A covariant framework is developed for photon surfaces in dynamical spherical spacetimes, recovering static limits and applied to collapse and accretion/evaporation models.
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Light ring stability in ultra-compact objects
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We prove the following theorem: axisymmetric, stationary solutions of the Einstein field equations formed from classical gravitational collapse of matter obeying the null energy condition, that are everywhere smooth and ultracompact (i.e., they have a light ring) must have at least two light rings, and one of them is stable. It has been argued that stable light rings generally lead to nonlinear spacetime instabilities. Our result implies that smooth, physically and dynamically reasonable ultracompact objects are not viable as observational alternatives to black holes whenever these instabilities occur on astrophysically short time scales. The proof of the theorem has two parts: (i) We show that light rings always come in pairs, one being a saddle point and the other a local extremum of an effective potential. This result follows from a topological argument based on the Brouwer degree of a continuous map, with no assumptions on the spacetime dynamics, and hence it is applicable to any metric gravity theory where photons follow null geodesics. (ii) Assuming Einstein's equations, we show that the extremum is a local minimum of the potential (i.e., a stable light ring) if the energy-momentum tensor satisfies the null energy condition.
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Derives a factorized leading term for the strong deflection angle near degenerate photon spheres using local expansion of the effective potential and Weyl tensor measures.
In dRGT massive gravity, static spherically symmetric black holes exhibit zero, one, or two photon spheres whose topological charges and stability patterns differ from Einstein gravity and from horizonless compact objects.
Presents classical GR solutions for black holes enveloped by ultra-relativistic orbiting particle shells that extend the photon sphere to arbitrary depth while remaining optically indistinguishable from standard black holes.
Upper bound r_γ ≤ [(n-1)M]^{1/(n-3)} and lower bound r_γ ≥ ((n-1)/2)^{1/(n-3)} r_H on the photon sphere for nD black holes with anisotropic matter obeying the weak energy condition and non-positive trace.
Black holes with infinitely degenerate horizons are proposed to be stable against Aretakis instability, potentially serving as end states.
Causal viscous relativistic hydrodynamics applied to neutron stars yields new axial mode families without perfect-fluid counterparts, including mode avoidance and long-lived modes.
Cuspy black hole shadows correspond to swallowtail thermodynamic free energy, with boundary self-intersections marking geometric phase transitions whose critical exponents fall in the mean-field class.
A necessary and sufficient condition for thermodynamic mimicry of Schwarzschild black holes is satisfied by a one-parameter family of self-similar systems including hillingar black holes, stiffest stars, and frozen stars, derived from photon-sphere control of junction conditions and the TOV equation
Analytic charged black holes in nonlinear electrodynamics with non-monotonic lapse functions support stable light rings and additional longer-lived quasinormal modes compared to Einstein gravity.
Lorentzian-Euclidean black holes produce excess inner-shadow intensity and accumulate energy at the horizon with backreaction unlike stable light rings.
ℓ-boson stars are constructed and their properties studied in asymptotically anti-de Sitter spacetime.
Numerical construction of Hayward boson stars shows that frozen states produce Schwarzschild-like shadows with no extra photon rings while non-frozen states show multiple photon rings inside the shadow.
A constructive algorithm yields NEC-obeying static spherical metrics with g_tt g_rr = -1, including a log-corrected Schwarzschild geometry that can mimic black holes.
In dyonic black holes, periodic orbits with identical rotation numbers but spanning different curvature regions generate radiatively distinct gravitational waveforms in EMRIs.
The paper decomposes simulation data of ultracompact boson stars into spherical harmonics as a first step toward characterizing non-spherical modes.
A scalar-NED coupled black hole metric is reconstructed from an effective geometry, yielding EHT bounds on magnetic charge, Hawking-Page transition, and topological equivalence to the Reissner-Nordström solution.
Topological numbers categorize black hole systems into universality classes based on thermodynamic behavior, with calculations for critical points and phase transitions.
Current and future observations can test whether dark compact objects are Kerr black holes or exotic alternatives, with null results strengthening the black hole paradigm.
citing papers explorer
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Photon spheres in dynamical space-times
A covariant framework is developed for photon surfaces in dynamical spherical spacetimes, recovering static limits and applied to collapse and accretion/evaporation models.
-
Strong-deflection expansion of the deflection angle near a degenerate photon sphere
Derives a factorized leading term for the strong deflection angle near degenerate photon spheres using local expansion of the effective potential and Weyl tensor measures.
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Topological charge and black hole photon spheres in massive gravity
In dRGT massive gravity, static spherically symmetric black holes exhibit zero, one, or two photon spheres whose topological charges and stability patterns differ from Einstein gravity and from horizonless compact objects.
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On Black Holes Surrounded by Radiation: I. Classical Considerations
Presents classical GR solutions for black holes enveloped by ultra-relativistic orbiting particle shells that extend the photon sphere to arbitrary depth while remaining optically indistinguishable from standard black holes.
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Bounds on the photon sphere radius for spherically symmetric black holes in n-dimensional Einstein gravity
Upper bound r_γ ≤ [(n-1)M]^{1/(n-3)} and lower bound r_γ ≥ ((n-1)/2)^{1/(n-3)} r_H on the photon sphere for nD black holes with anisotropic matter obeying the weak energy condition and non-positive trace.
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Taming the Aretakis instability: extremal black holes with multi-degenerate horizons
Black holes with infinitely degenerate horizons are proposed to be stable against Aretakis instability, potentially serving as end states.
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Axial Oscillations of Viscous Neutron Stars
Causal viscous relativistic hydrodynamics applied to neutron stars yields new axial mode families without perfect-fluid counterparts, including mode avoidance and long-lived modes.
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Gravity/thermodynamics correspondence via black hole shadows
Cuspy black hole shadows correspond to swallowtail thermodynamic free energy, with boundary self-intersections marking geometric phase transitions whose critical exponents fall in the mean-field class.
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As Cold as a Black Hole: Extended Photon Spheres
A necessary and sufficient condition for thermodynamic mimicry of Schwarzschild black holes is satisfied by a one-parameter family of self-similar systems including hillingar black holes, stiffest stars, and frozen stars, derived from photon-sphere control of junction conditions and the TOV equation
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Properties of black holes in non-linear electrodynamics
Analytic charged black holes in nonlinear electrodynamics with non-monotonic lapse functions support stable light rings and additional longer-lived quasinormal modes compared to Einstein gravity.
-
Shadow signatures and energy accumulation in Lorentzian-Euclidean black holes
Lorentzian-Euclidean black holes produce excess inner-shadow intensity and accumulate energy at the horizon with backreaction unlike stable light rings.
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$\ell$-Boson stars in anti-de Sitter spacetime
ℓ-boson stars are constructed and their properties studied in asymptotically anti-de Sitter spacetime.
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Light Rings, Accretion Disks and Shadows of Hayward Boson Stars
Numerical construction of Hayward boson stars shows that frozen states produce Schwarzschild-like shadows with no extra photon rings while non-frozen states show multiple photon rings inside the shadow.
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Energy conditions in static, spherically symmetric spacetimes and effective geometries
A constructive algorithm yields NEC-obeying static spherical metrics with g_tt g_rr = -1, including a log-corrected Schwarzschild geometry that can mimic black holes.
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Topologically equivalent yet radiatively distinct orbits in EMRI system
In dyonic black holes, periodic orbits with identical rotation numbers but spanning different curvature regions generate radiatively distinct gravitational waveforms in EMRIs.
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Search for growing angular modes in ultracompact boson star evolutions
The paper decomposes simulation data of ultracompact boson stars into spherical harmonics as a first step toward characterizing non-spherical modes.
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Scalar-Electromagnetic Couplings as Source of Deformed Black Hole: From Shadows to Thermodynamic Topology
A scalar-NED coupled black hole metric is reconstructed from an effective geometry, yielding EHT bounds on magnetic charge, Hawking-Page transition, and topological equivalence to the Reissner-Nordström solution.
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Topology of black hole thermodynamics: A brief review
Topological numbers categorize black hole systems into universality classes based on thermodynamic behavior, with calculations for critical points and phase transitions.
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Testing the nature of dark compact objects: a status report
Current and future observations can test whether dark compact objects are Kerr black holes or exotic alternatives, with null results strengthening the black hole paradigm.