The key event in the “modern” period (though largely unrecognised at the time) was the 1981 publication of Unruh’s paper “Experimental black hole evaporation” , which implemented an analogue model based on fluid flow, and then used the power of that analogy to probe fundamental issues regarding Hawking radiation from “real” general relativity black holes.
We believe that Unruh’s 1981 article represents the first observation of the now widely established fact that Hawking radiation has nothing to do with general relativity per se, but that Hawking radiation is instead a fundamental curved-space quantum field theory phenomenon that occurs whenever a horizon is present in an effective geometry.15 Though Unruh’s 1981 paper was seminal in this regard, it lay largely unnoticed for many years. Some 10 years later Jacobson’s article “Black-hole evaporation and ultrashort distances”  used Unruh’s analogy to build a physical model for the “trans-Planckian modes” believed to be relevant to the Hawking radiation process. Progress then sped up with the relatively rapid appearance of  and [377, 378]. (This period also saw the independent rediscovery of the fluid analogue model by one of the present authors , and the first explicit consideration of superfluids in this regard .)
The later 1990’s then saw continued work by Jacobson and his group [187, 188, 88, 90, 198], with new and rather different contributions coming in the form of the solid state models considered by Reznik [319, 318].16 This period also saw the introduction of the more general class of superfluid models considered by Volovik and his collaborators [402, 403, 213, 110, 407, 405, 406, 199, 409, 410], more precise formulations of the notions of horizon, ergosphere, and surface gravity in analogue models [389, 391], and discussions of the implications of analogue models regarding Bekenstein-Hawking entropy [390, 391]. Finally, analogue spacetimes based on special relativistic acoustics were considered in . By the year 2000, articles on one or another aspect of analogue gravity were appearing at the rate of over 20 per year, and it becomes impractical to summarise more than a selection of them.
Key developments in 2000 were the introduction, by Garay and collaborators, of the use of Bose-Einstein condensates as a working fluid [136, 137], and the extension of those ideas by the present authors . Further afield, the trans-Planckian problem also reared its head in the context of cosmological inflation, and analogue model ideas previously applied to Hawking radiation were reused in that context [205, 273].
That year also marked the appearance of a review article on superfluid analogues , more work on “near-horizon” physics , and the transference of the idea of analogue-inspired “multiple metric” theories into cosmology where they can be used as the basis for a precise definition of what is meant by a VSL (“variable speed of light”) cosmology . Models based on nonlinear electrodynamics were investigated in , based models were reconsidered in [193, 411], and “slow light” models in quantum dielectrics were considered in [235, 236, 231].
The most radical proposal to appear in 2000 was that of Laughlin et al. . Based on taking a superfluid analogy rather literally they mooted an actual physical breakdown of general relativity at the horizon of a black hole .
Additionally, the workshop on “Analogue models of general relativity”, held at CBPF (Rio de Janeiro) gathered some 20 international participants and greatly stimulated the field, leading ultimately to the publication of the book  in 2002.
This year saw more applications of analogue-inspired ideas to cosmological inflation [107, 263, 262, 207, 275], to neutron star cores , and to the cosmological constant [414, 416].
Closer to the heart of the analogue programme were the development of a “normal mode” analysis in [15, 16, 398], the development of dielectric analogues in , speculations regarding the possibly emergent nature of Einstein gravity [20, 398], and further developments regarding the use of  as an analogue for electromagnetism. Experimental proposals were considered in [19, 398, 331].
Vorticity was discussed in , and the use of BECs as a model for the breakdown of Lorentz invariance in . Analogue models based on nonlinear electrodynamics were discussed in . Acoustics in an irrotational vortex were investigated in .
The excitation spectrum in superfluids, specifically the fermion zero modes, were investigated in [412, 182], while the relationship between rotational friction in superfluids and super-radiance in rotating spacetimes was discussed in . More work on “slow light” appeared in . The possible role of Lorentz violations at ultra-high energy was emphasised in .
“What did we learn from studying acoustic black holes?” was the title and theme of Parentani’s article in 2002 , while Schützhold and Unruh developed a rather different fluid-based analogy based on gravity waves in shallow water [344, 345]. Super-radiance was investigated in , while the propagation of phonons and quasiparticles was discussed in [122, 121]. More work on “slow light” appeared in [124, 311].
The stability of an acoustic white hole was investigated in , while further developments regarding analogue models based on nonlinear electrodynamics were presented by Novello and collaborators in [102, 103, 282, 278, 126]. Analogue spacetimes relevant to braneworld cosmologies were considered in .
Though analogue models lead naturally to the idea of high-energy violations of Lorentz invariance, it must be stressed that definite observational evidence for violations of Lorentz invariance is lacking - in fact there are rather strong constraints on how strong any possible Lorentz violating effect might be [195, 194].
That year saw further discussion of analogue-inspired models for black hole entropy and the cosmological constant [419, 421], and the development of analogue models for FRW geometries [115, 114, 17, 105, 242]. There were several further developments regarding the foundations of BEC-based models in [18, 116], while analogue spacetimes in superfluid neutron stars were further investigated in .
Effective geometry was the theme in , while applications of nonlinear electrodynamics (and its effective metric) to cosmology were presented in . Super-radiance was further investigated in [26, 24], while the limitations of the “slow light” analogue were explained in . Vachaspati argued for an analogy between phase boundaries and acoustic horizons in . Emergent relativity was again addressed in .
The review article by Burgess , emphasised the role of general relativity as an effective field theory - the sine qua non for any attempt at interpreting general relativity as an emergent theory. The lecture notes by Jacobson  give a nice introduction to Hawking radiation and its connection to analogue spacetimes.
The year 2004 saw the appearance of some 30 articles on (or closely related to) analogue models. Effective geometries in astrophysics were discussed by Perez Bergliaffa , while the physical realizability of acoustic Hawking radiation was addressed in [95, 382]. More cosmological issues were raised in [382, 424], while a specifically astrophysical use of the acoustic analogy was invoked in [96, 97, 98].
BEC-based horizons were again considered in [149, 148], while backreaction effects were the focus of attention in [10, 9, 208]. More issues relating to the simulation of FRW cosmologies were raised in [118, 119].
Unruh and Schützhold discussed the universality of the Hawking effect , and a new proposal for possibly detecting Hawking radiation in a electromagnetic wave guide . The causal structure of analogue spacetimes was considered in , while quasinormal modes attracted attention in [31, 237, 64, 269]. Two dimensional analogue models were considered in .
There were attempts at modelling the Kerr geometry , and generic “rotating” spacetimes , a proposal for using analogue models to generate massive phonon modes in BECs , and an extension of the usual formalism for representing weak-field gravitational lensing in terms of an analogue refractive index .
Finally we mention the development of yet more strong observational bounds on possible ultra high energy Lorentz violation [196, 197].
The first few months of 2005 have seen continued and vigourous activity on the analogue model front.
More studies of the super-resonance phenomenon have appeared [25, 113, 209, 354], and a mini-survey was presented in . Quasinormal modes have again received attention in , while the Magnus force is reanalysed in terms of the acoustic geometry in . Singularities in the acoustic geometry are considered in , while back-reaction has received more attention in .
Interest in analogue models is intense and shows no signs of abating.
We shall in the next subsection focus more precisely on the early history of analogue models, and specifically those that seem to us to have had a direct historical connection with the sustained burst of work carried out in the last 15 years.
© Max Planck Society and the author(s)