AdS exotic compact objects imprint bulk-cone singularities from null geodesics and echoes from trapped waves on CFT Green functions, signaling no horizon.
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Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
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abstract
Ultracompact objects are self-gravitating systems with a light ring. It was recently suggested that fluctuations in the background of these objects are extremely long-lived and might turn unstable at the nonlinear level, if the object is not endowed with a horizon. If correct, this result has important consequences: objects with a light ring are black holes. In other words, the nonlinear instability of ultracompact stars would provide a strong argument in favor of the "black hole hypothesis," once electromagnetic or gravitational-wave observations confirm the existence of light rings. Here we explore in some depth the mode structure of ultracompact stars, in particular constant-density stars and gravastars. We show that the existence of very long-lived modes -- localized near a second, stable null geodesic -- is a generic feature of gravitational perturbations of such configurations. Already at the linear level, such modes become unstable if the object rotates sufficiently fast to develop an ergoregion. Finally, we conjecture that the long-lived modes become unstable under fragmentation via a Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the structure of the star, it is also possible that nonlinearities lead to the formation of small black holes close to the stable light ring. Our results suggest that the mere observation of a light ring is a strong evidence for the existence of black holes.
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representative citing papers
Time-domain evolutions demonstrate that the nonlinear scalar ergoregion instability saturates via a weakly turbulent direct cascade transferring energy to small scales and populating higher-order azimuthal modes on the stable light ring.
Excitation factors of long-lived quasinormal modes in horizonless compact objects scale with their small imaginary frequency, suppressing early contributions and producing a hierarchy where prompt ringdown uses ordinary modes and late echoes use cavity modes.
Black holes with infinitely degenerate horizons are proposed to be stable against Aretakis instability, potentially serving as end states.
A one-body conformal-factor correction stabilizes boson star-black hole initial data, enabling gravitational-wave analysis that shows higher multipoles can discriminate mixed mergers from pure black-hole binaries.
In quasi-topological gravity, neutron stars can surpass black-hole compactness with universal high-density behavior and theory corrections that stabilize radially unstable configurations from general relativity.
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.
AdS gravastar glued from AdS-Schwarzschild and de Sitter yields horizon-less signatures including specific bulk-cone singularities and echoes in dual CFT retarded Green functions.
High-frequency quasi-reflectionless scattering modes in the greybody factors of ultracompact horizonless objects are responsible for echoes in the time-domain response.
Numerical simulations of black hole-boson star binaries show that scalar self-interactions can suppress tidal disruption while radiative efficiency depends on the chosen potential.
The paper decomposes simulation data of ultracompact boson stars into spherical harmonics as a first step toward characterizing non-spherical modes.
No observational data can confirm the existence of black holes because general relativity imposes fundamental limits on what can be established about them.
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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Bulk-cone singularities and echoes from AdS exotic compact objects
AdS exotic compact objects imprint bulk-cone singularities from null geodesics and echoes from trapped waves on CFT Green functions, signaling no horizon.
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Weakly turbulent saturation of the nonlinear scalar ergoregion instability
Time-domain evolutions demonstrate that the nonlinear scalar ergoregion instability saturates via a weakly turbulent direct cascade transferring energy to small scales and populating higher-order azimuthal modes on the stable light ring.
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Excitation factors for horizonless compact objects: long-lived modes, echoes, and greybody factors
Excitation factors of long-lived quasinormal modes in horizonless compact objects scale with their small imaginary frequency, suppressing early contributions and producing a hierarchy where prompt ringdown uses ordinary modes and late echoes use cavity modes.
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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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Boson star-black hole binaries: initial data and head-on collisions
A one-body conformal-factor correction stabilizes boson star-black hole initial data, enabling gravitational-wave analysis that shows higher multipoles can discriminate mixed mergers from pure black-hole binaries.
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Neutron stars more compact than black holes in quasi-topological gravity: Equilibrium configurations and radial stability
In quasi-topological gravity, neutron stars can surpass black-hole compactness with universal high-density behavior and theory corrections that stabilize radially unstable configurations from general relativity.
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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.
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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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AdS gravastar and its signatures from dual conformal field theory
AdS gravastar glued from AdS-Schwarzschild and de Sitter yields horizon-less signatures including specific bulk-cone singularities and echoes in dual CFT retarded Green functions.
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Greybody factors, reflectionless scattering modes, and echoes of ultracompact horizonless objects
High-frequency quasi-reflectionless scattering modes in the greybody factors of ultracompact horizonless objects are responsible for echoes in the time-domain response.
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Black Hole-Boson Star Binaries: Gravitational Wave Signals and Tidal Disruption
Numerical simulations of black hole-boson star binaries show that scalar self-interactions can suppress tidal disruption while radiative efficiency depends on the chosen potential.
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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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On the impossibility of observational confirmation of black holes
No observational data can confirm the existence of black holes because general relativity imposes fundamental limits on what can be established about them.
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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.
- Axial Oscillations of Viscous Neutron Stars