Per definition, a black hole is an astronomical object with such a strong gravitational pull that nothing, not even light, can escape from it.
The “surface” of a black hole, called the event horizon, defines the limit where the speed required to evade it exceeds the speed of light, which is the speed limit in the cosmos. As a result, matter and radiation are trapped and cannot get out.
Two main classes of black holes have been studied extensively throughout the years. Stellar-mass black holes, three to dozens of times the mass of the Sun, are spread throughout our galaxy, the Milky Way, while supermassive monsters weighing between 100,000 to billions of solar masses are found in the centers of more giant galaxies, including our own.
How many black holes are out there in the Universe? This is one of the most relevant and pressing questions in modern astrophysics and cosmology.
Experts found surprising data: first, the researchers say that around 1% of the overall ordinary (baryonic) matter of the Universe is locked up in stellar-mass black holes.
Secondly, and perhaps more importantly, scientists have discovered that the number of black holes within the observable Universe is no less than 40 billion billions.
The count, performed by researchers from various institutions across Italy and the U.K., involved complex computations involving the formation rate and mass of stars as well as the metallicity of galaxies—or the presence of elements that are heavier than hydrogen and helium.
“The innovative character of this work is in the coupling of a detailed model of stellar and binary evolution with advanced recipes for star formation and metal enrichment in individual galaxies,” said Alex Sicilia, first author of the study and a researcher at the SISSA science facility in Italy, in a press release.
In one of the papers–more papers are set to follow–published in The Astrophysical Journal, the authors have examined the demographics of stellar-mass black holes, which are black holes with masses between a few to some hundred solar masses, that originated at the end of the life of massive stars.
As the authors of the research explain: “This important result has been obtained thanks to an original approach which combines the state-of-the-art stellar and binary evolution code SEVN developed by SISSA researcher Dr. Mario Spera to empirical prescriptions for relevant physical properties of galaxies, especially the rate of star formation, the amount of stellar mass and the metallicity of the interstellar medium (which are all important elements to define the number and the masses of stellar black holes).
Exploiting these crucial ingredients in a self-consistent approach, thanks to their new computation approach, the researchers have then derived the number of stellar black holes and their mass distribution across the whole history of the Universe.
Alex Sicilia, first author of the study, comments: “The innovative character of this work is in the coupling 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 one of the most robust, ab initio computation of the stellar black hole mass function across cosmic history.”
Calculating the number of black holes in the observable Universe is not the only topic investigated by scientists in this research.
In collaboration with Dr. Ugo Di Carlo and Professor Michela Mapelli from the University of Padua, researchers also explored the various formation channels of black holes of different masses, such as single stars, binary systems, and star clusters.
Based on their research, scientists suggest that the most massive stellar black holes form mainly from dynamical events in stellar clusters.
The study also investigated black hole “mergers” in which two of the cosmic monsters join together and stated that its results were a “starting point to investigate… the growth of supermassive black holes in high-redshift star-forming galaxies.” The researchers hope to further probe the growth of supermassive black holes in future work.
Black holes are regions of space where gravity is so strong that light can’t escape. They are thought to be formed when a star of sufficient mass dies and collapses in on itself.
This collapse means that all of the star’s matter collects into a single, tiny point in space. As an example, one could imagine a star 10 times more massive than the sun being packed into an area around the size of New York City.