Acoustic Horizons: Analogue Gravity Phenomena

• Acoustic horizons are the causal boundaries in a flowing medium where the local flow speed equals the sound speed, analogous to gravitational event horizons.
• They are modeled by mapping fluid perturbations to an effective Lorentzian metric, elucidating phenomena such as Hawking radiation, causal structures, and horizon thermodynamics.
• Experimental realizations in Bose–Einstein condensates, polariton fluids, and water tanks provide practical platforms to explore the dynamical, dissipative, and quantum aspects of analogue gravity.

An acoustic horizon is a causal boundary in a flowing medium beyond which sound (or other excitations obeying a similar equation) cannot propagate upstream, analogous to the event horizon of a gravitational black hole. The concept emerges naturally in diverse physical systems, including compressible hydrodynamics, general-relativistic fluid flows, Bose-Einstein condensates, polariton and superfluid platforms, and even magnetohydrodynamic and cosmological models. Acoustic horizons underpin a wealth of analogue gravity phenomena—ranging from Hawking-type radiation and nontrivial causal structures to horizon thermodynamics, information-theoretic paradoxes, and local “membrane” dissipation laws—that offer deep insight into both emergent and fundamental aspects of black-hole physics.

1. Definition and Formation of Acoustic Horizons

Acoustic horizons arise when the flow speed of a medium equals the local propagation speed of excitations, typically the sound speed in a barotropic or superfluid medium. For a flow with local velocity $v(x)$ and sound speed $c_s(x)$, the acoustic horizon is given by $|v(x_H)| = c_s(x_H).$

The effective wave equation for perturbations $\phi$ in such a background, for irrotational and barotropic flows, can always be written as

$$\frac{1}{\sqrt{-g}}\partial_\mu(\sqrt{-g}g^{\mu\nu}\partial_\nu\phi)=0,$$

where $g_{\mu\nu}$ is the so-called “acoustic metric,” whose structure depends on the flow and sound speed (see, e.g., (Mannarelli et al., 2020, Sarkar et al., 2013, Fabris et al., 2013, Wang et al., 2019)). The null hypersurface of this effective metric coincides with the acoustic horizon.

The formation of acoustic horizons is not restricted to Newtonian fluids. In general relativistic situations (e.g., accretion flows in a Kerr or Schwarzschild spacetime), the locations of acoustic horizons can differ significantly from optical (event) horizons, particularly in curved backgrounds or when relativistic pressure, angular momentum, or dissipation are important (Cheng et al., 10 Dec 2025, Pu et al., 2012, Maity et al., 2020, Vieira et al., 2021).

2. Acoustic Metrics, Analogue Gravity, and Surface Gravity

The mapping of fluid perturbation dynamics to an effective Lorentzian metric underlies the analogy with gravitation. The canonical form of the acoustic metric for a steady background flow is (in $(t, \mathbf{x})$ coordinates),

$$ds^2 = \frac{\rho_0}{c_s} \left[ -(c_s^2 - v^2)dt^2 - 2\mathbf{v}\cdot d\mathbf{x}\,dt + d\mathbf{x}^2 \right],$$

with $\rho_0$ the background density. This structure supports key causal features of general relativity, including horizons and ergoregions (Sarkar et al., 2013, Mannarelli et al., 2020, Fabris et al., 2013, Wang et al., 2019). In systems with rotation, the metric acquires off-diagonal $g_{t\phi}$ elements, supporting both acoustic horizons and ergosurfaces (Benone et al., 2014, Svetlichnyi et al., 2023).

The acoustic surface gravity, $\kappa$, is defined via the normal derivative of $c_s^2 - v^2$ at the horizon:

$$\kappa = \frac{1}{2}\left. \partial_n (c_s^2 - v^2)\right|_{H},$$

and dictates the analogue Hawking temperature via

$T_H = \frac{\hbar \kappa}{2\pi k_B}.$

This formalism extends straightforwardly to streaming superfluids, curved backgrounds, and magnetohydrodynamic flows (Gheibi et al., 2017, Giacobino et al., 16 Dec 2025).

3. Dynamical Properties, Dissipation, and Nonlinearity

Acoustic horizons act as one-way membranes for perturbations: upstream-propagating sound (or equivalent excitations) from the supersonic region cannot escape past the horizon to the subsonic exterior. For time-dependent or nonlinear backgrounds, the precise location and stability of the horizon are generally affected by higher-order corrections, including weak dispersion, viscosity, or nonlinear advection (Sarkar et al., 2013, Ray, 2019).

In the presence of dissipation (e.g., viscosity or three-body loss in a BEC), both the amplitude and energy flux of propagating disturbances are affected near the horizon. Viscosity can regularize divergences and shrink the radius of the sonic sphere, allowing a finite tunneling probability for phonon emission (analogue Hawking radiation) (Ray, 2019, Khlebnikov, 2023). In systems with dispersive corrections, even weak nonlocality can induce total opacity of the horizon to waves with upstream group velocity, killing horizon transmission exponentially (Sarkar et al., 2013).

Covariant kinetic theory provides a complementary picture: phonon emission at the horizon results in the irreversible growth of entropy at the expense of the bulk flow energy. Acoustic horizons thus exhibit a form of "quantum friction" and realize dissipation at a universal minimal viscosity-to-entropy-density ratio $\eta/s = 1/(4\pi)$, analogous to the black-hole membrane paradigm (Chiofalo et al., 2022, Mannarelli et al., 2020).

4. Analogue Black-Hole Phenomena and Quantum Signatures

Acoustic horizons replicate a myriad of classical and quantum black-hole phenomena:

• Hawking radiation: Stationary acoustic holes emit thermal phonons at $T_H$ (Mannarelli et al., 2020, Cheng et al., 10 Dec 2025). The emission can be derived via quantum tunneling, balance of entropy/energy fluxes, or mode-mixing in a pseudo-unitary scattering framework (Giacobino et al., 16 Dec 2025, Chiofalo et al., 2022).
• Quasi-normal modes and ringdown: Acoustic metrics display quasi-normal spectra, with mode frequencies scaling with surface gravity and effective potential structure (Chaverra et al., 2015, Vieira et al., 2021).
• Page curve and islands: The entanglement entropy of emitted Hawking phonons obeys a Page curve governed by the area of the acoustic horizon when the "island" prescription is applied, verifying information recovery in the analogue system (Cheng et al., 10 Dec 2025).
• Chaos bound: The exponential growth rate (Lyapunov exponent) of radial momentum for infalling test vortices or orbits at the horizon saturates $\lambda \leq 2\pi T_H$ in non-extremal cases, fully mirroring the chaos bound in holographic gravity (Wang et al., 2019, Singh et al., 20 May 2024).

In curved backgrounds (e.g., black holes in superfluid or polariton platforms), event horizons and acoustic horizons may coexist and interact, opening new possibilities for exploring information paradox analogues, QNM spectroscopy, and the dynamical interplay of causal structures (Cheng et al., 10 Dec 2025, Vieira et al., 2021, Giacobino et al., 16 Dec 2025).

5. Extensions: Rotating, Magnetoacoustic, and Cosmological Horizons

Acoustic horizons appear in a variety of advanced settings:

• Rotating condensates and fluids: Draining-bathtub analogues manifest rotating acoustic horizons, ergoregions, and standing-wave "cloud" solutions, with phenomena such as superradiance and confinement analogous to Kerr and Kerr-Newman spacetimes (Benone et al., 2014, Svetlichnyi et al., 2023).
• Magnetoacoustic and Alfvénic horizons: In ideal MHD, fast magnetoacoustic and Alfvén waves can experience horizons when the flow speed matches the relevant characteristic velocity, leading to quantum emission of magnephonons and Alphonons at detectable $T_H$ in laboratory setups (Gheibi et al., 2017).
• Cosmological horizons: In non-canonical inflationary models (tachyacoustic cosmology), the acoustic horizon provides the relevant causal boundary for primordial perturbation freezing, but constraints from Planck-scale physics preclude a sustained, large acoustic horizon, limiting the solution to the horizon problem (Lin et al., 2020).
• Wormhole and bounce backgrounds: Acoustic metrics embedded in black-bounce or wormhole spacetimes yield rich structures, including multi-horizon configurations, sonic shadows, and horizonless (wormhole) branches with analogue signatures distinguishable from true horizons (Pal et al., 2022).

6. Experimental Realizations and Observational Signatures

Acoustic horizons have been engineered and probed in a diversity of platforms:

• BECs and superfluids: Supersonic regions in controlled condensates (e.g., via phase engineering, convergent flows, or engineered loss) have revealed phase-slip dynamics, ring soliton emission, and analogue Hawking radiation (Khlebnikov, 2023, Chiofalo et al., 2022).
• Polariton and photon fluids: Driven polariton microcavities realize programmable curved effective metrics; acoustic horizons and their quantum optics signatures (such as quadrature squeezing and two-mode entanglement) can be measured with high precision (Giacobino et al., 16 Dec 2025).
• Water tanks and classical analogues: Tabletop experiments in fluids have demonstrated draining vortex horizons, mode reflection, and superradiance, with ringdown and QNM features directly observable (Benone et al., 2014).
• MHD tubes and astrophysical flows: Plasma setups and astronomical accretion flows exhibit sonic and magnetoacoustic horizons, with corresponding theoretical and potential observational signatures in emission spectra and shadow profiles (Gheibi et al., 2017, Pu et al., 2012, Maity et al., 2020).

Notably, entropic and viscous effects, Page curves, QNM spectra, and chaos bounds remain accessible at current or near-future resolution in cold-atom and polariton hydrodynamics experiments. The stability and realization of multi-horizon and horizonless (wormhole) sonic structures are areas of active investigation (Cheng et al., 10 Dec 2025, Pal et al., 2022, Singh et al., 20 May 2024).

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Acoustic horizons thus provide a rigorous, tunable, and deeply illuminating analogue of event horizons in general relativity, embodying not only hallmark causal and thermodynamic properties, but also serving as a testbed for quantum field theory in curved spacetime, information paradoxes, entropy bounds, and emergent gravitational phenomena (Sarkar et al., 2013, Mannarelli et al., 2020, Cheng et al., 10 Dec 2025, Chiofalo et al., 2022, Wang et al., 2019, Giacobino et al., 16 Dec 2025).