Modified Approaches to Quantum Gravity  

In General > s.a. BF theory; newton-cartan theory; phenomenology; theories of gravitation.
* Possibilities: Unitarity, renormalizability, and relativistic invariance are usually considered essential properties for any fundamental quantum field theory; In Hořava gravity unitarity and renormalizability are retained, while Lorentz invariance must emerge only as an approximate symmetry at low energies.
@ Strong-coupling limit: Henneaux et al PLB(82); Pilati PRD(82); PRD(83); Francisco & Pilati PRD(85); Rovelli PRD(87); Husain CQG(88); Kakas CQG(89) [matter]; Gamboa PRL(95) [2D]; Maeda & Sakamoto PRD(96)ht [expansion]; Sakamoto PRD(09)-a0905 [renormalized, and z = 2 Lifshitz point]; > s.a. modified general relativity [classical].
@ Weak-coupling limit: Varadarajan a1904 [lqg quantization, quantum propagation].
@ Scalar-tensor theory: Pimentel & Mora gq/00 [Bergmann-Wagoner theory]; Zhang & Ma PRD(11)-a1107, JPCS(12)-a1111, FrPh(13)-a1211 [loop quantization]; Steinwachs & van der Wild CQG(18)-a1712 [Wheeler–DeWitt equation, quantum gravity corrections]; > s.a. brans-dicke theory; no-boundary wave function; renormalization; scalar-tensor theories [conformal frames].
@ Modified dynamics: Smolin CQG(92) [G → 0]; Broda et al PLB(07) [abelian theory].
@ Hořava-lifshitz gravity: Orlando & Reffert CQG(09)-a0905; Shu & Wu a0906 [stochastic quantization]; Visser a0912-conf [power-counting]; Giribet et al JHEP(10)-a1006 [semiclassical]; Briscese et al FP(12)-a1205; D'Odorico et al PRL(14)-a1406 [asymptotic freedom]; Li et al PRD(14)-a1408, PRD(16)-a1511 [1+1]; Barvinsky et al PRD(16)-a1512 [projectable theory, perturbative renormalizability]; Glaser et al PRD(16)-a1605 [CDT approach]; Bellorín & Restuccia PRD(16)-a1606 [at the kinetic-conformal point]; Steinwachs a2004 [rev]; > s.a. 3D quantum gravity; dynamical triangulations; quantum cosmological models.
@ Other theories of gravity: Pagani & Percacci CQG(15)-a1506 [with torsion and non-metricity]; Álvarez & González-Martín JCAP(17)-a1610 [Weyl gravity]; > s.a. bimetric gravity; classical gravity and theories [incuding vector]; conformal gravity; higher-order gravity; massive gravity; non-local theories; quantum cosmology [varying constants]; teleparallel gravity [lqc approach]; types of quantum field theories [ultralocal].

Linearized Gravity > s.a. perturbations in general relativity and quantum cosmology / approaches to canonical quantum gravity and covariant quantum gravity.
@ Deformation quantization: Quevedo & Tafoya GRG(05)gq/04; García-Compeán & Turrubiates IJMPA(11)-a1109 [ground state Wigner functional and graviton propagator].
@ Other approaches: Bronstein PZS(36), translation GRG(12); Ashtekar et al PRD(91) [lqg]; Grigore CQG(00)ht/99; Shojai & Shojai PS(03)gq [Bohmian approach]; Bomstad & Klauder CQG(06)gq [projection operator]; Contreras et al a1612 [monoidal representation].
@ Ground state: Kuchař JMP(70) [canonical]; Hartle PRD(84), Hartle & Schleich in(87)-a2004 [Euclidean path integral].
@ Gravitons: Varadarajan PRD(02)gq [loop and Fock space]; Speziale JHEP(06)gq/05 [2-point function from spin foam, 3D model]; Skagerstam et al CQG(19)-a1806 [spontaneous graviton emission and/or absorption]; > s.a. quantum field theory in curved spacetime [graviton].
@ Stability: Moncrief GRG(79) [linearization instabilities]; Kuntz & da Rocha a1903 [instability due to runaway modes].
@ Related topics: Atkins & Calmet PLB(11)-a1003 [coupled to matter, S-matrix unitarity]; Magueijo & Benincasa PRL(11)-a1010 [chiral vacuum fluctuations]; Fewster & Hunt RVMP(13)-a1203 [with a cosmological constant]; > s.a. gravitomagnetism; quantum regge calculus.

Approaches Based on Different Frameworks > s.a. approaches to quantum gravity [including categorical]; quantum spacetime.
* Possibilities: Modify the underlying structure, such as (i) Twistors; (ii) Algebraic approaches, quantum groups, non-commutative geometry; (iii) Finkelstein and other fundamentally quantum approaches (plexars, quantum topology); (iv) Posets, as finite spatial topologies, or as causal sets; (v) Fundamentally discrete approaches.
@ General references: DeWitt & Molina-París MPLA(98)ht [from the space of histories]; Delfino et al JHEP(15)-a1210 [pure-connection formulation, Feynman rules].
@ Bohm / pilot-wave theory: Shtanov PRD(96)gq/95; Goldstein & Teufel in(01)qp/99; Shojai PRD(99)gq, & Golshani IJMPA(98), IJMPA(98)gq/99, & Shojai CQG(04)gq/03; Santini PhD(00)gq [canonical quantum gravity]; Pinto-Neto & Santini GRG(02); Kenmoku et al CQG(02) [3D spherical]; Shojai & Shojai proc(05)gq/04 [lqg]; Shojai et al IJMPA(05)gq [Einstein universe]; Carroll TMP(07) [fluctuations and entropy]; Sverdlov a1010; Vassallo & Esfeld FP(14)-a1308 [discrete]; Dürr & Struyve CQG(20)-a2003 [quantum Einstein equations]; > s.a. canonical quantum gravity.
@ And other hidden variables: 't Hooft CQG(99)gq [information dissipation].
@ And spectral geometry: Esposito 98-ht/97, ht/97-conf, CM(05)ht/03-proc [Euclidean]; Booß-Bavnbek et al Sigma(07)-a0708 [rev]; Kempf & Martin PRL(08)-a0708 [information theory and cutoffs]; Aasen et al PRL(13)-a1212 [2D case].
@ Causality-based: Schorn CQG(97), CQG(97); Rainer IJTP(00)gq/97, CQG(00)gq/99 [algebraic]; Christensen & Crane JMP(05) [causal sites]; Markes & Hardy JPCS(11)-a0910 [and entropy]; > s.a. causal sets; causality in quantum theory ["causaloids"].
@ Relational: Corichi et al MPLA(02)gq; Dreyer gq/04-GRF; Raptis IJTP(07) ['third quantization']; Dreyer in(06)gq, PoS-a0710 [internal relativity]; Anderson CQG(09)-a0809.
@ Deformed: Finkelstein LMP(96); Antonsen gq/97; Gavrilik eConf-gq/99 [quantum algebras]; Vacaru IUJP-a0801 [Lagrange-Finsler variables and Fedosov quantization]; de Vegvar EPJC(17)-a1605 [Hopf algebra methods on commutative manifolds]; > s.a. loop representation; modified versions of general relativity.
@ Related topics: Ghosh ht/02 [use all signatures]; Siino ht/06 [algebraic]; Finkelstein IJTP(08)gq/06 [homotopy approach]; Raptis IJTP(06) [Glafka meeting, iconoclastic approaches]; Alfaro et al CQG(11) [two-symmetric-tensor delta gravity]; Gegenberg & Husain CQG(13)-a1210 [solvable model]; Etesi a1712 [von Neumann algebra of a 4-manifold]; Singh a1901 [action for an asymmetric metric tensor using non-commutative geometry, string theory, and trace dynamics]; > s.a. FLRW quantum cosmology.
blue bullet Other approaches: see clifford algebra; non-commutative gravity; Non-Symmetric Gravity; types of symplectic structures.

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