ζ, ζ*), and they
    satisfy Gab = 0
    or Λgab; One
    naturally gets a complex g', but it is easy to impose reality.
  * Drawbacks: The equations are
    messy; Their nature (hyperbolic, elliptic?) is not known, and only some non-local
    solutions are known; There is a large, not really understood, gauge freedom.
  * Quantization: Might suggest to
    quantize only Ω, but not really believed.
  @ References: Kozameh et al AP(91);
    Kozameh & Newman GRG(91),
    in(91),
    et al JGP(92);
    Iyer et al JGP(96)gq/95 [holonomies and light cone cut functions];
    Frittelli et al JMP(95)gq,
    JMP(95)gq,
    JMP(95)gq,
    JMP(95),
    PRD(97),
    JMP(00);
Bordcoch et al a1201.
Other Versions > s.a. actions; canonical
  [including ADM] and initial-value formulation; einstein's equation;
  gravity theories; semiclassical gravity.
  * Motivation: Look for
    hyperbolic formulation to show that evolution is well-posed.
  * Euclideanized: It has no
    asymptotically Euclidean non-trivial solution in any dimension, with any topology
    [@ in Witten CMP(81)].
  @ Theory of embeddings:
    Deser et al PRD(76);
    Regge & Teitelboim in(77);
    Gibbons & Wiltshire NPB(87)ht/01;
    Paston & Franke TMP(07)-a0711 [canonical];
    Faddeev a0906 [action];
    Paston & Semenova IJTP(10)-a1003 [canonical, constraint algebra];
    Paston TMP(11)-a1111;
    Willison a1311 [Cauchy problem];
    Sheykin & Paston AIP(14)-a1402;
    Paston et al G&C(17)-a1705 [canonical];
    > s.a. branes; friedmann cosmology;
      quantum gravity; spherical symmetry.
  @ Group manifold: D'Adda et al AP(85);
    Regge PRP(86)-proc;
    Nelson & Regge IJMPA(89).
  @ Discretized versions: Boström et al gq/93;
    Regge & Williams JMP(00)gq;
    Castellani & Pagani AP(02)ht/01;
    Gambini & Pullin in(03)gq/01;
    Holfter  & Paschke JGP(03)ht/02 [Dirac operator];
    Huang et al a2006 [on graphs];
    > s.a. lattice gravity; regge calculus.
  @ Elastic / plastic deformations: Kokarev gq/02-GRF [elastic bending of spacetime];
    Fernández & Rodrigues 10 [distorted Lorentz vacuum];
    > s.a. spacetime structure.
  @ As a theory of paths: Teitelboim NPB(93), ZNA(97);
    Padmanabhan IJMPD(19)-a1908;
    > s.a. lines; loop variables;
      Paths.
  @ Other variables: Jadczyk IJTP(79) [conformal structure + scalar density];
    Grant CQG(96)gq/94 [volume-preserving vector fields];
    Godina et al GRG(00)gq/99 [2-spinor + Dirac fields];
    Novello JCAP(07)gq [two spinors];
    Barnett a1412 [gravitational field tensor].
  @ Related topics: Essén IJTP(90)
      [conformally invariant scalar gauge field theory];
    Bonanos JMP(91) [matrix-valued differential forms];
    't Hooft NPB(91) [chiral];
    Wallner JMP(95);
    Anderson gq/99 [no need for metric];
    Atkins a0803
      [cohomological version, in terms of cochain complex of (n+2)-tensors];
    Maharana a1004
      ['t Hooft's chiral alternative to the vierbein];
    Gomes et al CQG(11)-a1010 [as a 3D conformally invariant theory];
    Obukhov & Hehl PPN(14) [fundamental spinors];
    Vey CQG(15)-a1404 [n-plectic vielbein gravity];
    Adamo IJMPD(15)-a1505 [as a 2D CFT];
    Hehl et al IJMPD(16)-a1607-conf [pre-metric formulation, and electromagnetism];
    Herfray a1807-th [chiral, twistor, 3-forms].
  > Other: see 2D gravity;
    3D gravity; 3D general relativity; Cartan
    Geometry; conformal invariance; Ether;
    Faddeev Formulation; gravitational thermodynamics;
    higher-dimensional gravity; Modified Gravity [MOG];
    modified theories; Observers; quantum
    gravity; Shape Dynamics; simplex;
    unified theories; Weyl Space.
Formalism, Techniques
  > s.a. emergent gravity [analog models]; complex
  structures; geometry; hamilton-jacobi theory.
  @ And global differential geometry:
    Fischer & Marsden GRG(74);
    Eguchi, Gilkey & Hanson PRP(80);
    > s.a. differentiable manifolds.
  @ Global methods: Geroch GRG(71),
    in(71);
    Penrose 72;
    > s.a. causality; singularities.
  @ Metric from geodesics:
    Hojman & Rodrigues PLA(91);
    > s.a. riemann tensor
      [gab from curvature].
  @ Related topics: Szydłowski et al JMP(96) [with Jacobi metric];
    Marklund et al gq/97-MG8 [non-holonomic, non-rigid frames for rotating matter];
    Roček & van Nieuwenhuizen gq/06 [smoothing, models];
    Boroojerdian IJTP(13)-a1211 [z-graded tangent bundle and geometrization of mass];
    Struckmeier PRD(15)-a1411 [as en extended canonical gauge theory];
    Dray 14 [differential forms];
    Hilditch a1509 [dual foliation formulations];
    Hardy a1608 [operational formulation];
    Donoghue et al a1702 [as a quantum effective field theory];
    > s.a. category theory in physics [stacks];
fluids; numerical general relativity.
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