Size of black hole
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Understanding the Size of Black Holes
Maximum Size of Black Holes in an Accelerating Universe
Recent research has established a cosmological upper bound for the area of stable marginally outer trapped surfaces in black holes, considering factors such as angular momentum, gravitational waves, and matter. This bound is particularly relevant in the early Universe and is tighter than previous estimates that did not account for all these variables . This finding is crucial for understanding the maximum possible size of black holes in our accelerating Universe.
Volume Inside a Black Hole
The volume inside a spherical black hole can be defined by extending the intrinsic flat-spacetime characterization of the volume inside a two-sphere. For a collapsed object, this volume grows over time since the collapse, eventually reaching a large asymptotic form. This has significant implications for discussions on the information paradox, suggesting that the internal volume of black holes is much larger than previously thought .
Constraints on Primordial Black Holes
Primordial black holes (PBHs) formed in the early Universe are subject to various constraints based on their mass. PBHs smaller than approximately (10^{15}) grams would have evaporated by now due to Hawking radiation, while larger PBHs are constrained by gravitational lensing, dynamical effects, and their influence on large-scale structures. These constraints help define the possible sizes of PBHs and their role in the Universe's evolution .
Schwarzschild Black Holes: The Largest for a Given Mass
In Einstein gravity, static black holes are characterized by parameters such as the radii of the horizon, photon sphere, and black hole shadow. The Schwarzschild black hole, which saturates all known inequalities, is the largest for a given mass. This includes an upper bound on entropy for any quantum system with a given energy, making the Schwarzschild black hole a critical reference point for understanding black hole sizes .
Upper Limits on Primordial Black Hole Sizes
The size of primordial black holes formed in the early Universe is considerably smaller than the cosmological apparent horizon size. This is true for various formation scenarios, including those involving hydrodynamical processes. The maximum size of an overdense region that can form a black hole without becoming a separate closed universe is also derived, providing a clear upper limit on the size of primordial black holes .
Size of Rotating Black Holes
For rotating black holes, the sequence of inequalities relating the horizon, photon sphere, and shadow remains valid. However, rotation affects the actual and apparent sizes of black holes, making them smaller. This is particularly evident when comparing different types of rotating black holes, such as Kerr and Kerr-Newman black holes .
Universal Bounds on Black Hole Sizes
A series of universal inequalities have been proposed for static black holes in Einstein gravity, relating the outermost photon sphere area, shadow area, and horizon area. These inequalities provide a comprehensive framework for understanding the spatial size of black holes and are supported by evidence in spherically symmetric and general static cases .
Higher-Dimensional Black Holes
Higher-dimensional black holes exhibit novel properties compared to four-dimensional ones, primarily due to their horizons having two characteristic lengths of very different sizes. A long-distance world-volume effective theory captures the dynamics of these black holes, revealing complex horizon geometries and topologies. This approach provides a new framework for understanding the size and behavior of higher-dimensional black holes .
Conclusion
The size of black holes is influenced by various factors, including their formation process, rotation, and the cosmological context. From the maximum size constraints in an accelerating Universe to the unique properties of higher-dimensional black holes, recent research provides a comprehensive understanding of the limits and characteristics of black hole sizes. These insights are crucial for advancing our knowledge of black hole physics and their role in the Universe.
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