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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An exact analytic metric is constructed for rotating black holes embedded in generic dark matter halos with a central density spike that vanishes beyond a truncation radius near the horizon, generalizing prior spherical solutions.
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.
Viscous neutron stars have new families of axial oscillation modes without perfect-fluid counterparts, featuring mode avoidance and long-lived modes.
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.
Numerical relativity simulations of triple black hole systems reveal redshift effects and gravitational lensing in ringdown signals from head-on mergers, with no additional black hole formation from amplified waves.
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.
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.
Monopole bags in axionic backgrounds gravitationally collapse into horizonless states or dyonic regular black holes that evade singularities while retaining axionic hair.
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}.
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.
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.
Tidal Love numbers of regular black holes are generically nonzero, model-dependent, and can acquire logarithmic scale dependence at higher perturbative orders.
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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A metric solution for rotating black holes embedded in dark matter halos with central spikes
An exact analytic metric is constructed for rotating black holes embedded in generic dark matter halos with a central density spike that vanishes beyond a truncation radius near the horizon, generalizing prior spherical solutions.
-
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.
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Axial Oscillations of Viscous Neutron Stars
Viscous neutron stars have new families of axial oscillation modes without perfect-fluid counterparts, featuring mode avoidance and long-lived modes.
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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.
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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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The third wheel: ringdown and lensing of triple systems
Numerical relativity simulations of triple black hole systems reveal redshift effects and gravitational lensing in ringdown signals from head-on mergers, with no additional black hole formation from amplified waves.
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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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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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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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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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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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Can wormholes have vanishing Love numbers?
For a specific R=0 wormhole, the magnetic Love number for ℓ=2 vanishes to linear order in the regularization parameter under static axial gravitational perturbations.
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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 logarithmic correction to Schwarzschild in static spherical symmetry obeys all classical energy conditions and serves as an effective exterior for horizon-bearing and horizonless compact objects.
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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.
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Gravitational Atoms from Topological Stars
Bound states of a massive scalar field around topological stars form strictly normal modes, producing a hydrogen-like spectrum when the Compton wavelength exceeds the star size and localized states otherwise.
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GW250114: testing Hawking's area law and the Kerr nature of black holes
GW250114 data confirm the remnant black hole ringdown frequencies lie within 30% of Kerr predictions and that the final horizon area is larger than the sum of the progenitors' areas to high credibility.
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Echoes and quasinormal modes of asymmetric black bounces
Computes quasinormal modes and echoes for black bounce solutions, finding echoes only in certain symmetric horizonless cases and none in asymmetric models that recover Reissner-Nordström externally.
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Binary black holes in the heat of merger
Develops a frequency-domain approximant for tidal heating in nonspinning black hole binaries that extends to merger and includes horizon parameters.
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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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Tests of General Relativity with Binary Black Holes from the second LIGO-Virgo Gravitational-Wave Transient Catalog
No evidence for deviations from general relativity is found in LIGO-Virgo binary black hole events, with improved constraints on waveform parameters, graviton mass, and ringdown properties.
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Bayesian Analysis of Massive Boson Star Models for Sagittarius A* Using Near-Infrared Astrometry Data
Bayesian analysis shows current near-IR astrometry data cannot distinguish massive boson stars from Schwarzschild black holes for Sgr A*.
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Unveiling Inner Shadows and Polarization Signatures of Rotating Einstein-Gauss-Bonnet Black Holes
Numerical backward ray-tracing shows that the inner shadow size shrinks with the Gauss-Bonnet coupling while polarization direction near the shadow and photon ring shifts noticeably, and combining both observables yields stronger constraints than either alone.
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Buchdahl stars and bounds with cosmological constant
Generalized Buchdahl bounds on horizonless object compactness are derived in the presence of a cosmological constant, preserving universality while yielding method-dependent results.
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Black holes surrounded by dark matter spike: Spacetime metrics and gravitational wave ringdown waveforms
Black hole spacetimes in dark matter spikes are solved analytically from TOV equations; ringdown quasinormal frequencies differ from Schwarzschild by up to order 10^{-4}.
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Quasinormal modes of massless scalar and electromagnetic perturbations for Euler-Heisenberg black holes surrounded by perfect fluid dark matter
Quasinormal frequencies and greybody factors for massless scalar and electromagnetic perturbations are calculated for charged Euler-Heisenberg black holes surrounded by perfect fluid dark matter, showing that charge Q, nonlinear parameter a, dark matter parameter λ, and angular number l modify the有效