What shape is a black hole
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The Shape of Black Holes: An Overview
Rotating Black Holes: Rotational Thickening and Stabilization
Rotating black holes, also known as Kerr black holes, exhibit unique shapes due to their angular momentum. The rotation causes a phenomenon known as rotational thickening, where the central regions of the black hole widen. This effect limits the global shape of the black hole, making it more stable and closer to the geometry of an extreme Kerr horizon at nearly maximal rotational speeds 14. The shape of these black holes is determined by their horizon area and angular momentum, independent of the surrounding spacetime geometry or the presence of matter 14.
Photon Rings: Measuring the Shape
Gravitational lensing near black holes produces narrow photon rings, which can be used to measure the shape of black holes. The detailed shape and intensity profile of these rings can be directly measured using long interferometric baselines. The periodicity of the visibility amplitude as a function of baseline angle encodes the projected diameter at that angle, allowing for model-independent measurements of the shape of thin rings .
Event Horizons: Spherical and Non-Spherical Shapes
While black holes are often depicted as spherical, their actual shapes can vary. For instance, the event horizons of some black holes can be topologically spherical with punctures, forming shapes known as "black bottles." These black holes have non-compact event horizons of finite area, with the puncture pushed to infinity in the form of a cusp . Additionally, the shape of the shadow of a black hole, which is the dark area seen against the backdrop of the accretion disk, can vary depending on the observer's motion and the black hole's rotation. For static spherical black holes, the shadow appears circular, but for rotating Kerr black holes, the shape is dependent on the observer's motion and is never perfectly circular .
Super-Spinning Black Holes: Smaller Shadows
Astrophysical black holes can potentially violate the Kerr bound, having angular momentum greater than their mass (J > M). If this bound is violated, the shadow cast by the black hole changes significantly, becoming much smaller than in the case where J ≤ M. This smaller shadow can serve as a clear observational signature in the search for super-spinning black holes .
Theoretical Models: Beyond Spherical Shapes
Some theoretical models propose that black holes might not be perfectly spherical. For example, one model suggests replacing the spherical Schwarzschild black hole with a disk-shaped structure at the quantum level. This model posits that the energy and mass of the black hole are doubled due to the rotation of the gravitational field within it, leading to a disk-shaped black hole with concentric columns .
Conclusion
The shape of black holes is a complex and multifaceted topic influenced by factors such as rotation, gravitational lensing, and theoretical models. Rotating black holes exhibit rotational thickening, while photon rings provide a method for measuring their shape. Event horizons can vary from spherical to more complex shapes like black bottles, and super-spinning black holes have distinctively smaller shadows. Theoretical models continue to challenge the traditional spherical depiction, suggesting alternative shapes at the quantum level. Understanding these shapes is crucial for advancing our knowledge of black holes and their properties.
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