Shape of black hole
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The Shape of Black Holes: Insights from Recent Research
Super-Spinning Black Holes and Their Apparent Shape
Astrophysical black holes that violate the Kerr bound, where the angular momentum exceeds the mass (J > M), exhibit significant changes in their apparent shape. The shadow cast by such super-spinning black holes is notably smaller, approximately an order of magnitude, compared to black holes that adhere to the Kerr bound (J ≤ M). This distinct shadow can serve as a clear observational signature in identifying super-spinning black holes .
Photon Rings and Gravitational Lensing
General relativity predicts that gravitational lensing near black holes produces narrow "photon rings" in images. The detailed shape and intensity profile of these rings can be measured on 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 these thin rings. These measurements can then be compared to the precise predictions of general relativity .
Shadows of Nonsingular Black Holes
Nonsingular black holes, which avoid the curvature singularities predicted by general relativity, cast shadows that appear as dark zones covered by deformed circles. These black holes are characterized by mass (M), spin (a), and a deviation parameter (k). As the parameter k increases, the size of the shadow decreases, and the shadow becomes more distorted compared to the Kerr black hole shadow. This indicates that the deviation parameter significantly affects the apparent shape of the black hole's shadow .
Rotating Black Holes and Their Horizon Shape
The shape of rotating black holes, particularly their horizons, is influenced by their rotation. Rotation causes a widening of the central regions (rotational thickening) and limits the global shapes of the black holes. The geometry of these horizons tends to be close to the extreme Kerr horizon geometry at nearly maximal rotational speeds. These findings are based on the stability inequality and depend solely on the horizon area (A) and angular momentum (J), independent of the surrounding spacetime geometry or the presence of matter 45.
Light Ring Images and Accretion Disks
The silhouette of a black hole, when illuminated by an optically thin accretion disk, shows a distinctive central brightness depression (the shadow) with an outer edge consisting of strongly lensed, self-similar rings. The size and shape of this critical curve depend on the background geometry, while the pattern of bright and dark regions in the image is influenced by the physics of the accretion disk. The presence of additional light rings in the intermediate region between two critical curves can indicate the existence of black hole mimickers with multiple critical curves .
Gravitational Polarizability and Tidal Bulges
The gravitational polarizability of black holes, defined by the "shape" or "height" multipolar Love numbers, describes the height of the tidal bulge raised on the horizon by faraway masses. These Love numbers are compared to their electromagnetic analogs and provide insights into the shape of the tidal bulge raised by a test mass near the black hole horizon .
Black Hole Shadows with Soft Hairs
The presence of soft hairs, which are asymptotic symmetries of spacetime, can affect the near-horizon geometry of black holes. Although the soft hairs do not change the shape of the black hole shadow, they can alter the average size and position of the shadow. This suggests that the infrared structure of gravity theory can influence the observational characteristics of black hole shadows .
Black Holes with Bottle-Shaped Horizons
A new class of black holes, termed "black bottles," features non-compact event horizons with finite areas. These horizons are topologically spheres with one puncture, forming a cusp at infinity. The shape of these event horizons is unique and arises from specific solutions to the Plebanski-Demianski equations. These black holes can be either static or rotating, and their unique shape provides a distinct observational signature .
Conclusion
The shape of black holes, influenced by factors such as rotation, deviation parameters, and the presence of soft hairs, offers a rich field of study with significant implications for observational astronomy. Understanding these shapes helps in identifying and distinguishing between different types of black holes and their mimickers, providing deeper insights into the nature of these enigmatic objects.
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