Because we can not see black holes, it’s hard to know exactly how many are out there in the big, wide universe.
But that does not mean that we have no chance of trying to figure it out.
Star mass black holes are the collapsed nuclei of dead massive stars, and new research incorporating how these stars and binary stars form and evolve has been able to derive a new estimate of the star mass black ape population in the universe.
The number is pretty jaw-dropping: 40 quintillions or 40,000,000,000,000,000,000,000 black holes that roughly make up 1 percent of all normal matter in the observable universe.
“The innovative nature 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 of the International School of Advanced Studies (SISSA) in Italy.
“This is one of the first and one of the most robust, from the beginning calculation[s] of the mass function of the star’s black hole through cosmic history. “
Black holes are a big question mark that hangs 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 out there, it may help to 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 can provide clues about the growth and evolution of supermassive black holes millions or billions of times the mass of the Sun that make up the nuclei of galaxies.
Sicily and his colleagues took a computational approach. They included only black holes formed via the evolution of single or binary stars, and taking into account the role of black hole fusions, the number of which can be estimated based on gravitational wave data, and which produce black holes with slightly higher masses.
This allowed them to calculate the birth rate of black holes with stellar mass between five and 160 times the mass of the Sun over the life of the universe.
This birth rate suggests that there should be about 40 quintillion black holes with stellar mass scattered throughout the observable universe today, with the most massive black holes of stellar mass produced by binary black hole fusions in clusters of stars.
The team compared their results with gravity wave data and found that their estimate of the velocity of black hole fusions was in good agreement with the observational data. This suggests that star cluster aggregations are likely behind the black hole collisions we have seen.
By calculating the birth rate over time, scientists were also able to derive an estimate of the number of black holes with star mass in the early universe. This is of great interest, as observations of the distant universe have revealed supermassive black holes at a shockingly early time after the Big Bang.
It is unclear how these giants grew so large so fast. Some current issues concern the mass of the ‘seeds’ of the black holes from which they grew – whether they were black holes with light star mass or ‘heavy’ black holes with intermediate mass.
The team’s research will form the basis for examining these issues. This paper was the first in a series; future papers will examine medium-sized black holes and supermassive black holes to get a more complete picture of the distribution of black holes across the universe.
“Our work provides a robust theory for generating light seeds for (super) massive black holes at high redshifts and can provide a starting point for investigating the origin of ‘heavy seeds’, which we will pursue in a future paper,” says astrophysicist Lumen Boco from SISSA.
The team’s research has been published in The Astrophysical Journal.