Black hole shape
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The Shape of Black Holes: Insights from Recent Research
Rotating Black Holes and Their Shape
Rotational Thickening and Stabilization
Rotating black holes exhibit unique shape characteristics due to their angular momentum. Research indicates that rotation causes the central regions of black holes to widen, a phenomenon known as rotational thickening. This effect limits the global shape of the black hole, making it more stable and closer to the geometry of an extreme Kerr black hole at high rotational speeds1 2. The shape of these black holes is determined solely by their horizon area and angular momentum, independent of the surrounding spacetime geometry or the presence of matter1 2.
Apparent Shape and Observational Signatures
The apparent shape of black holes, especially those that might violate the Kerr bound (where angular momentum exceeds mass), can significantly differ from typical black holes. Such super-spinning black holes cast much smaller shadows, providing a clear observational signature that can be used to identify them5. This deviation in shape is crucial for astronomers searching for these exotic objects.
Photon Rings and Gravitational Lensing
Measuring the Shape of Photon Rings
Gravitational lensing near black holes produces narrow photon rings, which are critical for understanding black hole shapes. The detailed shape and intensity profile of these rings can be 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 precise, model-independent measurements of the ring's shape3.
Black Hole Shadows and Soft Hairs
The shadow of a black hole, formed by light bending due to strong gravity, provides insights into the near-horizon geometry. The presence of soft hairs, which are asymptotic symmetries of spacetime, can alter the average size and position of the black hole shadow without changing its shape. This suggests that while the shadow's shape remains consistent, its size and position can vary due to these additional gravitational effects10.
Unconventional Black Hole Shapes
Black Bottles and Non-Compact Horizons
A novel class of black holes, termed "black bottles," features non-compact event horizons with finite areas. These horizons are topologically spheres with a puncture pushed to infinity, forming a cusp. This unique shape arises from specific solutions within the Plebanski-Demianski framework and can describe both static and rotating black bottles7.
Primordial Black Holes and Inflationary Power Spectrum
The abundance and formation of primordial black holes are highly dependent on the shape of the inflationary power spectrum. Broad peaks in the power spectrum lead to a higher density of primordial black holes compared to narrow peaks. This relationship underscores the importance of the initial energy density profile in determining the formation and characteristics of these early-universe black holes8 9.
Conclusion
The shape of black holes is influenced by various factors, including rotation, gravitational lensing, and the presence of soft hairs. Rotating black holes exhibit rotational thickening and stabilization, while super-spinning black holes have distinct observational signatures. Photon rings and black hole shadows provide critical insights into their geometry. Additionally, unconventional shapes like black bottles and the dependence of primordial black hole formation on the inflationary power spectrum highlight the diversity of black hole shapes in different contexts. Understanding these shapes not only advances our knowledge of black hole physics but also aids in the search for and study of these enigmatic objects.
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Most relevant research papers on this topic
Shape of rotating black holes
Rotating black holes exhibit widening of central regions, precompact family formation, and enforced shaping, independent of surrounding geometry and matter presence.
Shape of black holes
Black hole rotation widens central regions, limits global shapes, and enforces close to extreme Kerr horizon geometry, independent of surrounding spacetime geometry and matter presence.
Measuring the shape of a black hole photon ring
The shape and intensity profile of black hole photon rings can be directly measured on long interferometric baselines, enabling model-independent measurements compared to general relativity predictions.
What is the Shape of a Black Hole
The shape of a black hole is related to its total energy and mass, and may be heard, but it is not fully understood.
Apparent shape of super-spinning black holes
Super-spinning black holes can violate the Kerr bound, resulting in a significantly smaller shadow, which can be used as an observational signature in the search for super-spinning black holes.
Highly CitedHow big is a black hole
A black hole's three-dimensional volume is large, potentially impacting the information paradox discussion.
Highly CitedBlack holes with bottle-shaped horizons
Black holes with bottle-shaped event horizons, called "black bottles," can be static or rotating, and may have acceleration horizons.
Abundance of Primordial Black Holes Depends on the Shape of the Inflationary Power Spectrum.
The abundance of primordial black holes depends on the shape of the inflationary power spectrum, with a broad peak yielding exponentially larger densities than a narrow peak, and the density of black holes dominated by masses equal to the cosmological horizon mass at horizon crossing.
Highly CitedThreshold for primordial black holes: Dependence on the shape of the cosmological perturbations
The threshold for primordial black hole formation depends on the shape of the initial energy density profile, with the abundance of black holes depending on the shape of the inflationary power spectrum of cosmological perturbations.
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