Infinite-derivative completions of quasitopological gravities are ghost-free, avoid strong coupling, and admit exact spherically symmetric vacuum regular black holes obeying a perturbative Birkhoff theorem.
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abstract
The study of regular black holes and black hole mimickers as alternatives to standard black holes has recently gained significant attention, driven both by the need to extend general relativity to describe black hole interiors, and by recent advances in observational technologies. Despite considerable progress in this field, significant challenges remain in identifying and characterizing physically well-motivated classes of regular black holes and black hole mimickers. This report provides an overview of these challenges, and outlines some of the promising research directions -- as discussed during a week-long focus programme held at the Institute for Fundamental Physics of the Universe (IFPU) in Trieste from November 11th to 15th, 2024.
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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.
Causal sets can approximate black hole horizons via discrete timelike curves and ladders tracing null geodesics, with a discrete expansion changing sign across the horizon in a 1+1D toy model.
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.
All integrable 2D Horndeski theories (and thus many regular black-hole metrics) arise as spherical reductions of pure d≥4 gravities, which the paper terms quasi-topological.
RG-improved black hole spacetimes with scale-dependent gravitational coupling are derived as vacuum solutions to 2D Horndeski master field equations, embedding prior works and exposing implementation discrepancies.
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.
Regular Vaidya solutions exist in effective gravitational theories that dynamically describe radiation-driven formation of regular black holes or mimickers without curvature singularities.
Charged Bardeen and Hayward metrics become singular under deformed Einstein–Maxwell equations; more stringent regularity conditions are derived and improved regular charged metrics are constructed.
Dynamical black-hole-to-white-hole transitions require a violation of the null convergence condition for the trapped region to disappear and the anti-trapped region to form.
Monopole bags in axionic backgrounds gravitationally collapse into horizonless states or dyonic regular black holes that evade singularities while retaining axionic hair.
A unified framework links the generating function for static black holes satisfying g_tt g_rr=-1 in extended quasi-topological gravity to thermodynamic mass and Wald entropy via an effective 2D dilaton theory.
GR coupled to a constrained scalar triplet and auxiliary three-form admits an exact continuous family of asymptotically flat geometrically regular black holes with de Sitter cores and topological hair.
Black holes with infinitely degenerate horizons are proposed to be stable against Aretakis instability, potentially serving as end states.
A spherically symmetric charged black hole evaporating via Hawking radiation can remain regular, with neither a singularity nor a Cauchy horizon, due to electromagnetic repulsion and energy-condition violation.
A regular black hole metric is constructed with sub-Planckian curvature controlled by the inner horizon radius and power-law rather than exponential mass inflation near the inner horizon.
Nonlocal form factors in D-dimensional gravity yield effective geometries whose nonlinear completion gives regular, asymptotically flat Schwarzschild deformations with de Sitter cores.
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.
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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Regular Black Holes in Nonlocal Quasitopological Gravity
Infinite-derivative completions of quasitopological gravities are ghost-free, avoid strong coupling, and admit exact spherically symmetric vacuum regular black holes obeying a perturbative Birkhoff theorem.
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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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Towards black-hole horizons and geodesic focusing in causal sets
Causal sets can approximate black hole horizons via discrete timelike curves and ladders tracing null geodesics, with a discrete expansion changing sign across the horizon in a 1+1D toy model.
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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.
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All $2D$ generalised dilaton theories from $d\geq 4$ gravities
All integrable 2D Horndeski theories (and thus many regular black-hole metrics) arise as spherical reductions of pure d≥4 gravities, which the paper terms quasi-topological.
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Effective geometrodynamics for renormalization-group improved black-hole spacetimes in spherical symmetry
RG-improved black hole spacetimes with scale-dependent gravitational coupling are derived as vacuum solutions to 2D Horndeski master field equations, embedding prior works and exposing implementation discrepancies.
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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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Regular Vaidya solutions of effective gravitational theories
Regular Vaidya solutions exist in effective gravitational theories that dynamically describe radiation-driven formation of regular black holes or mimickers without curvature singularities.
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Charging up regular black holes
Charged Bardeen and Hayward metrics become singular under deformed Einstein–Maxwell equations; more stringent regularity conditions are derived and improved regular charged metrics are constructed.
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Null geodesic defocusing in dynamical black-hole-to-white-hole transitions
Dynamical black-hole-to-white-hole transitions require a violation of the null convergence condition for the trapped region to disappear and the anti-trapped region to form.
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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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$g_{tt}g_{rr} =-1$ black hole thermodynamics in extended quasi-topological gravity
A unified framework links the generating function for static black holes satisfying g_tt g_rr=-1 in extended quasi-topological gravity to thermodynamic mass and Wald entropy via an effective 2D dilaton theory.
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Geometrically Regular Black Holes with Hedgehog Scalar Hair
GR coupled to a constrained scalar triplet and auxiliary three-form admits an exact continuous family of asymptotically flat geometrically regular black holes with de Sitter cores and topological hair.
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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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Radiating black holes in general relativity need not be singular
A spherically symmetric charged black hole evaporating via Hawking radiation can remain regular, with neither a singularity nor a Cauchy horizon, due to electromagnetic repulsion and energy-condition violation.
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Regular black hole with sub-Planckian curvature and suppressed exponential mass inflation
A regular black hole metric is constructed with sub-Planckian curvature controlled by the inner horizon radius and power-law rather than exponential mass inflation near the inner horizon.
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Eikonal, nonlocality and regular black holes
Nonlocal form factors in D-dimensional gravity yield effective geometries whose nonlinear completion gives regular, asymptotically flat Schwarzschild deformations with de Sitter cores.
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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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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.