Black-Hole Analogs and Mimickers  

In General > s.a. black holes [alternatives]; emergent gravity [and analog models of spacetime]; Fuzzballs; Gravastar; modified general relativity.
* Idea: Black-hole analogs and mimickers are objects that may solve Hawking's information-loss paradox and the singularity problem associated with black holes, while reproducing almost all of their classical properties; They are, however, generically unstable on relatively short timescales.
* Remark: Black holes are like spacetime rivers, in that their geometry can be viewed as if space were a moving medium rushing towards their singularities; Horizons are formed where the flow speed exceeds the speed of light such that nothing can escape anymore.
* Black-hole analogs: Systems in which perturbations of a medium (usually acoustic or electromagnetic waves) cannot leave a region of space, for reasons other than spacetime curvature due to gravity; Can often be modeled by an effective curved geometry, and are useful for studying features of black holes, including their possible realization in the lab.
* Black-hole-mimicking spacetimes: Curved spacetimes with properties that make them look like black holes in some ways; 2018, The existing stochastic background constraints from LIGO and Virgo show that black holes are very black, although some shades of grey may still be allowed; While 100% reflection from the horizon is ruled out, smaller reflection coefficients may still be possible depending on the spin of the object.
@ General references: Novello et al CQG(03)gq/02 [flowing dielectrics]; Jacobson & Koike cm/02-ch [thin film of 3He-A]; Novello et al ed-02; Schützhold CQG(08)-a1004 [rev]; Schützhold ASL(09)-a1004 [recreating fundamental effects]; Crowther et al Syn-a1811 [what we cannot learn].
@ In BECs: Fabbri NCC(13)-a1212-conf; Boiron et al PRL(15)-a1406 [momentum correlations]; Kühnel PRD(14)-a1312; Liao PRA(19) [of photons].
@ Radiation: Pauri & Vallisneri FP(99)gq ["electromagnetic radiation" as seen by inertial and accelerated observers]; Robertson JPB(12)-a1508 [introductory tutorial, including dispersion]; Unruh FP(14)-a1401 [measurement already performed in 2010]; Fabbri JPCS(15)-a1411 [in BECs]; Robertson JPB(12)-a1508 [theory]; Thébault ch(19)-a1610 [what can we learn?]; Drori et al PRL(19) [in an optical analogue]; Belgiorno et al PRD(20)-a2003 [master equation].
@ Black-hole-mimickers / doubles: Lemos & Zaslavskii PRD(08)-a0806; Guzmán & Rueda-Becerril PRD(09) [boson stars]; Kovács & Harko PRD(10)-a1011 [naked singularities, using accretion disks]; Stadnik et al EPJC(13)-a1210 [resonant scattering of light]; Marunović & Murković CQG(14)-a1308 [boson star and global monopole]; Cardoso et al PRL(16) + news pw(16)apr [mimicking the ringdown signal from black-hole merger]; Holdom & Ren PRD(17)-a1612; Barausse et al CQG(18) [partial reflectivity bounds]; Konoplya et al PRD(19)-a1905 [Schwarzschild stars]; Salvio & Veermäe JCAP(20)-a1912 [quadratic gravity, horizonless ultracompact objects]; Lemos & Zaslavskii a2007 [compact objects, quasiblack holes]; Novikov & Repin AR(21)-a2009 [wormholes, comparing spatial properties]; Mazza et al JCAP(21)-a2102 [rotating solutions]; Herdeiro et al JCAP(21)-a2102 [Proca stars].
@ Distinguishing from black holes: Akhmedov et al PRD(16)-a1601 [energy levels for massive fields]; Cardoso et al PRD(19)-a1907; Mitra et al ASS(21)-a1908; Shaikh et al a2102 [shadows].
@ Perturbations: Eskin JMP-a0905 [of Kerr black holes]; > s.a. Ergoregion [instabilities].

Analogs, for Acoustic Waves > s.a. sound [differential-geometric approach].
* Idea: Fluid-dynamic analogs of general-relativistic black holes, where the behavior of sound waves in a moving fluid acts as an analog for scalar fields in a gravitational background; The analog of the event horizon occurs where the bulk speed of the fluid coincides with the propagation speed of acoustic waves in the fluid.
* Motivation: Acoustic horizons possess many of the properties normally associated with the event horizons of general relativity, including Hawking radiation, and have received much attention because it would seem to be much easier to experimentally create an acoustic horizon than to create an event horizon; However, so far (2005) the Hawking temperature seems to be too low for radiation to be detected, \(T^{~}_{\rm H} \approx (10^{-9}/r^{~}_{\rm H}({\rm m}))\) K, because the thermal noise would be too high.
@ Reviews, intros: Visser gq/99-proc; Cardoso phy/05-conf; Jacobson & Parentani SA(05)dec; Lemos a1312-ch [rotating].
@ General references: Jacobson & Volovik PRD(98)cm, JETPL(98)gq [3He]; Parentani IJMPA(02)gq-proc; Cadoni CQG(05) [2D]; Cadoni & Mignemi PRD(05)gq [with singular source]; Ge & Sin JHEP(10)-a1001 [for relativistic fluids]; Ge et al IJMPD(11)-a1010 [from supercurrent tunneling, Josephson effect]; Vieira & Bezerra a1406; Ge et al PRD(15)-a1508 [not merely an analogy]; Michel et al PRD(16)-a1511 [no-hair theorems]; Patrick a2009-PhD [bathtub vortices].
@ Rotating: Basak & Majumdar CQG(03)gq/02; Visser & Weinfurtner CQG(05)gq/04 [Kerr, equatorial]; Horstmann et al PRL(10)-a0904 [rotating ions in rings, and Hawking radiation]; Lemos a1312-proc [survey and phenomenology]; Garza et al CQG(18)-a1802.
@ Other specific examples: Hossenfelder PLB(16) [planar black hole in AdS].
@ Back-reaction: Balbinot et al PRL(05)gq/04, PRD(05)gq/04, NCB(06)gq; Fagnocchi JPCS(06)gq, gq/06-conf.
@ Radiation: Unruh PRL(81), PRD(95)gq/94 [and high-energy dispersion relations]; Visser CQG(98)gq/97; Sakagami & Ohashi PTP(02)gq/01 [experimental model using a Laval nozzle]; Giovanazzi PRL(05)phy/04; Balbinot et al RNC(05)gq/06; Barceló et al CQG(06)gq, PRL(06)gq [quasi-particle creation, no need for trapped region]; Kim & Shin JHEP(07)-a0706, Bécar et al IJMPA(10)-a0808 [anomaly-cancellation method]; Horstmann et al NJP(11)-a1008 [with ions]; Finazzi & Parentani PRD(11)-a1012 [spectral properties], JPCS(11)-a1102 [robustness of spectrum]; Giovanazzi PRL(11)-a1101 [entanglement entropy and mutual information production rates]; Weinfurtner et al PRL(11) [measurement]; Zhang & Zhao PLB(11) [rotating]; Steinhauer nPhys(14) + news ns(14)oct [observation]; Eskin a1906-CM [new examples]; Mannarelli et al PRD(21)-a2011 [temperature and phonon emission].
@ Thermodynamics: Kim et al JKPS(06)gq/05 [entropy and superradiance]; Zhang AHEP(16)-a1606 [2D].
@ Quasinormal modes: Berti et al PRD(04); Cardoso et al PRD(04) [rotating]; Lepe & Saavedra PLB(05) [and area spectrum]; Saavedra MPLA(06)gq/05; Abdalla et al CQG(07)-a0706 [Laval nozzles]; Dolan et al PRD(12)-a1105 [rotating, draining bathtub model].
@ Other topics: Liberati et al CQG(00)gq [surface gravity]; Schützhold & Unruh PRD(02)gq [gravity waves]; Rosquist GRG(04)gq/03 [and electromagnetic waves]; Choy et al PRD(06) [superradiant energy flow]; Rousseaux et al NJP(10) [surface waves on moving water]; Lombardo & Turiaci PRL(12)-a1206 [decoherence], PRD(13)-a1208 [as open quantum systems]; Benone et al PRD(15)-a1412 [acoustic clouds around black-hole analogs].
@ In Bose-Einstein condensates: Garay et al PRL(00)gq, PRA(01)gq/00; Barceló et al CQG(01)gq/00, IJMPA(03)gq/01 [black-hole radiation]; Visser et al ht/01-conf; Weinfurtner MSc-gq/04; Carusotto et al NJP(08)-a0803 [radiation, numerical]; Lahav et al PRL(10)-a0906 + news physorg(11)jan [created in the lab]; Rinaldi PRD(11)-a1106, IJMPD(13)-a1112 [entropy]; Finazzi PhD(11)-a1208; Anderson et al PRD(14)-a1404 [gray-body factor and infrared divergences]; Steinhauer nPhys(16)-a1510 + comment Leonhardt AdP(18)-a1609, response Steinhauer a1609 [observation, and particle entanglement].
@ Laval nozzles: Furuhashi et al CQG(06)gq; Okuzumi & Sakagami PRD(07)gq [quasinormal ringing, simulations].
@ Other variations: Mayoral et al NJP(11) [white holes, in flowing atomic Bose-Einstein condensates]; Prain & Faraoni a1403 [re turbulent fluid flow].

Analogs, for Electromagnetic Waves
* Optical black holes with fluid vortices: Trap light inside vortices of fluid that whirl at speeds close to the speed of light in the medium (possibly real slow); According to Leonhardt, light brought to a standstill in a gas should produce a singularity analogous to the event horizon of a black hole, and emit pairs of photons similar to Hawking radiation.
* Optical black holes with dielectrics: A dielectric characterized by an electric permittivity εij and magnetic permeability μij, in terms of which the effective fields inside the material are described by the field strength I ab = Z abcd Fcd (> see electromagnetism in media), will behave like a curved geometry gab if Z abcd = (g/γ)1/2 (gac gbdgad gbc).
@ General references: Reznik PRD(00)gq/97 [radiation]; Schäfer & Sauerbrey ap/98 [high-intensity lasers]; Unruh & Schützhold PRD(03)gq [slow light]; Philbin et al Sci(08)-a0711, a0711 + news pw(08)mar [event horizon in optical fibers]; Nation et al PRL(09) + news sn(09)aug [using an array of SQUIDs]; Pereira & Moraes CEJP(11)-a0910 [flowing liquid crystal and equatorial section of Schwarzschild metric]; Belgiorno et al PRL(10) + Dudley & Skryabin Phy(10), comment Schützhold & Unruh PRL(11)-a1012 [using ultrashort laser pulse filaments], comment Liberati et al PRD(12)-a1111; news ns(13)sep [plastic black hole].
@ Radiation: Srinivasan & Padmanabhan gq/98 [from a moving mirror]; Schützhold & Unruh PRL(05)qp/04 [electromagnetic waveguide, proposal].
@ Optical black holes: Leonhardt & Piwnicki PRA(99)phy, PRL(00)cm/99 + pn(00)jan; Visser PRL(00)gq; Brevik & Halnes PRD(02)gq/01; Leonhardt Nat(02)phy/01, gq/01-ch; Royston & Gass gq/02 [with radial flow]; De Lorenci et al PRD(03); Hegde & Vishveshwara a1209 [analytical theory, and effectiuve Schwarzschild solution]; Finazzi & Carusotto PRA(14) [non-linear optical media]; Tinguely & Turner a1909 [Kerr-Newman black hole].
@ Dielectrics: Schützhold et al PRL(02)qp/01; Belgiorno et al PRD(11)-a1003, news disc(10)sep [and Hawking effect]; Belgiorno et al NJP(10)-a1006; Bittencourt et al CQG(14)-a1401 [static dielectrics].
@ Other types: Nguyen et al PRL(15) [analog black hole for microcavity polaritons]; Pikulin & Franz PRX(17) [solid state, black hole on a chip]; Blencowe & Wang PTRS(20)-a2003 [on a superconducting chip].

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