According to a recent study of our cosmos, there are around 40 billion black holes in the observable universe.
Black Holes are fascinating yet mysterious cosmic objects scattered across the Universe. Unfortunately, we don’t know much about them, and we barely managed to snap a photo of one located in another galaxy.
But, what exactly are black holes?
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.
Some theories suggest that Black Holes are the remnants of other universes. In contrast, other studies suggest these cosmic monsters might serve as tunnels in space, allowing us to reach unimaginable distances with ease. Whether the above studies are right or wrong remains to be seen. However, a group of scientists from SISSA–Scuola Internazionale Superiore di Studi Avanzati or International School for Advanced Studies–has calculated the number of Black Holes in the observable Universe: 40 billion billions.
The quest to answer the question of how many black holes exist in the cosmos was taken on by SISSA doctoral student Alex Sicilia, supervised by Professor Andrea Lapi and Dr. Lumen Boco, together with other collaborators from the Italian School as well as from different national and international institutions.
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.
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.
Calculating the number of Black holes in the Universe
“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),” the researchers explained.
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 has shown that such events are required to explain the mass function of merging black holes as estimated from gravitational wave observations by the LIGO/Virgo collaboration.