Interior structure of black holes
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Rotating Black Holes: Mass Inflation and Inner Horizon Instabilities
Research on the interior structure of rotating black holes shows that as matter falls in, both ingoing and outgoing streams of particles become highly focused along specific directions near the inner horizon. This leads to a phenomenon called mass inflation, where the energy density and curvature grow rapidly due to hyper-relativistic counter-streaming between these streams. This process eventually causes a collapse at an extremely small radius, where rotational motion becomes significant compared to radial motion. These findings are supported by fully nonlinear, dynamical solutions that describe the interior from just above the inner horizon inward, and they remain valid even for slowly rotating black holes with small accretion rates, where angular effects are negligible compared to radial ones 126.
Charged and Non-Rotating Black Holes: Core Structures and Regularization
For charged or non-rotating black holes, some models propose that the interior can be regular, avoiding singularities. One approach suggests a core made of a condensate of Higgs and Z bosons, with negative pressures preventing gravitational collapse. The size of this core depends on the black hole's mass and charge, and in extreme cases, the core can fill the entire interior, potentially avoiding singularities altogether. This model also predicts that during black hole mergers, the core of a nearly extremal black hole might be exposed, leading to new observable phenomena .
Other models for charged black holes consider matching the interior to a de Sitter spacetime at a transition layer near the Cauchy horizon, or describing evaporation with non-stationary null shells. These approaches aim to keep the curvature bounded and provide alternative ways to model the black hole interior 45.
Symmetries and Quantum Aspects of Black Hole Interiors
Recent work has uncovered new symmetries in the dynamics of black hole interiors, specifically a Poincaré algebra of conserved charges. This symmetry allows the evolution of the interior geometry to be described in terms of geodesics and special curves in a lower-dimensional anti-de Sitter space. Importantly, these symmetries can guide the choice of regularization and quantization schemes, and some approaches inspired by loop quantum cosmology suggest that singularities can be resolved, leading to a transition from a black hole to a white hole while preserving these symmetries .
Schwarzschild Black Holes: Interior Geometry and Singularities
The interior of a Schwarzschild (non-rotating, uncharged) black hole is fundamentally different from its exterior. Inside the event horizon, the roles of time and space coordinates switch, leading to new conceptual challenges. While the classical solution predicts a singularity at the center, the true nature of the interior remains uncertain and is still debated. Various coordinate systems and interpretations have been explored, but a complete and realistic description of the interior is still lacking .
Exotic and Higher-Dimensional Black Hole Interiors
Studies of more exotic black holes, such as helical or distorted black holes, reveal that their interiors can evolve toward stable spacelike singularities, often of the Kasner type. In some cases, oscillatory behavior is observed near the horizon, especially at low temperatures or with small deformations. For charged helical black holes, the inner Cauchy horizon does not exist, and the system evolves directly to a singularity. Distorted black holes, including "caged" black holes, also show unique interior structures, with general solutions describing their behavior near the horizon and singularity 910.
Conclusion
The interior structure of black holes is complex and varies depending on rotation, charge, and other properties. Key features include mass inflation near the inner horizon for rotating and charged black holes, possible regular cores that avoid singularities, and rich symmetry structures that may play a role in quantum gravity. Despite significant progress, the true nature of black hole interiors—especially the fate of singularities—remains an open and active area of research.
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