|  Origin of Black-Hole Entropy | 
Proposals Based on  Black-Hole Microstates
  > s.a. black-hole entropy [reviews].
  @ Fluctuations: Gerlach PRD(76);
    York PRD(83),
    in(84);
    Pavón & Rubí PRD(88);
    Frolov PRL(95);
    Sorkin & Sudarsky CQG(99)gq;
    Gour & Medved CQG(03)gq;
    Requardt a0708.
  @ Spacetime microstructure: Scardigli CQG(97)gq [foam];
    Padmanabhan PRL(98)ht;
    Garattini Ent(00)gq [foam, Schwarzschild-de Sitter space],
    IJMPD(02)gq/00;
    Bergamin & Grumiller IJMPD(06)gq-GRF;
    Tanaka EPJP(14)-a1304 [non-commutative spacetime];
    Davidson a1907 [horizon tiling configurations].
  @ Horizon microstates: Sfetsos & Skenderis NPB(98)ht/97;
    Brown PRD(98);
    Epp & Mann MPLA(98)gq,
    Epp gq/98/PRD [tetrad approach];
    Cvetič & Larsen PRL(99) [rotating];
    Soloviev PRD(00)ht/99;
    Dou & Sorkin FP(03)gq,
    Rideout & Zohren in(08)gq/06 [causal links];
    Das & Shankaranarayanan CQG(07)gq;
    Silva & Landim EPL(11)-a1003 [based on fuzzy space];
    Ghosh IJTP(11)-a1008 [Shannon entropy];
    Chung NPB(12).
  @ Horizon conformal field theory:
    Carlip gq/95,
    gq/96-conf,
    PRD(97)gq/96 [3D],
    NPPS(97)gq,
    PRL(99)ht/98,
    CQG(99)gq,
    NPPS(00)gq/99;
    Park NPB(02)ht/01 [deformations];
    Carlip CQG(05)ht/04 [stretched horizon],
    CQG(05)gq-in,
    IJTP(07)gq/06-conf [horizon constraints and symmetry algebra];
    Carlip AIP(12)-a1207 [rev].
@ Loop quantum gravity: 
    Garay & Mena CQG(03),
    Ghosh & Mitra PLB(05)gq/04 [and Immirzi parameter];
    Domagała & Lewandowski CQG(04)gq;
    Meissner CQG(04)gq;
    Swain IJMPD(05)gq-GRF;
    Mitra a0705-conf [state counting];
    Tamaki CQG(07)-a0707;
    Jacobson CQG(07)-a0708 [and renormalization];
    Sahlmann PRD(07)-a0709;
    Barbero & Villaseñor CQG(09)-a0810;
    Agulló et al JCAP(09)-a0903 [conformal-field-theory perspective],
    PRD(09)-a0906;
    Bianchi CQG(11)-a1011 [and polymer physics];
    Cao & Barrau a1111 [for large black holes];
    Dasgupta Sigma(13)-a1203 [and semiclassical states];
    Bianchi a1204 [non-extremal black holes];
    Livine & Terno CQG(12)-a1205;
    Ghosh & Perez a1210 [scaling];
    Ghosh et al PRD(14)-a1309 [statistics and holography];
    Frodden et al EPL(14)-a1312 [complex variables];
    Bodendorfer CQG(14)-a1402 [and entanglement entropy];
    Han a1402;
    Pigozzo et al CQG(20)-a2001 [and Immirzi parameter].
  @ Spin-foam quantum gravity: García-Islas CQG(08)-a0809;
    García-Islas CQG(10)-a1004 [statistical calculation].
  @ Strings, M-theory: Horowitz & Strominger PRL(96) [near extremal];
    Strominger & Vafa PLB(96);
    Horowitz & Marolf PRD(97)ht/96;
    Maldacena et al JHEP(97)ht,
    Horowitz & Roberts PRL(07) [extremal, in M-theory];
    Lowe PRL(98);
    Dabholkar IJMPD(06).
  > Related topics: see 2D
    and 3D black holes; 2D gravity;
    quantum black holes; spin-foam models.
Other Origin and Approaches > s.a. gravitational thermodynamics;
  horizons; non-extensive statistical mechanics.
  * Ways of forming the black hole:
    S is the log of the number of quantum mechanically distinct ways that
    the black hole could have been made, or information lost in the creation of the
    black hole; This leads to an estimate of the quantum black hole level spacing.
  * Entanglement entropy:
    Black-hole entropy is semi-classical, in the sense that it comes from tracing
    out over internal degrees of freedom of quantum fields in a classical geometry,
    without even backreaction – related to the "brick wall model"
    idea; It appears to be proportional to A for fields in the ground
    state or in coherent/squeezed states, not in excited states.
  * Species problem: The fact that
    the Bekenstein-Hawking entropy is independent of the number of particle species,
    while entanglement entropy gives a contribution that appears to depend on the
    number of species.
  @ Ways of forming the black hole: & Bekenstein; & Hawking;
    Zurek & Thorne PRL(85);
    Mukhanov FP(03);
    Hsu & Reeb PLB(08)-a0706 [and monster states];
    Zhang et al GRG(11)-a1102.
  @ Entanglement entropy: Sorkin in(83);
    Bombelli et al PRD(86);
    Srednicki PRL(93);
    Jacobson gq/94 [and induced gravity];
    Kabat NPB(95)ht [and 1-loop corrections];
    Muller & Loustó PRD(95);
    Ahmadi et al CJP(06)ht/05-in [deviations from area law];
    Ansari NPB(08)gq/06 [in lqg];
    Emparan JHEP(06) [holographic derivation];
    Das & Shankaranarayanan JPCS(07)gq/06,
    Das et al PRD(08)-a0705,
    CJP(08)-a0708-conf [power-law corrections in different states];
    Das et al a0806-ch [rev];
    Braunstein et al PRL(13)-a0907 [and information retrieval];
    Arenas & Tejeiro NCB(10)-a1108;
    Solodukhin LRR(11);
    Giaccari et al EPJC(18)-a1512 [finite];
    Garfinkle a1901 [as a consequence of excision].
  @ Species problem: Brout IJMPD(09)-a0802;
    Dvali & Solodukhin a0808;
    Chen et al EPJC(18)-a1609,
    Rojas & Arenas a1712 [proposed solutions].
  @ And brick wall: Mukohyama et al PRD(97)gq;
    Mukohyama PhD(98)gq,
    gq/99-conf;
    Jing & Yan PRD(99)gq [Kerr];
    Garattini MPLA(04) [and spacetime foam].
  @ Contributions from other fields: Donnelly & Wall PRD(12)-a1206 [gauge-field contribution].
  @ Other proposals:
    Yang MPLA(01) [topological invariants];
    Badiali JPA(06)gq/05 [spacetime order];
    Banerjee et al a0804 [horizon chiral Virasoro algebra];
    Hadad & Kupferman a1006-wd [Wald and statistical entropies];
    Nicolini & Singleton PLB(14)-a1409 [as the statistical entropy of a gas of voxels];
    Kalogeropoulos a1711,
    IJGMP(18)-a1712 [mesoscopic treatment, from horizon topology];
    Xiao PRD(20)-a1910 [microscopic derivation];
    Roy Chowdhury et al a2103 [from the Einstein boundary term].
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