How large is a black hole
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Understanding the Size of Black Holes
Black Hole Volume and Growth Over Time
The volume inside a black hole, particularly a spherical one, can be surprisingly large. This volume grows over time since the collapse of the black hole, eventually reaching a simple asymptotic form. This growth has a compelling geometrical interpretation and may be relevant to discussions on the information paradox .
Physical Limits on Black Hole Mass
There is a physical limit to the mass a black hole can achieve through luminous accretion of gas. This limit is approximately (5 \times 10^{10} M_{\odot}) for typical parameters, but can reach up to (2.7 \times 10^{11} M_{\odot}) in extreme cases, such as with maximal prograde spin. Black holes can grow beyond this limit through non-luminous means like mergers, but they cannot become luminous accretors again .
Supermassive Black Holes (SMBHs)
Supermassive black holes, which are found in the centers of large galaxies, have masses ranging from millions to billions of times that of the Sun. These SMBHs could form from the merger of smaller black holes or the collapse of stellar clusters or large gas clouds. Understanding their mass is crucial for studying their formation and evolution .
Constraints on Primordial Black Holes (PBHs)
Primordial black holes, which could have formed in the early universe, are subject to various constraints based on their mass. Those smaller than approximately (10^{15}) grams would have evaporated by now due to Hawking radiation. Larger PBHs are constrained by their gravitational effects, accretion, and influence on large-scale structures. These constraints help in understanding the role of PBHs in the universe and their contribution to dark matter 45.
Maximum Size in an Accelerating Universe
In the context of an accelerating universe with a positive cosmological constant, there is a cosmological upper bound for the area of stable marginally outer trapped surfaces. This bound is tighter when considering factors like angular momentum, gravitational waves, and matter, providing a concrete limit to the size of black holes, especially in the early universe .
Stupendously Large Black Holes (SLABs)
SLABs, with masses greater than (10^{11} M_{\odot}), have been less studied but are conceivable given the evidence for black holes up to nearly (10^{11} M_{\odot}) in galactic nuclei. These SLABs could have been seeded by primordial black holes and are constrained by their dynamical and lensing effects, background radiation generation, and gamma-ray emissions from WIMP annihilation .
Conclusion
Black holes vary significantly in size, from primordial black holes to supermassive black holes in galactic centers. Their growth is limited by physical constraints, and their sizes are influenced by various factors, including accretion processes, mergers, and the cosmological context. Understanding these constraints and the potential sizes of black holes helps in probing fundamental aspects of black hole physics and the early universe.
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