How many black holes in the universe
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Estimating the Number of Black Holes in the Universe
Stellar-Mass Black Holes in Globular Clusters
Globular clusters, dense collections of stars, are expected to host numerous stellar-mass black holes. Recent studies have confirmed the presence of such black holes in clusters like M22 (NGC 6656), with estimates ranging from 5 to 100 black holes per cluster. Theoretical models suggest that these clusters can retain a significant number of black holes over time, with retention rates around 10% leading to the presence of multiple black holes even after billions of years.
Black Holes at the Galactic Centre
Observations at the center of our Galaxy have revealed a dozen stellar-mass black holes, suggesting that thousands more could be present in this region alone. This finding aligns with predictions from stellar dynamics, indicating a high concentration of black holes in galactic centers.
Supermassive Black Holes in Galaxies
Nearly every massive galaxy is believed to host a supermassive black hole at its center. These black holes can range from millions to billions of solar masses. For instance, the supergiant elliptical galaxy M87 contains a black hole of 6.3 billion solar masses, while NGC 3842 and NGC 4889 host black holes with masses around 10 billion solar masses . These discoveries highlight the prevalence of supermassive black holes in the universe and their significant role in galactic evolution .
Intermediate-Mass Black Holes and Dark Matter
Intermediate-mass black holes (IMBHs), with masses between 30 and 300,000 solar masses, are hypothesized to exist in large numbers, potentially constituting all cold dark matter . Estimates suggest that the visible universe, comprising approximately 10^11 halos, could host a vast number of these IMBHs, contributing significantly to the universe's entropy .
Isolated Black Holes in the Milky Way
Apart from black holes in binary systems, it is estimated that our Galaxy alone could harbor around 100 million isolated black holes (IBHs). These IBHs, although difficult to detect due to weak accretion flows, represent a substantial population of black holes within the Milky Way.
Conclusion
The universe is teeming with black holes of various masses and types. From stellar-mass black holes in globular clusters and galactic centers to supermassive black holes in the cores of galaxies, and potentially vast numbers of intermediate-mass black holes constituting dark matter, black holes are a fundamental component of the cosmos. While precise numbers are challenging to ascertain, current estimates suggest that there are millions to billions of black holes scattered throughout the universe, each playing a crucial role in the dynamics and evolution of their respective environments.
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