We can't see black holes, so it's hard to know how many are out there.
We can try to figure it out.
New research has been able to derive a new estimate of the population of stellar-mass black holes in the Universe.
The number is 40 quintillion, or 40,000,000,000,000,000,000 black holes, and it makes up 1 percent of all the normal matter in the observable Universe.
Alex Sicilia of the International School of Advanced Studies (SISA) in Italy says that the innovative character of the work is in the combination of a detailed model of stellar and binary evolution with advanced recipes for star formation and metal enrichment in individual galaxies.
This is one of the first and most robust computations of the stellar black hole mass function.
Black holes are a lot of question marks over our understanding of the Universe. If we have a good idea of how many black holes are out there, that could help answer some of the questions.
Estimate the history of massive stars is one approach. We could calculate the number of black holes that should be in a given volume of space.
The growth and evolution of supermassive black holes millions or billions of times the mass of the Sun could be seen by this knowledge.
The computational approach was taken by Sicilia and his colleagues. They only included black holes that form via the evolution of single stars, and taking into account the role of black hole mergers, whose numbers can be estimated based on the data from the wave data.
The birthrate of stellar-mass black holes is between five and 160 times the mass of the Sun.
The birthrate suggests that there should be 40 quintillion stellar-mass black holes scattered throughout the observable Universe today, with the most massive stellar-mass black holes produced by black hole mergers in clusters of stars.
The team found that their estimate of the rate of black hole mergers was in agreement with observational data. The black hole collision we've seen is likely the result of star cluster mergers.
The researchers were able to estimate the number of black holes in the early Universe by calculating the birthrate over time. The observations of the distant Universe have shown supermassive black holes at a very early time after the Big bang.
It's not clear how these giants grew so fast. Some current questions concern the mass of the black hole'seeds' from which they were grown, whether they were light stellar-mass black holes or heavy intermediate-mass black holes.
The team's research will give them a basis for their investigation. The paper was the first in a series and will investigate intermediate-mass black holes and supermassive black holes for a more complete picture of the black hole distribution across the Universe.
"Our work provides a robust theory for the generation of light seeds for black holes at high redshift, and can constitute a starting point to investigate the origin of 'heavy seeds', that we will pursue in a forthcoming paper," says astrophysicist Lumen Boco of SISSA.
The research was published in a journal.
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