Black holes event horizon form and behave
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Formation and Behavior of Black Holes' Event Horizons
Non-Smooth Structures on Event Horizons: Creases, Corners, and Caustics
The event horizon of a dynamical black hole is typically a non-smooth hypersurface. Various non-smooth structures can form on the horizon, including creases, corners, and caustic points. Creases and corners are spacelike submanifolds of dimensions 2 and 1, respectively, while caustic points form a set of dimension at most 1. These structures can undergo qualitative changes, known as "perestroikas," which describe events such as the merger of black holes or the nucleation and collapse of holes in toroidal horizons. These non-smooth features might also contribute to black hole entropy through quantum entanglement entropy .
Instabilities at the Event Horizon of Extreme Black Holes
Extreme black holes, such as the Reissner–Nordstrom (RN) black hole, exhibit instabilities at their event horizons when perturbed by a massless scalar field. Numerical studies show that these instabilities generally lead to a non-extreme RN solution. However, finely tuned perturbations can result in a time-dependent extreme black hole where the event horizon remains smooth, but observers crossing it at late times experience large gradients. This indicates that such black holes can admit a C1 extension across an inner (Cauchy) horizon .
Event Horizon Dynamics in Black Hole Mergers
The formation and evolution of the event horizon during a black hole binary merger, especially when the black holes are charged, reveal significant insights into modified gravity theories and dark matter candidates. The presence of charge influences the merger properties, including the growth in the area of the event horizon and the duration of the merger. Analytical and numerical analyses in the extreme mass ratio (EMR) limit provide a detailed description of the horizon's time evolution and the critical points where horizons touch 34.
Horizon Dynamics of Rotating and Distorted Black Holes
Simulations of rotating black holes distorted by ingoing gravitational radiation show the formation of multiple concentric marginally outer trapped surfaces. These surfaces appear and disappear in pairs, and their world tubes are spacelike during dynamic regimes, approaching a null hypersurface at early and late times. The event horizon itself is well-behaved, aligning with the apparent horizon before and after the dynamic regime, with no new generators entering the event horizon during the simulation .
Primordial Black Hole Formation and Accretion Dynamics
During the radiation-dominated phase of the early Universe, horizon-size density fluctuations can collapse to form primordial black holes (PBHs). The threshold for black hole formation is higher than previously thought, and the accretion onto newly formed black holes is generally insignificant. Small black holes cause a fluid bounce that halts further accretion, while larger ones do not. The scaling of black hole mass with the distance from the formation threshold applies to PBH formation, similar to near-critical gravitational collapse .
Quantum Fluctuations and Observations with the Event Horizon Telescope
Quantum modifications to black hole descriptions, extending beyond the horizon, can influence observable regions. These modifications, behaving like metric fluctuations, can cause rapid time variability in the shadow of a black hole. The Event Horizon Telescope (EHT) can potentially observe these effects, especially in larger black holes like the one in the M87 galaxy. Such observations could provide insights into the quantum structure of black holes and the consistency of quantum evolution with unitarity .
Magnetic Field Structure Near the Event Horizon
EHT observations of polarized emission around the supermassive black hole in M87 reveal the structure of magnetic fields near the event horizon. The low fractional linear polarization suggests internal Faraday rotation within the emission region. The observed polarization patterns indicate organized, poloidal magnetic fields, and comparisons with general relativistic magnetohydrodynamic (GRMHD) simulations identify models that match the observations. These models suggest a mass accretion rate onto the black hole in M87 of (3-20) x 10^-4 Msun yr-1 .
Conclusion
The event horizon of black holes exhibits complex behaviors and structures, influenced by various factors such as charge, rotation, and quantum effects. Understanding these dynamics through numerical simulations, theoretical models, and observations like those from the EHT provides deeper insights into the nature of black holes and the fundamental laws governing their behavior.
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