Narrow Results

Recent developments in simulating fundamental quantum field theoretical effects in the kinematical context of analogue gravity are reviewed. Specifically, it is argued that a curved spacetime generalization of the Unruh–Davies effect — the Gibbons–Hawking effect in the de Sitter spacetime of inflationary cosmological models — can be implemented and verified in an ultracold gas of bosonic atoms.

This work addresses the analogy between the speed of sound of a viscous, barotropic, and irrotational fluid and the equation of motion for a non-massive field in a curved manifold. It will be shown that the presence of viscosity implies the introduction, into the equation of motion of the gravitational analogue, of a source term which entails the flow of energy from the non-massive field to the curvature of the spacetime manifold. The stress–energy tensor is also computed and it is found not to be constant, which is consistent with such energy interchange.

Beltrami-shaped graphene sheets have been recently proposed as analogs of curved spacetimes with Hawking–Unruh effects detected through typical condensed matter measurements involving scanning tunneling microscopes and spectroscopy. However, such deformed sheets, if ever fabricated, will contain large strain-induced pseudo-magnetic fields with important guiding effects on the motion of the electrons in the conduction band. Besides, possible surface polariton and plasmon modes are known to be important players in the radiative heat transfer which takes place in the natural near-field nanoscale experimental conditions. Therefore, we suggest that the latter class of experiments could shed light on phenomena related to the black hole membrane paradigm instead.

In this paper, we model a canonical acoustic thin-shell wormhole (CATSW) in the framework of analogue gravity systems. In this model, we apply cut and paste technique to join together two spherically symmetric, analogue canonical acoustic solutions, and compute the analogue surface density/surface pressure of the fluid using the Darmois–Israel formalism. We study the stability analyses by using a linear barotropic fluid (LBF), Chaplygin fluid (CF), logarithmic fluid (LogF), polytropic fluid (PF) and finally Van der Waals Quintessence (VDWQ). We show that a kind of analog acoustic fluid with negative energy is required at the throat to keep the wormhole stable. It is argued that CATSW can be a stabile thin-shell wormhole if we choose a suitable parameter values.

We propose a nonrelativistic fluid analogue model for a scalar field coupled to a classical source. The generic analogue gravity model involves the phonon-field which is coupled to the acoustic metric. We work in the special relativity limit of the acoustic analogue. By assuming a time-dependent external potential on the fluid system, we are able to model a source term for the scalar field. Upon quantization, phonon creation due to the source is studied.

In black hole evaporation process, the mass of the hole anti-correlates with the Hawking temperature. This indicates that the smaller holes have higher surface gravity. For analogue Hawking effects, however, the acoustic surface gravity is determined by the local values of the dynamical velocity of the stationary background fluid flow and the speed of propagation of the characteristic perturbation embedded in the background fluid, as well as by their space derivatives evaluated along the direction normal to the acoustic horizon, respectively. The mass of the analogue system — whether classical or quantum — does not directly contribute to extremize the value of the associated acoustic surface gravity. For general relativistic axially symmetric background fluid flow in the Schwarzschild metric, we show that the initial boundary conditions describing such accretion influence the maximization scheme of the acoustic surface gravity and associated analogue temperature. Aforementioned background flow onto black holes can assume three distinct geometric configurations. Identical set of initial boundary conditions can lead to entirely different phase-space behavior of the stationary flow solutions, as well as the salient features of the associated relativistic acoustic geometry. This implies that it is imperative to investigate how the measure of the acoustic surface gravity corresponding to the accreting black holes gets influenced by the geometric configuration of the inflow described by various thermodynamic equations of state. Such investigation is useful to study the effect of Einstenian gravity on the nonconventional classical features as observed in Hawking like effect in a dispersive medium in the limit of a strong dispersion relation.

Linear perturbation of general relativistic accretion of low angular momentum hydrodynamic fluid onto a Kerr black hole leads to the formation of curved acoustic geometry embedded within the background flow. Characteristic features of such sonic geometry depend on the black hole spin. Such dependence can be probed by studying the correlation of the acoustic surface gravity $\kappa$ with the Kerr parameter $a$. The $\kappa$–$a$ relationship further gets influenced by the geometric configuration of the accretion flow structure. In this work, such influence has been studied for multitransonic shocked accretion where linear perturbation of general relativistic flow profile leads to the formation of two analogue black hole-type horizons formed at the sonic points and one analogue white hole-type horizon which is formed at the shock location producing divergent acoustic surface gravity. Dependence of the $\kappa$–$a$ relationship on the geometric configuration has also been studied for monotransonic accretion, over the entire span of the Kerr parameter including retrograde flow. For accreting astrophysical black holes, the present work thus investigates how the salient features of the embedded relativistic sonic geometry may be determined not only by the background spacetime, but also by the flow configuration of the embedding matter.

Black D3-branes are known to admit an effective hydrodynamic description when low frequency and long wavelength perturbations are introduced into the system. We use this perturbed nonextremal black D3-brane as background metric to study the emergence of acoustic black holes, following the same holographic approach in constructing the acoustic black hole in asymptotically Anti-de-Sitter (AAdS) background spacetime. We show that the acoustic black hole which appears on the timelike cutoff surface in the nonextremal black D3-brane also admits a holographic dual description. The duality includes the dynamical connection between the acoustic black hole and the bulk gravity, a universal equation relating the Hawking-like temperature and the real Hawking temperature, and a phonon/scalar channel quasi-normal mode correspondence.

Perturbations in a draining vortex can be described analytically in terms of confluent Heun functions. In the context of analogue models of gravity in ideal fluids, we investigate analytically the absorption length of waves in a draining bathtub, a rotating black hole analogue, using confluent Heun functions. We compare our analytical results with the corresponding numerical ones, obtaining excellent agreement.

Recent experimental progresses in controlling classical and quantum fluids have made it possible to realize acoustic analogs of gravitational black holes, where a flowing fluid provides an effective spacetime on which sound waves propagate, demonstrating Hawking-like radiation and superradiance. We propose the exciting possibility that new hydrodynamic systems might provide insights to help resolve mysteries associated with quantum gravity, including the black hole information-loss paradox and the removal of spacetime singularities.

In the framework of acoustic geometry, we investigate the transonic accretion onto gravitating astrophysical objects to establish that such a configuration is a unique example of an classical analogue system realized naturally in the Universe. Only for an accreting astrophysical black hole, on which we mainly concentrate in our work, both kind of horizons, gravitational as well as acoustic (generated due to transonicity of accreting fluid) are simultaneously present in the same system, which indicates that accreting astrophysical black holes are the most ideal candidate to study theoretically and to compare the properties of these two different kind of horizons. Such a system is also unique in the aspect that accretion onto the black holes represents the only classical analogue system found in the nature so far, where the analogue Hawking temperature may exceed the actual Hawking temperature.