GW241011 data shows consistency with Kerr black holes for both quadrupole and octupole moments and delivers the first observational bounds on spin-induced octupole deviations.
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Testing the nature of dark compact objects: a status report
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abstract
Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.
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A vector-supported compact object in modified gravity relaxes dissipatively without oscillatory ringdown because a hidden chiral symmetry converts perturbations into one-way transport.
Disk-induced dissipation drives rapid orbital plane alignment followed by slower eccentricity damping in extreme mass-ratio inspirals, with relativistic effects producing accumulating deviations from Keplerian orbits even at large separations.
First search for high-frequency gravitational waves via inverse Gertsenshtein conversion in Earth's magnetic field with VLA and ALMA sets new upper limits h_c ≲ 10^{-18} from 1 GHz to 1 THz.
A covariant framework is developed for photon surfaces in dynamical spherical spacetimes, recovering static limits and applied to collapse and accretion/evaporation models.
Non-singular gravity with regulator ℓ induces a PBH mass gap ~c²ℓ/G and a stronger Carr criterion δ_H > 2GM_gap/R_H - 1 when R_H ~ ℓ.
A classification of admissible energy density profiles with bounded Kretschmann scalar yields a unified framework for regular static spherically symmetric spacetimes satisfying the weak energy condition, recovering known models and producing new families with hypergeometric and other closed forms.
Axial tidal Love numbers for black holes in anisotropic fluid environments are derived analytically and numerically, with non-compact support density profiles producing logarithmic terms that obstruct standard tidal matching due to the lack of a strictly vacuum exterior.
3D GRMHD simulations of accretion onto a JMN-1 horizonless singularity produce a magnetically arrested disk with an accretion rate of ~3e-6 Eddington matching M87* observations and EHT-consistent images, plus central brightness inside the shadow as a discriminant from black holes.
GreyRing model based on greybody factors reproduces numerical relativity ringdown signals with mismatches of order 10^{-6} and enables a new post-merger consistency test of general relativity applied to GW250114.
Higher-order terms in an infinite tower of higher-derivative gravity regularize a 5D Proca-Maxwell system, creating frozen regular cores that mimic extremal black holes and satisfy all energy conditions.
An exact Debye-series decomposition of the scattering matrix for waves on compact stars reveals distinct Regge-Debye pole families that dominate amplitudes differently in neutron-star-like and ultracompact regimes.
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
AdS exotic compact objects imprint bulk-cone singularities from null geodesics and echoes from trapped waves on CFT Green functions, signaling no horizon.
Exact non-singular black holes from the phantom DBI field evaporate to gram-mass relics, opening a new mass window for primordial black holes as dark matter.
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.
Tailored temporal modulation of incoming signals enables complete absorption by black holes via excitation of complex-plane resonances, storing energy for later release through virtual absorption modes.
Stable neutron-star configurations denser than black holes exist in quasi-topological gravity and may produce detectable gravitational-wave echoes.
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.
Head-on black-hole mergers next to a companion show Doppler- and redshift-shifted ringdown, lensing magnification behind the lens, and delayed echo images, with only tentative signs of resonant mode excitation.
Neutron stars in quasi-topological gravity can be more compact than black holes and radially stable across several equations of state.
Spinning test particles around rotating hairy black holes show finite-time instability in localized regions of the (spin, hair-parameter) plane that reorganize the strong-field phase space compared to Kerr.
Introduces Debye series and Debye-QNMs to decompose waveforms from Schwarzschild-star models, achieving early-time convergence and organizing ringdown plus echo packets into individual propagation channels.
A rotating black-hole metric with a dark-matter halo is derived, but the proposed halo profiles do not make the spacetime asymptotically flat as claimed.
citing papers explorer
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Testing the Kerr hypothesis beyond the quadrupole with GW241011
GW241011 data shows consistency with Kerr black holes for both quadrupole and octupole moments and delivers the first observational bounds on spin-induced octupole deviations.
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Relaxation without ringdown for a compact object in modified gravity
A vector-supported compact object in modified gravity relaxes dissipatively without oscillatory ringdown because a hidden chiral symmetry converts perturbations into one-way transport.
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Orbital evolution of asymmetric binaries within accreting environments
Disk-induced dissipation drives rapid orbital plane alignment followed by slower eccentricity damping in extreme mass-ratio inspirals, with relativistic effects producing accumulating deviations from Keplerian orbits even at large separations.
-
Search for High-Frequency Gravitational Waves via Geomagnetic Conversion with Radio Telescopes
First search for high-frequency gravitational waves via inverse Gertsenshtein conversion in Earth's magnetic field with VLA and ALMA sets new upper limits h_c ≲ 10^{-18} from 1 GHz to 1 THz.
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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.
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Carr criterion and mass gaps in non-singular primordial black hole formation
Non-singular gravity with regulator ℓ induces a PBH mass gap ~c²ℓ/G and a stronger Carr criterion δ_H > 2GM_gap/R_H - 1 when R_H ~ ℓ.
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Families of regular spacetimes and energy conditions
A classification of admissible energy density profiles with bounded Kretschmann scalar yields a unified framework for regular static spherically symmetric spacetimes satisfying the weak energy condition, recovering known models and producing new families with hypergeometric and other closed forms.
-
Axial tidal Love numbers of black holes in matter environments
Axial tidal Love numbers for black holes in anisotropic fluid environments are derived analytically and numerically, with non-compact support density profiles producing logarithmic terms that obstruct standard tidal matching due to the lack of a strictly vacuum exterior.
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GRMHD accretion beyond the black hole paradigm: Light from within the shadow
3D GRMHD simulations of accretion onto a JMN-1 horizonless singularity produce a magnetically arrested disk with an accretion rate of ~3e-6 Eddington matching M87* observations and EHT-consistent images, plus central brightness inside the shadow as a discriminant from black holes.
-
Novel ringdown tests of general relativity with black hole greybody factors
GreyRing model based on greybody factors reproduces numerical relativity ringdown signals with mismatches of order 10^{-6} and enables a new post-merger consistency test of general relativity applied to GW250114.
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Proca-Maxwell System in an Infinite Tower of Higher-Derivative Gravity
Higher-order terms in an infinite tower of higher-derivative gravity regularize a 5D Proca-Maxwell system, creating frozen regular cores that mimic extremal black holes and satisfy all energy conditions.
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Scattering from compact objects: Debye series and Regge-Debye poles
An exact Debye-series decomposition of the scattering matrix for waves on compact stars reveals distinct Regge-Debye pole families that dominate amplitudes differently in neutron-star-like and ultracompact regimes.
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Rotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
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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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Exact, non-singular black holes from a phantom DBI Field as primordial dark matter
Exact non-singular black holes from the phantom DBI field evaporate to gram-mass relics, opening a new mass window for primordial black holes as dark matter.
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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.
-
Total absorption of tailored incoming signals by black holes
Tailored temporal modulation of incoming signals enables complete absorption by black holes via excitation of complex-plane resonances, storing energy for later release through virtual absorption modes.
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Neutron stars more compact than black holes as a probe of strong-field gravity
Stable neutron-star configurations denser than black holes exist in quasi-topological gravity and may produce detectable gravitational-wave echoes.
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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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Ringdown and lensing of triple systems
Head-on black-hole mergers next to a companion show Doppler- and redshift-shifted ringdown, lensing magnification behind the lens, and delayed echo images, with only tentative signs of resonant mode excitation.
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Neutron stars more compact than black holes in quasi-topological gravity: Equilibrium configurations and radial stability
Neutron stars in quasi-topological gravity can be more compact than black holes and radially stable across several equations of state.
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Spin-Hair Induced Chaos of Spinning Test Particles in Rotating Hairy Black Holes
Spinning test particles around rotating hairy black holes show finite-time instability in localized regions of the (spin, hair-parameter) plane that reorganize the strong-field phase space compared to Kerr.
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Ringdown and echoes from compact objects: Debye series and Debye quasinormal modes
Introduces Debye series and Debye-QNMs to decompose waveforms from Schwarzschild-star models, achieving early-time convergence and organizing ringdown plus echo packets into individual propagation channels.
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A metric solution for rotating black holes embedded in dark matter halos with central spikes
A rotating black-hole metric with a dark-matter halo is derived, but the proposed halo profiles do not make the spacetime asymptotically flat as claimed.
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Gravitational Waves from a Black Hole Falling Radially into a Thin-Shell Traversable Wormhole
Analytic gravitational waveforms from radial test-particle infall into a thin-shell traversable wormhole exhibit a characteristic pulse-gap structure from repeated throat crossings and lie within reach of ground-based detectors at ~500 Mpc.
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Gravitational Properties of the Monopole Bag
Monopole bags in axionic backgrounds gravitationally collapse into horizonless states or dyonic regular black holes that evade singularities while retaining axionic hair.
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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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Ringing of rapidly rotating black holes in effective field theory
Leading-order cubic-curvature corrections to scalar quasinormal modes of black holes with spins up to 0.99M are computed numerically for modes up to l=5 with relative errors below 10^{-4}.
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Rindler Physics with a UV Cutoff on the Lattice
Lattice regularization of Rindler QFT shows the Unruh effect survives operationally for distant observables even though exact thermality is lost at the state level, with wave packets reflected at a stretched horizon of order the cutoff.
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The relativistic restricted three-body problem: geometry and motion around tidally perturbed black holes
Increasing tidal deformation around a black hole drives bound geodesics through weak chaos, plunging, unbinding, and eventual depletion of all bound motion, with semi-analytic critical amplitudes for each transition.
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Tidal Love numbers for regular black holes
Tidal Love numbers of regular black holes are generically nonzero, model-dependent, and can acquire logarithmic scale dependence at higher perturbative orders.
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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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Reflectionless and echo modes in asymmetric Damour-Solodukhin wormholes
In asymmetric Damour-Solodukhin wormholes, reflectionless and echo modes share asymptotic spectral properties parallel to the real frequency axis with matching spacing, and reflectionless modes lie closer to the axis yielding larger echo amplitudes.
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On the universality of late-time ringdown tail
Analytical proof establishes universality of late-time ringdown tails for any effective potential decaying as 1/r², with different power-law behavior for 1/r^α (1<α<2), covering charged black holes, Kerr, exotic objects, modified gravity, and environmental matter distributions.
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Probing near-zone magnetic fields with extreme mass-ratio inspirals
A magnetized Schwarzschild background shifts EMRI orbital dynamics and GW waveforms, with B~10^9 G producing ~1.3 rad dephasing over one year for a 10^6 M_sun system.
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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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On Black Holes Surrounded by Radiation II: Thermodynamics
Hillingar black holes thermodynamically mimic ordinary black holes of mass M, sharing temperature and entropy under thermal equilibrium.
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Massive scalar fields in eccentric regime: Detectability and constraints from LISA observations of extreme mass-ratio inspirals
Computes scalar and tensor fluxes for eccentric EMRIs with massive scalars, quantifies dephasing, and shows via Fisher matrix that LISA can constrain scalar charge and mass.
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Broadened Lensing Rings of Compact Boson Stars: Enhanced Imprint of Accretion Flow in Images and Visibilities
Compact boson stars exhibit broader gravitational lensing rings than black holes, resulting in images and visibilities that depend more strongly on the structure of the accretion flow.
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$\tt BlackHawk$ $\tt v3.0$: Hawking Radiation from Regular Black Holes
BlackHawk v3.0 adds Hawking temperatures and greybody factors for multiple regular black hole metrics to an existing public code via numerical routines.
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Approaching the surface of an Exotic Compact Object
Near an Exotic Compact Object surface, vacuum Einstein equations yield chaotic oscillations with walls becoming cliffs, driving runaway squeezing that continues to fuzzball monopoles in string theory.
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Testing solitonic boson star interpretations of Sagittarius A* with near-infrared flare astrometry
Fitting GRAVITY flare astrometry to solitonic boson star models requires masses larger than 4.3 million solar masses, with more diffuse models yielding values closer to the standard black hole mass and thus placing stringent but incomplete constraints on such interpretations of Sgr A*.
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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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Spectroscopy of analogue black holes using simulation-based inference
Simulation-based inference reliably extracts physical parameters from noisy spectra of analogue black holes.
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Probing Kerr Symmetry Breaking with LISA Extreme-Mass-Ratio Inspirals
LISA EMRIs can constrain deviations from Kerr equatorial symmetry to 10^{-2} and axial symmetry to 10^{-3} using Analytic Kludge waveforms and Fisher analysis.
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Are Black Holes Fuzzballs? Probing Horizon-Scale Structure with LISA
LISA can constrain non-axisymmetric mass quadrupole deformations at the 10^{-3} level and axisymmetric mass octupole deformations at the 10^{-2} level in EMRI signals to test fuzzball proposals.
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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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$\ell$-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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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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Dynamical Tidal Response of Non-rotating Black Holes: Connecting the MST Formalism and Worldline EFT
Renormalized dynamical tidal response functions for non-rotating black holes in GR carry inevitable ambiguities from renormalization scheme and flow initial condition, yielding scheme-dependent dynamical tidal Love numbers after MST-worldline EFT matching.