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Because we can’t see black holes, it’s hard to know exactly how many there are in the big, wide Universe.
But that doesn’t mean we have no way of trying to figure it out.
Stellar-mass black holes are the collapsed cores of dead massive stars, and new research incorporating how these stars and binaries form and evolve has been able to derive a new population estimate of stellar-mass black holes. of the universe.
The number is quite staggering: 40 quintillion, or 40,000,000,000,000,000,000 black holes, or about 1% of all normal matter in the observable Universe.
“The innovative character of this work lies in the coupling of a detailed model of stellar and binary evolution with advanced recipes for star formation and metal enrichment in individual galaxies”, explains astrophysicist Alex Sicilia from the International School for Advanced Studies (SISSA) in Italy. .
“It is one of the first, and one of the strongest, ab-initio calculation[s] of the mass function of the stellar black hole throughout cosmic history.”
Black holes are a huge question mark hanging over our understanding of the Universe – or rather, a lot of question marks. But if we have a good idea of how many black holes there are, it might help answer some of these questions.
One approach is to estimate the history of massive stars in the Universe. We would then be able to calculate the number of black holes that should be in a given volume of space.
This knowledge could provide clues to the growth and evolution of supermassive black holes millions or billions of times the mass of the Sun, forming the core of galaxies.
Sicilia and her colleagues took a computational approach. They included only black holes that form through the evolution of single or binary stars, and taking into account the role of black hole mergers, the number of which can be estimated from gravitational wave data, and which produce black holes of slightly higher masses.
This allowed them to calculate the birth rate of stellar-mass black holes between five and 160 times the mass of the Sun over the lifetime of the Universe.
This birth rate suggests that there should be around 40 quintillion stellar-mass black holes scattered throughout the observable Universe today, with the most massive stellar-mass black holes produced by binary black hole mergers in star clusters.
The team compared their results to gravitational wave data and found that their estimate of the black hole merger rate was in good agreement with observational data. This suggests that star cluster mergers are likely to cause the black hole collisions we have seen.
By calculating the birth rate over time, the researchers were also able to derive an estimate of the number of stellar-mass black holes in the early Universe. This is of great interest, since observations of the distant Universe have revealed supermassive black holes at a surprisingly early time after the Big Bang.
It’s unclear how these behemoths got so big so quickly. Some current questions concern the mass of the “seeds” of black holes from which they sprouted – whether they be light stellar-mass black holes or intermediate-mass “heavy” black holes.
The team’s research will provide a basis for investigating these questions. This article was the first in a series; future papers will study intermediate-mass black holes and supermassive black holes for a more complete picture of the distribution of black holes in the Universe.
“Our work provides a robust theory for the generation of light seeds for high redshift (super)massive black holes, and may provide a starting point for investigating the origin of ‘heavy seeds’, which we will pursue in a future paper,” says astrophysicist Lumen Boco of SISSA.
The team’s research has been published in The Astrophysical Journal.
