Number of black holes in the universe
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Estimated Number of Black Holes in the Universe
Black Holes in the Milky Way Galaxy
Research using population synthesis models and star formation histories estimates that the Milky Way contains about 120 million (1.2 × 10^8) single black holes and about 9.3 million (9.3 × 10^6) black holes in binary systems, with average masses of 14 and 19 solar masses, respectively . Other studies suggest that the number of isolated black holes in our galaxy could be as high as 100 million (10^8) . Some models, considering the evolution of massive stars and the effects of nucleosynthesis, propose that the number of low-mass black holes in the Milky Way could reach around one billion (10^9) . These estimates show that the Milky Way alone likely hosts hundreds of millions to a billion stellar-mass black holes 346.
Black Holes in the Visible Universe
Scaling up from the Milky Way, the visible universe is estimated to contain about 100 billion (10^11) galactic halos, each with a mass around one trillion (10^12) solar masses . If the number of black holes per galaxy is similar to the Milky Way, the total number of stellar-mass black holes in the observable universe could be on the order of 10^19 to 10^20 . This estimate assumes that most galaxies have similar star formation histories and black hole production rates as our own.
Supermassive Black Holes and Their Prevalence
Observations indicate that nearly all massive galaxies host a supermassive black hole at their center, with masses ranging from millions to tens of billions of solar masses 789. For example, some galaxies have been found to contain black holes with masses up to 10 billion solar masses . The number of supermassive black holes is therefore roughly equal to the number of massive galaxies, which is in the range of hundreds of billions in the observable universe 289.
Primordial and Intermediate-Mass Black Holes
There is ongoing research into the existence of primordial black holes, which could have formed in the early universe. While their exact abundance is uncertain and subject to many observational constraints, if they exist in significant numbers, they could contribute to the total black hole population and even to dark matter 210. Intermediate-mass black holes, with masses between stellar and supermassive black holes, are also considered in some models, but their numbers are less well constrained .
Black Hole Remnants from Mergers
Gravitational wave observations have revealed that black hole mergers produce remnant black holes, and the number density of these remnants is estimated to be about 60,000 per Milky Way–like galaxy . This population is a direct result of binary black hole mergers observed by LIGO and Virgo .
Conclusion
Current research suggests that the Milky Way contains hundreds of millions to a billion stellar-mass black holes, and scaling this up, the observable universe likely contains at least 10^19 to 10^20 such black holes. Nearly every massive galaxy also hosts a supermassive black hole, adding hundreds of billions more to the total count. The exact number of primordial and intermediate-mass black holes remains uncertain, but they could further increase the total. Overall, black holes are among the most numerous and significant compact objects in the universe 2345+4 MORE.
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