Gravity as an Emergent Phenomenon  

In General > s.a. emergence [including emergent spacetime]; entropic gravity; gravitation; gravitational thermodynamics; models for spacetime.
* Idea: The gravitational field as a metric, and its dynamics, are not fundamental but arise as an effective theory from some other microscopic degrees of freedom; The first widely known proposal was Sakharov's 1967 induced gravity, but many other theories have been proposed since.
* 2016: A popular idea is that spacetime and gravity emerge together from the entanglement structure of an underlying microscopic theory.
* Issue: Arguments against emergent gravity have used the Weinberg-Witten Theorem, but many current proposals are based on theories that are not Lorentz-covariant.
@ Reviews: Padmanabhan AIP(07)-a0706; Sindoni Sigma(12)-a1110 [overview]; Carlip a1207 [challenges]; Padmanabhan AIP(12)-a1208 [conceptual aspects]; Padmanabhan MPLA(15)-a1410.
@ General references: Hu gq/96-conf [geometro-hydrodynamics]; Barceló et al IJMPD(01)gq; Padmanabhan IJMPD(08) [conceptual]; Jenkins IJMPD(09)-a0904-GRF [difficulties]; Lee IJMPA(09)gq/06 [from interacting simplices]; Padmanabhan a0910-conf, JPCS(11)-a1012; Hu IJMPD(11)-a1010-fs [and non-equilibrium thermodynamics of classical matter]; Sindoni a1105 [group-field-theory perspective]; news dw(11)aug; Kolekar & Padmanabhan PRD(12) [action principle from fluid-gravity correspondence]; Wetterich PLB(12)-a1203 [ambiguity in which collective field is the effective metric]; Consoli & Pluchino a1311 [experimental signatures]; Marolf PRL(15)-a1409 [need for kinematic non-locality]; Bhattacharya & Shankaranarayanan IJMPD(15)-a1505-GRF.
@ Bose-Einstein condensate: Consoli CQG(09); Jannes PhD(09)-a0907; Sindoni PRD(11)-a1011 [multi-BEC hydrodynamics]; Belenchia et al PRD(14)-a1407.
@ Other condensed-matter-type proposals: Volovik 03 [low-energy behavior of Fermi liquids]; Girelli et al PRD(09)-a0806; Jannes in(09)-a0810 [collective excitations]; Xu & Hořava PRD(10)-a1003 [Bose Liquid on an fcc lattice]; > s.a. quantum-spacetime proposals.
@ From entanglement: Verlinde a1611 + news es(16)nov [and additional, dark gravitational force]; Lelli et al MNRAS(17)-a1702 [and the galaxy Radial Acceleration Relation]; Tortora et al a1702 [and early-type galaxies]; Hossenfelder a1703 [covariant version]; > s.a. entangled systems.
@ U(1) gauge fields on a non-commutative spacetime: Lee et al JHEP(12)-a1206; Yang IJMPA(15)-a1312 [quantization].
@ From fundamental spinors: Kober PRD(09)-a0812; Finster a1409 [causal action principle, system of relativistic fermions].
@ Other proposals: Sernelius IJMPA(09)-a0804 [as Casimir interaction]; Padmanabhan ASL(09)-a0807-in [from a theory of null vectors, and the cosmological constant]; Kabe a1002/PRD; Marin a1206, a1206 [from "arrangement field theory" on a non-ordered space-time]; Sexty & Wetterich NPB(12)-a1208 [2D non-linear σ-model, numerical simulations]; de Souza a1212 [from discrete interactions, and cosmological acceleration]; Matsueda a1310 [from the Fisher information metric]; Anastasiou et al PRL(14)-a1408 [as convolution of left and right Yang-Mills theories]; Diaz et al a1609 [gravity from bilocality]; > s.a. cosmological expansion.
@ Phenomenology, tests: Brouwer et al MNRAS(17)-a1612 [using weak gravitational lensing]; Iorio EPJC(17)-a1612 [and anomalous perihelion precessions]; Pardo a1706 [isolated dwarf galaxies].
> Related topics: see Deformations; duality; lensing; non-commutative gravity and spacetime; topology change; Weinberg-Witten Theorem.
> Related theories: see Induced Gravity; Matrix Models; modified electromagnetism; non-commutative gauge theories [electromagnetism].
> Online resources: see Wikipedia page.

Effective Metrics and Analog Gravity Models > s.a. lorentzian geometry; quantum field theory effects in curved spacetime; sound.
* Idea: The propagation of perturbations in certain condensed matter systems is equivalent to the dynamics of quantum fields in curved spacetimes.
@ Reviews: Barceló et al LRR(05)gq; Unruh & Schützhold ed-07; Visser & Weinfurtner PoS-a0712; Visser & Molina-Paris NJP(10)-a1001; Barceló et al LRR(11); Leonhardt et al NJP(12) [focus issue]; Visser a1206-proc.
@ General references: Barceló et al CQG(01)gq [field modes in non-trivial background]; Visser et al GRG(02)gq/01-proc; Barceló et al NJP(04)gq [causal structure]; Okninski gq/05 [from scalar field]; Weinfurtner et al JPA(06)gq/05-conf [analog of Klein-Gordon field in curved spacetime]; Milgrom PRD(06) [particles and mass]; Liberati et al a0909-proc [general aspects, diffeomorphism invariance]; Smolyaninov & Hung JOSA(11)-a1104 [modeling the flow of time]; Chaline et al LNP(13)-a1203 [surface waves and dispersive horizons]; Fewster & Liberati GRG(14)-a1402 [GR20 report]; Hossenfelder & Zingg a1704 [the class of metrics that have an analogue]; > s.a. Elasticity; finsler geometry; Lorentz-Fitzgerald Contraction; optics [optical geometry]; qft effects in curved spacetimes; sound [acoustic geometry]; spacetime [geometry]; types of quantum field theories.
@ In a dielectric: De Lorenci & Klippert PRD(02) [electromagnetism in non-linear media]; Novello & Perez Bergliaffa AIP(03)gq [flowing dielectric]; Cacciatori et al NJP(10) [refractive index perturbations]; Thompson & Frauendiener PRD(10)-a1010 [dielectric, general metrics]; Fathi & Thompson a1602.
@ In a boson gas / BEC: Chapline & Mazur APPB(14)gq/04 [superfluid condensate model, and spinning cosmic strings]; Liberati et al PRL(06), CQG(06)gq/05 [quantum gravity analog from BECs]; Weinfurtner et al PRD(07)gq [boson gas, signature change]; Bravo et al EPJQT-a1406 [simulation of gravitational waves]; Szpak a1410-GRF [ultra-cold atoms in optical lattices]; > s.a. bose-einstein condensates.
@ From non-linear field theory: Novello & Goulart CQG(11)-a1102 [Lagrangian for field + analog metric]; Goulart et al CQG(11)-a1108.
@ Other models: Sepehri et al a1606 [f(R) analog from defects in graphene].
@ Phenomenology: Klidis & Spyrou CQG(00) [in astrophysics]; Faccio et al ed-13.
> Specific types of metrics: see black-hole analogs; de sitter space; FLRW models [condensed matter analogs].

Quantum Gravity > s.a. approaches; models of spacetime [including internal relativity]; renormalization; time.
@ General references: Hu gq/06-talk [and stochastic gravity]; Hedrich a0902; Hu JPCS(09)-a0903; Mannheim GRG(11)-a0909, MPLA(11)-a1005-conf [intrinsically quantum-mechanical gravity and cancellation of cosmological-constant and zero-point energies]; Yang MPLA(10)-a1007 [using a spacetime symplectic structure], JPCS(12)-a1111 [and background-independent quantum gravity]; Mandrin a1505 [non-dynamical approach]; > s.a. gravity theories.
@ Quantum gravity simulation: Conti PRA(14)-a1406 [non-paraxial non-linear optics].

Gravity is the thermodynamic limit of the statistical mechanics of "atoms of spacetime."
— Thanu Padmanabhan, during a talk at Perimeter Institute.

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