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. The intriguing issue has recently been addressed by the SISSA Ph.D. student Alex Sicilia, supervised by Prof. Andrea Lapi and Dr. Lumen Boco, together with other collaborators from SISSA and from other national and international institutions.
In a first paper of a series just published in The Astrophysical Journal, the authors have investigated 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. According to the new research, a remarkable amount around 1% of the overall ordinary (baryonic) matter of the Universe is locked up in stellar mass black holes. Astonishingly, the researchers have found that the number of black holes within the observable Universe (a sphere of diameter around 90 billions light years) at present time is about 40 trillions, 40 billion billions (i.e., about 40 x 1018, i.e. 4 followed by 19 zeros!).
The estimate of the number of black holes in the observable Universe is not the only issue investigated by the scientists in this piece of research. In collaboration with Dr. Ugo Di Carlo and Prof. Michela Mapelli from University of Padova, they have also explored the various formation channels for black holes of different masses, like isolated stars, binary systems and stellar clusters. According to their work, the most massive stellar black holes originate mainly from dynamical events in stellar clusters.