Analogue Gravity

Dear Colleagues,

Analogue models of gravity give us the rare opportunity to test kinematic aspects of gravity in a laboratory. Otherwise, we have almost no hope of verifying the long-standing pioneering predictions about the behavior of fields in the vicinity of black holes or cosmological horizons. In the last few years, great advances have been made in observing gravitational phenomena. Gravitational waves are routinely detected, which unravel, amongst other things, the mystery of masses of compact objects. It has been established through observational data that supermassive black holes reside at the center of almost all galaxies. The event horizon telescope is testing the validity of Einstein’s theory of gravity or some of its proposed modifications in the regions of extreme gravity close to a black hole. Still, some phenomena remain elusive. For example, how do we isolate signatures of black hole radiation from the sea of cosmic microwave background radiation and X-rays emitted during the accretion process?

Analogue models are simple physical systems where certain phenomena bear a correspondence with kinematic aspects of gravity. Usually, such systems exhibit a flow. The propagation of perturbations of an appropriately chosen variable describing the system can be likened with the propagation of a similar field in general curved spacetime. The correspondence with gravity can be established if we imagine that the said perturbation evolves in an effective analogue spacetime having an analogue metric. The analogue metric is naturally defined by the physical variables of the system, and by suitably choosing them, we can design metrics of our choice. This vastly expands the scope of Lorentzian geometry in physics beyond that of gravitation and lends a new perspective on the analogue systems themselves.

Since the inception of this beautiful idea with Unruh’s seminal paper on acoustic Hawking radiation in 1981, the field has seen much progress on all fronts—theory, simulations, and experiments. Analogues of Schwarzschild and Kerr spacetimes and the spacetime of an expanding Universe have been created in the laboratory. The first theoretically proposed analogue model of gravity was a classical inviscid fluid, and the relevant field was that of acoustic perturbations in the velocity potential. Soon, various novel analogue models were examined, such as gravity waves in water and quantum fluids like superfluid liquid helium, Bose–Einstein condensates, photon fluids, polariton fluids, quark gluon plasma produced in heavy ion collisions, etc. The greatest successes of this field have been the observation of Hawking radiation through correlations in density perturbations on either side of the analogue of a black hole horizon in a Bose–Einstein condensate and the detection of super-radiance through the scattering of surface waves from a draining bathtub vortex. These observations have confirmed that black hole radiation depends only on the geometry of spacetime.

We are now standing at the threshold of a new era of research in analogue gravity. It is now time to look back on the achievements and search for new gravitational phenomena that may reveal themselves through this correspondence, in previously unimagined ways. We welcome your ideas and opinions.

Prof. Dr. Banibrata Mukhopadhyay
Dr. Chandrachur Chakraborty
Guest Editors

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• acoustic horizon
• black holes
• Bose–Einstein condensation
• expanding Universe in analogue systems
• Hawking radiation
• laboratory test of gravity
• super-radiance