How many black holes are there in the universe? This is one of the most relevant and pressing questions in modern astrophysics and cosmology. Recently, Alex Sicilia, a SISSA PhD student directed by Professor Andrea Lapi and Dr. Lumen Boco, explored this interesting issue with other collaborators from SISSA and other national and international institutions.
In the first in a series just published in the Astrophysical Journal, the authors studied the statistics of stellar-mass black holes with masses ranging from a few to hundreds of solar masses that originate at the end of the life of massive stars. The innovation of this work is the combination of detailed models of the evolution of stars and binary stars with advanced formulations for star formation and metal enrichment in individual galaxies. This is one of the earliest and most powerful de novo calculations of the mass function of stellar black holes throughout the history of the universe.
According to the new study, about 1 percent of the ordinary (baryonic) matter of the universe as a whole is locked into a stellar-mass black hole. Surprisingly, the researchers found that the number of black holes in the observable universe (spheres about 90 billion light-years in diameter) is currently about 40 billion, or 19 zeros after 4! This important result is achieved thanks to an ingenious approach that combines SEVN, a state-of-the-art stellar and binary evolution code developed by SISSA researcher Dr. Mario Spera, with empirical prescriptions for galaxy-related physical properties, particularly the rate of star formation, the quantity of stellar masses, and the metallicity of the interstellar medium (all important factors that define the number and mass of stellar black holes).
Estimating the number of black holes in the observable universe isn't the only question scientists investigated in this study. In collaboration with Dr. Hugo DiCarlo and Professor Michella Mapelli of the University of Padua, they also explored various formation channels for black holes of different masses, such as isolated stars, binary star systems, and star clusters. According to their work, stellar black holes with the greatest mass originate primarily from dynamic events in star groups. Specifically, the researchers showed that such events are needed to explain the mass function of the condensed black hole estimated by the LIGO/Virgo collaboration from gravitational wave observations.
This work provides a powerful theory for the generation of light seeds for highly redshifted (super)massive black holes, and could constitute a starting point for studying the origin of heavy seeds.
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