Frozen Star States & Black Hole Mimicry

Updated 18 March 2026

Frozen Star States are ultracompact, horizonless configurations supported by anisotropic fluids (ρ+pₙ=0) that mimic the exterior of Schwarzschild black holes.
They inherit black-hole-like thermodynamic properties with a Hawking temperature and entropy (S ≈ A/4) derived from unique Euclidean instanton tunneling processes.
Observationally, these objects produce photon ring shadows, gravitational wave echoes, and distinct quasi-normal modes that set them apart from classical black holes.

A frozen star is a non-singular, ultracompact solution to Einstein's equations that, while horizonless, possesses an exterior geometry and observable signatures that closely mimic those of a black hole. These configurations are supported by extremely anisotropic fluids, most notably characterized by a maximally negative radial pressure equation of state ( $\rho+p_r=0$ ), leading to near-infinite redshift at a critical surface just outside the would-be event horizon. Theoretical construction, thermodynamics, dynamical formation, microphysical sourcing, and phenomenology of frozen stars reveal their deep connections to semiclassical gravity, black-hole mimicry, and the quantum polymer paradigm (Brustein et al., 2023).

1. Theoretical Framework and Defining Properties

Frozen stars are static, spherically symmetric, horizonless configurations whose interiors are supported by fluids obeying $\rho + p_r = 0$ (ultra-negative radial pressure) and $p_\perp \ll p_r$ (vanishing tangential pressure) (Brustein et al., 2023, Brustein et al., 2024). In Schwarzschild coordinates, the metric takes the form: $ds^2 = -f(r)dt^2 + f(r)^{-1}dr^2 + r^2 d\Omega^2,$ where $f(r) = \epsilon^2 \ll 1$ inside the star, yielding a mass function $m(r) = \frac{1}{2}r(1-\epsilon^2)$ and energy profile $\rho = -p_r = (1-\epsilon^2)/(8\pi r^2)$ . The spacetime is glued at $r=R(1-\lambda)$ (interior) through a thin transition layer of width $\sim\lambda R$ to a Schwarzschild exterior at $r=R(1+\lambda)$ .

The global solution thus possesses:

Interior with nearly constant, exponentially redshifted $g_{tt}=\epsilon^2$ and no trapped surfaces.
A thin shell ("quantum layer") mediating the transition to vacuum Schwarzschild.
No event horizon; the redshift at the or near the surface is so large that, operationally, processes are frozen to a distant observer.

The frozen star is not singular: curvature scalars remain finite throughout the solution. When extended to Einstein–Born–Infeld or extended gravity frameworks, and with various matter sectors (scalar, Proca, Dirac, or perfect fluid), analogous ultracompact, horizonless objects with "frozen" interiors exist across diverse model families (Zhao et al., 19 Feb 2025, Chen et al., 31 Dec 2025, Tan et al., 11 Sep 2025).

2. Thermodynamics and Semiclassical Quantities

Frozen stars remarkably inherit the thermodynamic character of black holes with only perturbatively small deviations. Key thermodynamic properties are (Brustein et al., 2023):

Temperature ( $T_{FS}$ ): Using a near-horizon pair production analysis or surface-gravity argument, $T_{FS} \simeq 1/(8\pi M)$ up to corrections of order $\epsilon^2$ and $\lambda$ , coinciding with the Hawking temperature of a Schwarzschild black hole.
Entropy ( $S_{FS}$ ): Both first-law and Euclidean action (Gibbons–Hawking) methods yield $S_{FS} \approx A/4$ , where $A$ is the surface area. The detailed Euclidean action evaluates to $I=-\pi R^2$ , implying $S_{FS} = \pi R^2 = A/4$ .

In the quantum tunneling framework, the formation probability for a shell to tunnel into a specific microstate of a frozen star is $e^{-A/4}$ , but since there are $e^{A/4}$ microstates, the total probability is order unity (Brustein et al., 2023, Brustein et al., 4 Aug 2025). The radiation emitted follows a Page-curve behavior, with off-diagonal corrections arising at the Page time, restoring unitarity in the Hawking-like emission channel.

3. Dynamical Formation, Quantum Tunneling, and Polymer Connection

The formation mechanism for frozen stars fundamentally differs from classical black-hole formation. The classical collapse of matter fails to generate the required anisotropic stress configuration. Instead, the formation is described by a quantum tunneling process, modeled as a Euclidean instanton interpolating between a collapsing shell and the frozen-star interior (Brustein et al., 4 Aug 2025):

The tunneling action, calculated via the Euclidean path integral (Gibbons–Hawking prescription), gives $\mathrm{Im}\,S_E = 0$ after cancellation of interior and exterior contributions, resulting in a unit probability for transition. The phase transition is entropically driven due to the vastly larger number of frozen-star microstates relative to classical configurations.
The string-theoretic polymer model interprets the frozen interior as the classical limit of a highly quantum system of closed fundamental strings at maximal entropy, with a stress tensor matching $\rho+p_r=0$ , $p_\perp=0$ (Brustein et al., 2024).

This dual quantum-classical description links semiclassical gravity, stringy microphysics, and black-hole information phenomenology.

4. Model Realizations and Extensions

The frozen-star paradigm generalizes to a variety of field-theoretical models:

Boson, Proca, and Dirac Fields: Frozen states have been constructed for Bardeen–boson stars, Bardeen–Proca stars, Hayward–boson stars, and Bardeen–Dirac stars. The critical parameter (charge, coupling, self-interaction) determines when the oscillation frequency $\omega \to 0$ and the frozen regime is attained (Zhao et al., 19 Feb 2025, Zhang et al., 20 Mar 2025, Chen et al., 31 Dec 2025, Yue et al., 2023, Huang et al., 2023).
Neutron Stars and Nonlinear Electrodynamics: Sufficiently strong nonlinear magnetic charge or higher-derivative gravity leads to neutron-star solutions where, at a critical charge or coupling, a "frozen" configuration appears with infinite redshift at the surface ("critical horizon") (Tan et al., 11 Sep 2025, Tan et al., 29 Dec 2025).
Dark Matter Anisotropic Fluids: Relaxing the strict $p_r = -\rho$ EoS and considering more general anisotropic profiles with dark-matter sourcing yields a broad frozen-star family, distinguished by horizonless but extremal-like surfaces and often satisfying all standard energy conditions (Yue et al., 21 Jan 2026).
Higher-Curvature Gravities: In five (and four) dimensions, with an infinite tower of higher-derivative (quasi-topological) terms, frozen states generically arise as $\omega \to 0$ boson/proca-star solutions with regular interiors, near-extremal compactness, and no event horizon (Ma et al., 2024, Chen et al., 31 Dec 2025).

These constructions are characterized by the convergence of $g_{tt}$ and $1/g_{rr}$ to zero at a finite, model-dependent critical radius $r_c$ , beyond which the external observer cannot access the internal region in finite time.

5. Observational Signatures and Mimicry of Black Holes

Frozen stars are designed to be "black hole mimickers." Their observables are:

Optical/Photon Ring and Shadow: Frozen stars generically possess two light rings (null circular geodesics), an inner stable and an outer unstable one, bracketing the critical horizon (Huang et al., 19 Mar 2025, Zhao et al., 19 Feb 2025, Zhang et al., 20 Mar 2025). The outer light ring determines the shadow, which is almost indistinguishable from a black hole of the same mass if the critical radius lies just outside $3M$.
Gravitational Wave Echoes: Infalling matter or wavepackets reflect off the high-redshift surface, producing gravitational wave "echoes" with separation times determined by the light-travel time from the exterior to the critical surface (Yue et al., 21 Jan 2026).
Quasi-Normal Modes and Ringdown: The quasi-normal mode spectrum includes both standard damped ringdown modes and, due to the "frozen" interior surface, additional long-lived internal fluid modes whose frequencies can be parametrically small and damping rates suppressed (Brustein et al., 2023, Brustein et al., 2024).
Astrophysical Mass–Radius Curves: Frozen neutron stars and other compact objects terminate the usual $M$ – $R$ sequences at the extremal configuration, with the minimum metric function at the surface approaching zero, closing the gap between the neutron star sequence and the black-hole boundary (Tan et al., 29 Dec 2025).

These signatures suggest tests in both electromagnetic (e.g., EHT shadow, photon rings) and gravitational-wave (e.g., echoes, ringdown spectrum) channels.

6. Stability, Perturbations, and Defrosting

In the strict frozen star, the equation of state enforces internal ultra-stability—no internal oscillatory modes can be excited—so the object is dynamically "bald" (Brustein et al., 2023). However, "defrosting," i.e., perturbing the EoS so $p_r + \rho = \varepsilon \rho$ , allows the emergence of discrete, nonrelativistic internal fluid modes with frequencies $\omega^2 \sim \varepsilon/R^2$ . Their lifetimes are parametrically long, $\tau \sim 1/\varepsilon^2$ , and the sound speed is non-relativistic $c_s \sim \varepsilon$ (Brustein et al., 2024). These long-lived QNMs could be observationally relevant, providing another discriminatory channel compared to true black holes.

Mode stability analyses typically show that ultracompact frozen stars with only an outer unstable light ring and an inner stable light ring are susceptible to ergoregion-like instabilities in fully dynamical evolution, especially for configurations possessing both stable and unstable light rings (Brihaye et al., 11 Jul 2025, Liu et al., 11 Dec 2025).

7. Cosmological and Model Extensions

Frozen-star solutions have been embedded in dynamical, cosmological spacetimes—most notably, generalizations of Janis–Newman–Winicour (JNW) solutions—where cosmological versions preserve the infinite-redshift surface at finite radius in a time-evolving background (Kastor et al., 2016). Further, model independence is observed: in higher-derivative gravities, dark-matter-anisotropic fluids, and for generic equations of state satisfying (or perturbing) the ultra-negative radial pressure criterion, frozen-star states robustly emerge as universal endpoints of compact-object sequences (Yue et al., 21 Jan 2026, Tan et al., 29 Dec 2025).

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