Extensions and Modifications of General Relativity  

In General > s.a. einstein's equation [approximations]; general relativity; gravity; models of topology change; quantum gravity.
* Types of modifications: Some modified theories preserve general covariance (e.g., higher-order theories), others don't (e.g., Horava's).
@ Limits of theory: Tavakol & Zalaletdinov FP(98)gq/97; Ahluwalia MPLA(98)gq, gq/98; Okorokov gq/99; Chapline et al IJMPA(03)gq/00 [black hole event horizon]; Ishak et al PRD(06)ap/05 [from cosmic acceleration]; Bertolami a0905-GRF [and cosmological constant].
@ Parametrizations: Caldwell et al PRD(07) [cosmological version of Post-Newtonian parameter]; > s.a. modified newtonian gravity.
@ Constraints, phenomenology: Aguirre et al CQG(01)hp; Mortlock & Turner MNRAS(01)ap [from lensing]; Barausse & Sotiriou PRL(08)-a0803 [Kerr perturbations]; Skordis PRD(09)-a0806 [and cosmology].
@ Neighbors of general relativity: Bengtsson JMP(91) [action, metric]; Bengtsson & Boström CQG(92); Bengtsson & Peldán IJMPA(92); Capovilla NPB(92); Bengtsson CQG(93), GRG(96)gq/95 [connection and curvature]; Krasnov PRL(08)-a0711 [deformations of Ashtekar constraint algebra]; Kong PLB(08) [deformation].
@ Strong-coupling limit: Isham PRS(76), in(84); Pilati in(82); Tsacoyeanes PRD(87); Goldberg GRG(88); Helfer et al GRG(88); Salopek CQG(98)gq, CQG(99)gq/98 [applications]; Frasca IJMPD(06)ht/05 [expansion, vacuum ADM]; > s.a. initial-value formulation; quantum version.

Degenerate Metrics > s.a. geometrodynamics; models in canonical general relativity; solutions; types of metrics and singularities.
* Idea: Use variables (typically including connections) whose inverse does not appear in the equations.
@ References: Jacobson & Romano CQG(92)gq; Canarutto JMP(98) [degenerate tetrads + Maxwell-Dirac].

Signature Change > s.a. quantum geometrodynamics; lorentzian geometries [type-changing metrics].
* Motivation: Problems in causality implied by isotropy and homogeneity, and quantum cosmology and the initial singularity.
@ Precursor: Sakharov JETP(84)/SPU(91).
@ General references: Dray et al GRG(91); Ellis GRG(92), et al CQG(92); Dereli & Tucker CQG(93); Ellis in(93); Hayward gq/93; Kossowski & Kriele CQG(93), CQG(93), comment Dray & Hellaby GRG(96)gq; Hellaby & Dray PRD(94)gq; Carfora & Ellis IJMPD(95)gq/94; Ellis & Piotrkowska IJMPD(94); Kossowski & Kriele PRS(94); Kriele CQG(94); Hayward gq/96, PRD(95)gq/96; Dray et al GRG(97)gq/96, GRG(01)gq/00 [Colombeau theory]; Iliev PS(02)gq/98 [metric connection]; Stewart CQG(01) [well-posed problems]; Borowiec et al IJGMP(07) [Lagrangian formalism]; Otway JGP(08).
@ Matter fields: Dray et al PRD(93)gq [scalar]; Romano PRD(93)gq [scalar + spinor]; Dray et al CQG(95)gq [scalar], comment Hayward gq/95; Ghafoori-Tabrizi et al IJMPA(00)gq/99 [FRW model with scalar]; > s.a. types of quantum field theories.
@ Distributional approach: Hartley et al GRG(00)gq/97; Mansouri & Nozari GRG(00); Steinbauer mp/01-in; > s.a. distributions.
@ Applications: Hayward CQG(92), CQG(93) [cosmology]; Embacher PRD(95)gq/94, CQG(95)gq/94 [compactification].
@ Related topics: Hosoya & Ding CQG(93) [2+1]; Hayward CQG(94) [weak solutions]; Egusquiza CQG(95)gq; Embacher PRD(95)gq [action], CQG(96)gq/95, gq/95-in; Kriele & Martin CQG(95) [singularities]; Martin PRD(95)gq [perturbations]; Alty & Fewster CQG(96)gq [initial value]; Dray JMP(96)gq [Einstein's equation]; Hellaby et al IJMPD(97)gq/99 [black holes]; Mohseni PLA(00) [6D]; Kamleh gq/00 [and generalized functions]; Mars et al PRL(01)gq/00, Gibbons & Ishibashi CQG(04)ht [and branes].
@ And quantum gravity: Martin PRD(94)gq; Carlini & Greensite PRD(95)gq [and sqrt action]; Darabi & Rastkar GRG(06)gq/04 [and cosmological constant].

Other Modifications > s.a. einstein equation; kaluza-klein theory; physics [visions]; quasilocal general relativity; strings; supergravity.
@ Cosmological constant as integration constant: Padmanabhan IJMPD(06)gq; Afshordi a0807 [matter source Tab – gabT]; > s.a. unimodular relativity.
@ With varying constants: Barrow et al PLB(02)ap [G and ].
@ Complex: Woodhouse IJTP(77); Boyer et al in(80); Ko et al PRP(81); Esposito 95, gq/95-in, gq/95-in; Esposito & Stornaiolo NCB(96)gq/95 [linear equations]; Batakis ht/96/PLB; Kao ht/00 [extra scalar].
@ Infrared modifications: Piazza a0904-GRF, a0907 [and "ultra-strong equivalence principle"]; Gao a0905 [UV and IR, ADM formalism].
@ Non-local theories: Arkani-Hamed et al ht/02 [and cosmological constant]; Soussa & Woodard CQG(03)ap [and MOND]; Barvinsky PLB(03)ht; Barvinsky PRD(05)ht [action, and strong coupling]; Hamber & Williams MPLA(06)gq/05, Deser & Woodard PRL(07)-a0706 [and cosmology]; Mashhoon AdP(07)ht/06; Nojiri & Odintsov PLB(08)-a0708, Koivisto PRD(08) [and cosmology]; Hehl & Mashhoon PLB(09)-a0812, PRD(09)-a0902 [framework and effective dark matter]; > s.a. dark matter types.
@ Analogs: Visser et al GRG(02)gq/01-in; Okninski gq/05 [from scalar field]; > s.a. black-hole analogs, sound.
@ Related proposals: Brodbeck et al JMP(99)gq/98 [as part of larger theory]; Gambini & Pullin PRL(00)gq, CQG(00)gq [from → limit]; Magueijo & Smolin CQG(04) [doubly general relativity]; Piron FP(05); Bañados CQG(07)ht [with gab = 0 ground state]; Broda et al PLB(07)gq [abelian]; Verozub AdP(08)-a0801 [invariant under geodesic mappings]; > s.a. Horava-Lifshitz Gravity.
> Based on different mathematical structures: see non-commutative gravity; Octonions; Topos Theory.

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