Testing the nature of dark compact objects: a status report

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.

76 pages + references. Invited review article for Living Reviews in Relativity. v3: Overall improvements and references added, a few typos corrected. Version to appear in LRR

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• Imaging a semi-classical horizonless compact object with strong redshift
• Slowly-rotating compact objects: the nonintegrability of Hartle-Thorne particle geodesics
• New Non-Abelian Reissner-Nordström Black Hole Solutions in the Generalized SU(2) Proca Theory And Some Astrophysical Implications
• General formulae for the periapsis shift of a quasi-circular orbit in static spherically symmetric spacetimes and the active gravitational mass density
• Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review
• Novel triple barrier potential for axial gravitational perturbations of a family of Lorentzian wormholes
• Physical black holes in fourth-order gravity
• Re-examining the stability of rotating horizonless black shells mimicking Kerr black holes
• Gravitational wave echoes search with combs
• Hearts of Darkness: the inside out probing of black holes
• Compact objects in quadratic Palatini gravity generated by a free scalar field
• On the universality of late-time ringdown tail
• Hidden symmetry between rotational tidal Love numbers of spinning neutron stars
• Scale invariant elastic stars in General Relativity
• Gravitational wave constraints on extended dark matter structures
• Piercing of a solitonic boson star by a black hole
• Probing Spin-Induced Quadrupole Moments in Precessing Compact Binaries
• On turbulence for spacetimes with stable trapping
• Kinematic and energy properties of dynamical regular black holes
• Perturbative and non-linear analyses of gravitational turbulence in spacetimes with stable light rings
• Divergences in gravitational-wave emission and absorption from extreme mass ratio binaries
• Superradiant instabilities of massive bosons around exotic compact objects
• Bounds on tidal charges from gravitational-wave ringdown observations
• Probing Bosonic Stars with Atomic Clocks
• Bound on photon circular orbits in general relativity and beyond
• Physical and Theoretical Challenges to Integrable Singularities
• Modeling and detecting resonant tides of exotic compact objects
• Numerical relativity surrogate models for exotic compact objects: the case of head-on mergers of equal-mass Proca stars
• Cosmology and nuclear-physics implications of a subsolar gravitational-wave event
• Bound States of the Schwarzschild Black Hole
• Thermodynamic Consistent Description of Compact Stars of Two Interacting Fluids: The Case of Neutron Stars with Higgs Portal Dark Matter
• Models of cosmological black holes
• Classical Love for Quantum Blackholes
• Black holes, multiple propagation speeds, and energy extraction
• Superradiant instabilities by accretion disks in scalar-tensor theories
• Can light-rings self-gravitate?
• Interior Metric of Slowly Formed Black Holes in a Heat Bath
• Whispers from the quantum core: the ringdown of semiclassical stars
• Unruh-DeWitt Detector Differentiation of Black Holes and Exotic Compact Objects
• Fundamental Physics and Cosmology with TianQin
• Total light bending in non-asymptotically flat black hole spacetimes
• Applications of the close-limit approximation: horizonless compact objects and scalar fields
• Systematic biases from ignoring environmental tidal effects in gravitational wave observations
• Analytical investigations on formations of hairy neutral reflecting shells in the scalar-Gauss-Bonnet gravity
• Inverse problem of analog gravity systems
• Shadow Cast by the Kerr MOG Black Hole under the Influence of Plasma and Constraints from EHT Observations
• Compact Objects in Alternative Gravities
• Burst particle creation in gravitational collapse to a horizonless compact object
• Electromagnetic signatures of strong-field gravity from accreting black holes
• Are there echoes of gravitational waves?
• Highly compact Proca stars with quartic self-interactions
• Assessing the stability of ultracompact spinning boson stars with nonlinear evolutions
• Kicks in charged black hole binaries
• Solar Diffraction of LIGO-Band Gravitational Waves
• Are Astrophysical "Black" Holes Leaky?
• Euclidean methods and phase transitions for the strongest deformations compatible with Schwarzschild asymptotics
• Remarks on the light ring images and the optical appearance of hairy black holes in Einstein-Maxwell-dilaton gravity
• Echoes from bounded universes
• Imaging fermionic dark matter cores at the center of galaxies
• Echoes from corpuscular black holes
• Radius and equation of state constraints from massive neutron stars and GW190814
• Spherically symmetric black holes in metric gravity
• Tidal Love numbers of gravitational atoms
• Self-Gravitating Matter in Stationary and Axisymmetric Black Hole Spacetimes
• Holographic Dual of The Weak Gravity Conjecture
• A mechanism for a "leaky" black hole to catalyze galaxy formation
• An exact model for evaporating primordial black holes in cosmological space-time
• Gravitational Echoes from Braneworlds
• Effect of quantum deformed black hole on BH shadow in two-dimensional Dilaton gravity
• Efficient search method of anomalous reflection by the central object in an EMRI system by future space gravitational wave detectors
• Fate of initially bound timelike geodesics in spherical boson stars
• Inaccessibility of traversable wormholes
• On the Interaction between Ultralight Bosons and Quantum-Corrected Black Holes
• Inverse problem of analog gravity systems II: Rotation and energy-dependent boundary conditions
• Three-dimensional general relativistic Poynting-Robertson effect. IV. Slowly rotating and non-spherical quadrupolar massive source
• Model-independent search for the quasinormal modes of gravitational wave echoes
• Ultracompact vector stars
• Accretion discs onto supermassive compact objects: a portal to dark matter physics in active galaxies
• Universality of the Buchdahl sphere
• Fermionic Love of Black Holes in General Relativity
• Quasinormal modes of a charged black hole with scalar hair
• CLAP for modified gravity: scalar instabilities in binary black hole spacetimes
• Wormholes from beyond
• Theory-agnostic searches for non-gravitational modes in black hole ringdown
• Post-adiabatic waveforms from extreme mass ratio inspirals in the presence of dark matter
• Dynamical Tidal Response of Non-rotating Black Holes: Connecting the MST Formalism and Worldline EFT
• Shadows and optical appearance of black bounces illuminated by a thin accretion disk
• Influence of dark matter on the structure of strange quark stars in one-fluid model
• Instabilities of scalar fields around oscillating stars
• Parametrized Tidal Dissipation Numbers of Non-rotating Black Holes
• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations
• Extreme mass ratio inspirals around topological stars
• Vacuum Spacetime With Multipole Moments: The Minimal Size Conjecture, Black Hole Shadow, and Gravitational Wave Observables
• Spectral instability of horizonless compact objects within astrophysical environments
• Evaporation of Echoing Black Holes
• Matching slowly rotating spacetimes split by dynamic thin shells
• On thermodynamics of compact objects
• Photon escape in the extremal Kerr black hole spacetime
• Search for sub-solar mass binaries with Einstein Telescope and Cosmic Explorer
• THESEUS$-$BTA cosmological tests using Multimessenger Gamma-Ray Bursts observations
• Black hole spectroscopy and nonlinear echoes in Einstein-Maxwell-scalar theory
• Quantum Gravitational Hair in Gravastars and Observational Tests
• Padé metrics for black hole perturbations and light rings
• Overlapping of photon rings in black hole imaging
• Quasinormal Modes and Stability of Firewalls
• The Impact of Dark Matter on Gravitational Wave Detection by Space-based Interferometers
• Aspects of Quantum Gravity Phenomenology and Astrophysics
• Probing light scalars with not quite black holes
• Splitting the Echoes of Black Holes in Einstein-nonlinear Electrodynamic Theories
• Korteweg-de Vries Integrals for Modified Black Hole Potentials: Instabilities and other Questions
• Horizonless star based on regular black hole with finite radius and its observational signatures
• Gravitational Atoms from Topological Stars
• Holographic Bound on Area of Compact Binary Merger Remnant
• Transient electromagnetic sources can detect solitary black holes in Milky Way galaxy
• Accretion Process as a Probe of Extra Dimensions in MOG Compact Object Spacetimes
• Shedding Light on Gravity: Black Hole Shadows and Lensing Signatures in Lorentz Gauge Theory
• Slowly rotating ultracompact Schwarzschild star in the gravastar limit
• AdS gravastar and bulk-cone singularities
• Compact scalar field solutions in EiBI gravity
• Einstein's hole argument and Schwarzschild singularities
• Beyond Buchdahl's limit: bilayered stars and thin-shell configurations
• Bounded compactness from G(E)UP
• Bifurcations in Bosonic Stars: chains and rings from spherical solutions
• Probing the horizon of black holes with gravitational waves
• Horizon-bound objects: Kerr--Vaidya solutions
• Lensing by black holes within astrophysical environments
• An infinite class of exact rotating black hole metrics of modified gravity
• From de Sitter to anti-de Sitter singularity regularization: Theory and phenomenology
• Shadow of quadrupole-deformed compact objects in a local dark matter shell
• Testing the Existence of Event Horizons against Rotating Reflecting Surfaces
• Exotic Encounters: Buchdahl's Conditions and Physical Black Holes
• Gravitational field of scalar lumps in higher-derivative gravity
• A Conceptual Model for the Origin of the Cutoff Parameter in Exotic Compact Objects
• Hydrodynamic and Rayleigh-Plateau instabilities of Q-strings
• Exploring the viability of pseudo Nambu-Goldstone boson as ultralight dark matter in a mass range relevant for strong gravity applications
• QNM families: classification and competition
• Novel black holes with scalar hair in the Einstein-Maxwell-Scalar Theory with positive coupling
• Ten years of extreme gravity tests of general theory of relativity with gravitational-wave observations
• Detectability of Massive Boson Stars using Gravitational Waves from Fundamental Oscillations
• Ergoregion instability in bosonic stars: scalar mode structure, universality, and weakly nonlinear effects
• Shadows of Lorentzian traversable wormholes
• Pseudospectrum of Reissner-Nordström black holes: quasinormal mode instability and universality
• Echoes and quasinormal modes of asymmetric black bounces
• Curvature and dynamical spacetimes: can we peer into the quantum regime?
• Spacing Statistics of Energy Spectra: Random Matrices, Black Hole Thermalization, and Echoes
• Message in a bottle: energy extraction from bouncing geometries
• An Ultra-compact Object from Semi-classical Gravity
• Nonsingular, lump-like, scalar compact objects in $(2+1)$-dimensional Einstein gravity
• Boson Stars in Bumblebee Gravity and Their Gravitational Waveforms from Extreme-Mass-Ratio Inspirals
• Probing black holes via quasinormal modes in a dark energy-induced dark matter
• Buchdahl limit of compact stars in presence of Weyl anomaly
• Is GW190521 a gravitational wave echo of wormhole remnant from another universe?
• Black hole destabilization via trapped quasi-normal modes
• Limits on the existence of totally reflective exotic compact objects with current and future gravitational-wave detectors
• Equatorial orbits and imaging of hairy cubic Galileon black holes
• Space-based Gravitational Wave Observatories
• Neutron stars more compact than black holes as a probe of strong-field gravity
• Fundamental oscillations as a tool to distinguish boson stars from neutron stars and black holes
• QED Effects on Kerr Black Hole Shadows immersed in uniform magnetic fields
• Quasinormal modes for charged Lifshitz black holes with scalar hair
• Horizons, throats and bounces in hybrid metric-Palatini gravity with a non-zero potential
• The Resurgence of the G(2) Group for the Strong Sector and the Emergence of Dark Matter
• Accreting Schwarzschild-like compact object: Plasma-photon interaction and stability
• Hair imprints of the gravitational decoupling and hairy black hole spectroscopy
• Binary black holes in the heat of merger
• Perturbations of Einstein--Maxwell--phantom spacetime: Instabilities of charged Ellis--Bronnikov wormholes and quasinormal modes of black holes
• Semi-classical imprints on quasinormal mode spectra
• First Overtone Mode in the Ringdown Signal of GW231028
• Novel very-high-frequency quasi-periodic oscillations of compact, non-singular objects
• Excitation factors for horizonless compact objects: long-lived modes, echoes, and greybody factors
• Ringdown of a black hole sourced by a Burkert-density effective anisotropic source
• Stabilization of spherically symmetric static Boson stars by Coleman-Weinberg self-interaction potential
• Matter environments around black holes: geodesics, light rings, and ultracompact configurations
• Real-Time Holography and Hybrid WKB for BTZ Wormholes
• Preliminary forecasting constraint on scalar charge with LISA in non-vacuum environments
• Geometric properties of slowly rotating black holes embedded in matter environments
• Spontaneous particle creation by oscillating compact stars
• Energy conditions in static, spherically symmetric spacetimes and effective geometries
• Neutron stars more compact than black holes in quasi-topological gravity: Equilibrium configurations and radial stability
• Causal structure of black holes immersed in a Chaplygin-like dark fluid environment: Horizons and singularities
• Quantum Hairy Black Hole Formation and Horizon Quantum Mechanics
• The landscape of massive black-hole spectroscopy with LISA and Einstein Telescope
• Quasibound states of analytic black-hole configurations in three and four dimensions
• Regular scalar field around reflecting stars and black holes, and reflecting star polarization
• Tidal Love numbers of a nonexotic compact object with a thin shell
• Absorption spectroscopy of quantum black holes with gravitational waves
• The Effect of Mission Duration on LISA Science Objectives
• Constraints on horizonless objects after the EHT observation of Sagittarius A*
• Microscopic Origin of Bekenstein-Hawking Entropy in $(2+1)$ Gravity: A Thermo Field Dynamics Approach
• Scattering from compact objects: Debye series and Regge-Debye poles
• Black hole shadows in nonminimally coupled Weyl connection gravity
• Spectral Bifurcations in Quasinormal Modes of Regular BTZ Black Holes
• Isotropic Coordinates for Generalized Schwarzschild-like Solutions
• An upper limit to the lifetime of stellar remnants from gravitational pair production
• Luminosity of accretion disks in compact objects with quadrupole
• Dynamical gravastars from the interaction between scalar fields and matter
• Constraint-satisfying binary boson star initial data via XCFC
• Relativistic Accretion Flow in a Generic Class of Spherically Symmetric Static Spacetime
• Lorentzian-Euclidean singularity-free solutions to gravitational collapse
• Piercing of a boson star by a black hole
• Black holes and their horizons in semiclassical and modified theories of gravity
• P-stars in the gravitational wave era
• Black Holes and Baryon Number Violation: Unveiling the Origins of Early Galaxies and the Low-Mass Gap
• Propagation and emission of gravitational waves in the weak-field limit within the Palatini formalism
• Asymptotic Schwarzschild solutions in $f(R)$ gravity and their observable effects on the photon sphere of black holes
• The hole picture
• Scattering of massive particles from black holes and naked singularities
• Soliton boson stars, Q-balls and the causal Buchdahl bound
• Timing of echoes in electromagnetic radiation from hot spots orbiting a Kerr black hole impostor
• Implications of the quantum nature of the black hole horizon on the gravitational-wave ringdown
• There and back again: Outspiraling motion in non-Kerr compact objects
• Impact and detectability of spin-tidal couplings in neutron star inspirals
• Predictions on observing hot holographic quark star with gravitational waves
• Quasinormal spectra of a wormhole family: overtone features and a parameter-controlled redshift
• Compact elastic objects in general relativity
• AthenaK simulations of the binary black hole merger GW150914
• Gravitational wave memory in wormhole spacetimes
• Nontopological Electromagnetic Hedgehogs
• Light ring images of double photon spheres in black hole and wormhole space-times
• Bulk-cone singularities and echoes from AdS exotic compact objects
• AdS gravastar and its signatures from dual conformal field theory
• Model-agnostic search of gravitational wave echoes in LVK data
• Macroscopic backreaction of the trace anomaly on classical vacuum backgrounds
• $\ell$-Boson stars in anti-de Sitter spacetime
• Black hole mimickers as relativistic stars calculated from the Tolman-Oppenheimer-Volkoff equations
• Apparent horizon as a membrane
• Nonlinear evolution of the ergoregion instability: Turbulence, bursts of radiation, and black hole formation
• Multi-ring images of thin accretion disk of a regular naked compact object
• Resonances, black hole mimickers and the greenhouse effect: consequences for gravitational-wave physics
• Polar perturbations of dilaton-Euler-Heisenberg black holes
• Quantum fields in boson star spacetime
• Tidal deformability of ultracompact Schwarzschild stars and their approach to the black hole limit
• Light rings, gravitational lensing, and ISCOs of exotic compact objects in Einstein-scalar-Maxwell theories
• Tests for the existence of horizon through gravitational waves from a small binary in the vicinity of a massive object