How big are black holes
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Understanding the Size of Black Holes
Volume Inside a Black Hole
The volume inside a black hole is surprisingly large. When a spherical black hole forms from a collapsed object, its internal volume grows over time, eventually reaching a significant size. This growth can be understood through a geometrical interpretation that extends the concept of volume from flat spacetime to the curved spacetime inside a black hole .
Maximum Mass of Black Holes
Black holes have a physical limit to their mass, beyond which they cannot grow through luminous accretion of gas. This limit is approximately 5 × 10¹⁰ solar masses (M☉) under typical conditions, but can reach up to 2.7 × 10¹¹ M☉ in extreme cases, such as with maximal prograde spin. The largest black holes observed so far are close to but below this limit. Black holes can exceed this mass through non-luminous means like mergers, but they will not become luminous accretors again .
Primordial Black Holes
Primordial black holes (PBHs) span a wide range of masses, from as small as 10⁻⁵ grams to as large as 10⁵⁰ grams. Those smaller than approximately 10¹⁵ grams would have evaporated by now due to Hawking radiation. Larger PBHs are constrained by various factors, including gravitational lensing, dynamical effects, and their influence on large-scale structure. These constraints help us understand the role of PBHs in the early Universe and their contribution to dark matter Carr2020Carr2009.
Supermassive Black Holes in Galaxies
Observations indicate that nearly every massive galaxy hosts a supermassive black hole at its center, with masses ranging from millions to billions of solar masses. For instance, the giant elliptical galaxy Messier 87 has a black hole with a mass of 6.3 billion M☉, while NGC 3842 and NGC 4889 host black holes with masses around 9.7 billion M☉. These black holes are significantly more massive than predicted by standard correlations between black-hole mass and the properties of their host galaxies, suggesting unique evolutionary processes for the largest galaxies McConnell2011Begelman2003.
Formation of Supermassive Black Holes
Supermassive black holes, with masses up to 10⁹ M☉, are found in the centers of galaxies and were already present less than a billion years after the Big Bang. Their formation is still a mystery, but one plausible scenario is direct collapse, which can produce seed masses of 10⁵-10⁶ M☉. High-resolution simulations show that these black holes can grow rapidly through turbulent accretion and mergers, reaching significant masses within a relatively short time Latif2013Pacucci2015.
Cosmological Constraints on Black Hole Size
In the context of our accelerating Universe, there are cosmological upper bounds on the size of black holes, particularly when considering angular momentum, gravitational waves, and matter. These bounds are tighter than previous estimates and are especially relevant for understanding black holes in the early Universe .
Conclusion
Black holes vary greatly in size, from tiny primordial black holes to supermassive black holes at the centers of galaxies. Their growth is influenced by various factors, including accretion limits, mergers, and cosmological constraints. Understanding these factors helps us comprehend the diverse and complex nature of black holes in our Universe.
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