|  Quasilocal Energy in General Relativity | 
In General > s.a. stress-energy pseudotensors.
  * Motivation: The fundamental notion
    of energy in classical physics is quasilocal; Use in black-hole thermodynamics.
  * History: A quasilocal energy
    had been defined for spherically symmetric solutions by Tolman and Møller,
    but the field expanded in the 1980s, after a more general one was proposed by
    Penrose, based on twistor methods.
  * Criteria: (i) It must
    vanish for gab
    = ηab;
    (ii) It must agree with known standard definitions for spherical symmetry;
    (iii) The spi limit must be MADM;
    (iv) The scri limit must be MBondi; (v) For
    an apparent horizon, it must equal Mirred;
    (vi) It must be positive, and monotonic in a suitable sense [@ Christodoulou & Yau
    in(88)].
  @ References: Schmekel MoG-a0708 [brief review];
    Szabados LRR(09) [rev];
    Anderson PRD(10)-a1008 [Hamiltonian, constraints, and initial-boundary problem];
    Sun et al a1307
      [optimal choice of reference, and angular momentum];
    Wang a1510-ln;
    Chen et al GRG-a1811.
Various Expressions > s.a. bel-robinson tensor.
  * Ambiguities: Bergqvist
    showed that there are infinitely many definitions satisfying the criteria,
    which differ by boundary terms for finite regions, reflecting different choices
    of physical processes [& Nester].
  * Tolman expression: For a
    stationary field, if V is a region of space containing matter,
MT:= ∫V d3x |g|1/2 gab Tab .
  @ References: Tolman PR(30),
    62;
    Papapetrou PRIA(47) [simpler];
    Landau & Lifshitz v2, ch11 [simplest].
  * Møller expression:
MM:= ∫V d3x χ00i,i , where χ00i:= |g|1/2 (8πG)−1 g0a gib (g0b,a − g0a,b) .
  @ References:
    Tolman & Møller; Florides GRG(94);
    Lessner GRG(96);
    Xulu MPLA(00)gq [Kerr-Newman].
  * Ashtekar-Hansen mass: For a 2-sphere B
    of area A and induced metric σij
    [@ Ashtekar & Hansen JMP(78)],
MAH:= (8πG)−1 (A/16π)1/2 ∫B d2x |σ|1/2 σij σkl Cijkl .
* Brown-York mass: If H is the trace of the extrinsic curvature of the boundary S of a compact spatial hypersurface,
MBY = (8πG)−1 ∫S (H0−H) d2s .
  * Christodoulou-Ruffini black hole
    irreducible mass: Given by MCR
    = (A/16πG2)1/2.
  @ Penrose twistor expression:
    Penrose PRS(82),
    in(86);
    Tod CQG(86);
    Mason CQG(89);
    Godazgar & Kaderli a1807 [modification, and Kerr-Schild metrics].
  @ Bartnik expression: Bartnik PRL(89);
    Koc gq/96;
    Jauregui JGP(19)-a1806 [smoothing the boundary conditions].
  @ Liu-Yau expression:
    Yu a0706 [small- and large-sphere limits];
    Ó Murchadha a0706 [as energy rather than mass];
    Miao et al CMP(10) [problems, also for Brown-York expression].
  @ Expressions: Hawking JMP(68);
    Christodoulou & Yau in(88);
    Katz et al CQG(88);
    Katz & Ori CQG(90);
    Bergqvist & Ludvigsen CQG(91);
    Dougan & Mason PRL(91);
    Bergqvist CQG(92),
    CQG(93);
    Helfer CQG(92);
    Szabados CQG(93);
    Hayward PRD(94)gq/93;
    Chen & Nester CQG(99)gq/98;
    Beetle & Fairhurst AIP(99)gq;
    Epp PRD(00)gq [and angular momentum];
    Hayward gq/00 [as Noether charge];
    Chen et al gq/02-proc [spinor];
    Zhang AMS-gq/06;
    So IJMPD(07)gq/06;
    Wang & Yau CMP(09);
    So & Nester PRD(09)-a0901;
    Zhang CQG(09)-a0905;
    Ó Murchadha et al a0905-wd;
    Liu et al CQG(11)-a1105 [and choice of reference];
    Katz & Khuri MPLA(12)-a1201;
    Wang a1211-conf;
    Faraoni a1510 [Newtonian aspect of Hawking quasilocal energy];
    Álvarez et al a1811 [note].
Related Topics
  * Martinez conjecture:
    The Brown-York quasilocal energy at a black hole outer horizon is twice its
    irreducible mass, (A/4π)1/2.
  @ Martinez conjecture: Jing & Wang PRD(02)gq/01 [and string theory].
  @ Positivity: Liu & Yau PRL(03)gq,
    JAMS(06)m.DG/04,
    O'Murchadha et al PRL(04)gq/03 [Kijowski M];
    Shi & Tam JDG(02)m.DG/03.
  @ Bounds: Shi & Tam CMP(07)m.DG/05 [Brown-York and Bartnik M].
  @ For cosmological models: Chen et al MPLA(07)-a0705-conf [Bianchi models, FLRW models];
    Nester et al PRD(08)-a0803 [Bianchi models];
    Afshar CQG(09) [FLRW models];
    Lapierre-Leonard et al PRD(17)-a1710 [Brown-York mass].
  @ For other types of solutions: Balart PLB(10) [regular black holes, and Komar charge];
    Wu et al GRG(12) [spherically symmetric];
    Schmekel a1807 [rotating].
  @ In modified gravity theories: Faraoni CQG(16)-a1508 [scalar-tensor gravity];
    Chakraborty & Dadhich JHEP(15)-a1509 [Lanczos-Lovelock gravity];
    Faraoni & Coté a1907,
    Giusti & Faraoni CQG(20)-a2005 [scalar-tensor gravity].
  @ Other topics: Wang & Yau PRL(09)-a0804 [energy-momentum surface density];
    Yang & Ma PRD(09)-a0812 [in lqg];
    Wang & Yau CMP(10) [limit at spatial infinity];
    Chen et al CMP(11)-a1002 [limit at null infinity];
    Frauendiener & Szabados CQG(11)-a1102 [post-Newtonian limit].
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