The next generation of analogue gravity experiments

We observe the time dependence of the Hawking radiation in an analogue black hole. Soon after the formation of the horizon, there is little or no Hawking radiation. The Hawking radiation then ramps up during approximately one period of oscillation, until it reaches the quantity expected for spontaneous emission. This is similar to a black hole created from gravitational collapse. The spectrum remains approximately constant at the spontaneous level for some time, similar to a stationary black hole. An inner horizon then forms, in analogy with a charged black hole. The inner horizon causes stimulated Hawking radiation. Both types of stimulation predicted by Ted Jacobson and coworkers likely contribute, but the monochromatic stimulation probably contributes more than does the black-hole lasing.

Quantum fluids of light can be created in an optical cavity or in paraxial propagation. In such conditions propagating photons behave like massive particles, because they acquire a small effective mass. Moreover they interact with each other when the medium in which they propagate contains a nonlinear material. In the past few years two such systems, exciton polaritons in a semiconductor microcavity and photons in a hot Rubidium vapour have proved to be interesting platforms for the study of quantum hydrodynamics, including Bose-Einstein condensation, superfluidity, Cerenkov waves, or quantum turbulence. Topological excitations like vortices can be created in a polariton fluid with a defect or injected with engineered orbital angular using a specific laser excitation. 

Based on these properties, Professor Giacobino will show that these systems are very promising for the study of analogue gravity physics in a fluid of light. As proposed in several theoretical works, cavity polaritons can be considered for the emulation of Hawking physics. A closed 2D event horizon, appearing in the polariton superfluid where the fluid velocity becomes larger than the speed of sound in the fluid can be used to simulate the physics of black holes. Moreover, by injecting orbital angular momentum in the flow, the physics of rotating black holes can be studied, in particular Penrose process such as extracting rotation energy from the black hole or acceleration of excitations passing close to a rotating black hole.

Superradiance is the amplification of waves scattered by a rapidly rotating object, first introduced by Roger Penrose in 1969 for rotating black holes. This phenomenon is not peculiar of the field of astrophysics: a few years after Penrose's proposal, in 1971, Zel’dovich showed that electromagnetic waves scattered by a metallic rotating cylinder get amplified whenever their angular frequency ω satisfies the condition ω<mΩ, where m is the waves angular frequency and Ω is the cylinder angular velocity. Theoretical predictions of rotating superradiance amplification have been presented for many other physical systems as in hydrodynamics, nonlinear optics and Bose-Einstein condensates. However, the only measurement of superradiance has been recently reported in water waves, in a draining bathtub experiment.

In this talk, Professor Faccio elucidates how the process of Penrose-like superradiance can be observed in superfluids of light, and shows how it can be realised in a true optics experiment. Thanks to a novel analysis based on Noether currents, they show that superradiant scattering occurs in a 2D rotating nonlinear superfluid of light. This analysis addresses the full Nonlinear Schrödinger (NLS) equation without making use of the analogy with curved space-time and accounting for quantum pressure, always present in Bose-Einstein condensates and photon fluid analogues. Results of numerical simulations demonstrate the observability of the phenomenon is a nonlinear optics experiment.

This work deepens the understanding of the superradiance scattering in superfluids, reporting an experimental proposal to observe superradiated light, and unveils novel phenomena in the field of nonlinear optics.

While the original analogy between condensed matter systems and curved space-time geometries was originally derived under strict conditions, recent analogue gravity experiments suggest that vortex flows outside the analogue regime and rotating black holes still share similar fundamental effects.

In this talk, Theo Torres will focus on light-bending, superradiant scattering and quasi-normal modes emission in the presence of dispersive effects.

Using tools originally developed in black hole physics, Theo Torres will characterise theoretically the quasi-normal mode spectrum emitted by a perturbed vortex flow. This is done by extending and reinterpreting the notion of light-rings. An experiment will be presented where the relaxation spectrum of a vortex flow was measured and found to be in agreement with the light-ring prediction. 
Finally, relying on the fact that the quasi-normal modes are fully characterised by the properties of the background flow, Theo will introduce a new and non-invasive flow measurement technique applicable to fluids and superfluids alike. This method, that is called analogue black hole spectroscopy, will be presented theoretically and tested experimentally.

These theoretical and experimental results exhibit a new facet of the fluid-gravity analogy and shine new light on fundamental processes of rotational systems.

The current focus of analogue gravity research is on the behaviour of classical and quantum fields on fixed background spacetimes. However, in many cases of interest, including the scattering of high-amplitude waves from small compact objects or the extreme case of late-stage Hawking evaporation, the spacetime geometry is altered significantly by interactions with a field. Dr Gooding will discuss a recent analogue rotating black hole experiment in a vortex flow that exhibits back-reaction, and also admits a relatively simple theoretical description. Dr Gooding will then comment on future back-reaction experiments, and the possibility of using analogue systems to provide experimental guidance for quantum gravity.  

In this talk, Dr Rousseaux will first review the way to build analogue space-times in fluid mechanics by looking at the flow phase diagram and the corresponding analogue experiments performed during the last 13 years in the associated flow regimes (Nice 2008–2010, Vancouver 2011, Poitiers 2013–2017). Then, they will discuss the effect of non-linearities and capillarity on analogue Hawking radiation in hydraulic channels and their impact on the interpretation of the above experiments with some guidelines for future work. Thin films like the circular jump with different dispersive properties will be discussed in a second part with the presentation of a brand new system for the next generation of analogue gravity experiments in hydrodynamics.

Experiments on analogues of gravity have extended the theory of quantum phenomena in gravitational fields to other areas of physics, confirming them there, but their primary goal is to learn something new for astrophysics. This lecture explains how analogues of gravity may shed light on one of the greatest challenges of theoretical cosmology: the enigma of dark energy. 

In 1968 Zeldovich published the conjecture that the cosmological constant – now known as dark energy – is a consequence of vacuum fluctuations. While he got the correct structure of Einstein’s cosmological term, he got a vastly incorrect quantitative value. In its modern version, the theory is off by 120 orders of magnitude from the measured value. 

What is the problem? While all real experiments on forces of the quantum vacuum are explicable by renormalised vacuum fluctuations, gravity was thought to perceive the total, bare vacuum energy. One might regard the measured value of the cosmological constant as the result of an experiment disproving this idea. In fact, as the lecture will explain, the most successful theory of vacuum forces in real materials, Lifshitz theory, may be adapted to account for the correct order of magnitude of the measured cosmological constant [https://doi.org/10.1016/j.aop.2019.167973]. This adaptation is possible due to the analogy between gravitational fields and electromagnetic media, and heavily draws on insights gained in the physics of horizons.

To demarcate the limits of experimental knowledge Dr Thébault probes the limits of what might be called an experiment. By appeal to examples of scientific practice from astrophysics and analogue gravity, they demonstrate that the reliability of knowledge regarding certain phenomena gained from an experiment is not circumscribed by the manipulability or accessibility of the target phenomena. Rather, the limits of experimental knowledge are set by the extent to which strategies for what we call ‘inductive triangulation’ are available: that is, the validation of the mode of inductive reasoning involved in the source-target inference via appeal to one or more distinct and independent modes of inductive reasoning. When such strategies are able to partially mitigate reasonable doubt, we can take a theory regarding the phenomena to be well supported by experiment. When such strategies are able to fully mitigate reasonable doubt, we can take a theory regarding the phenomena to be established by experiment. There are good reasons to expect the next generation of analogue experiments to provide genuine knowledge of unmanipulable and inaccessible phenomena such that the relevant theories can be understood as well supported.