From The International School for Advanced Studies [Scuola Internazionale Superiore di Studi Avanzati](IT): “There are 40 billion billions of Black Holes in the Universe!” 


From The International School for Advanced Studies [Scuola Internazionale Superiore di Studi Avanzati](IT)


Nico Pitrelli
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Donato Ramani
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There are 40 billion billions of Black Holes in the Universe!

Image by PIxabay

With a new computational approach SISSA researchers have been able to
make the fascinating calculation. Moreover, according to their work, around
1% of the overall ordinary (baryonic) matter is locked up in stellar mass
black holes. Their results have just been published in the prestigious The
Astrophysical Journal

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!).

A new method to calculate the number of black holes

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.”

What’s the origin of most massive stellar black holes?

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 The University of Padua [Università degli Studi di Padova](IT),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. Specifically, the researchers have shown that such
events are required to explain the mass function of coalescing black holes as
estimated from gravitational wave observations by the LIGO/Virgo collaboration.

Caltech/MIT Advanced aLigo at Hanford, WA(US), Livingston, LA(US) and VIRGO Gravitational Wave interferometer, near Pisa(IT).

Lumen Boco, co-author of the paper, comments: “Our work provides a robust
theory for the generation of light seeds for (super)massive black holes at high
redshift, and can constitute a starting point to investigate the origin of ‘heavy
seeds’, that we will pursue in a forthcoming paper.

A multidisciplinary work carried out in the context of “BiD4BESt – Big Data
Application for Black Hole Evolution Studies”

Prof. Andrea Lapi, Sicilia’s supervisor and coordinator of the Ph.D. in
Astrophysics and Cosmology at SISSA, adds: “This research is really
multidisciplinary, covering aspects of, and requiring expertise in stellar
astrophysics, galaxy formation and evolution, gravitational wave and multi-messenger astrophysics; as such it needs collaborative efforts from various
members of the SISSA Astrophysics and Cosmology group, and a strong
networking with external collaborators.”

Alex Sicilia’s work occurs in the context of a prestigious Innovative Training
Network Project “BiD4BESt – Big Data Application for Black Hole Evolution
Studies” co-PIed by Prof. Andrea Lapi from SISSA (H2020-MSCAITN-2019
Project 860744), that has been funded by the European Union with about 3.5
million Euros overall; it involves several academic and industrial partners, to
provide Ph.D. training to 13 early stage researchers in the area of black hole
formation and evolution, by exploiting advanced data science techniques.

See the full article here.


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International School for Advanced Studies, Trieste. Credit: Mike Peel (

The International School for Advanced Studies [Scuola Internazionale Superiore di Studi Avanzati] (IT) (SISSA) is an international, state-supported, post-graduate-education and research institute, located in Trieste, Italy.

SISSA is active in the fields of mathematics, physics, and neuroscience, offering both undergraduate and post-graduate courses. Each year, about 70 PhD students are admitted to SISSA based on their scientific qualifications. SISSA also runs master’s programs in the same areas, in collaboration with both Italian and other European universities.


SISSA was founded in 1978, as a part of the reconstruction following the Friuli earthquake of 1976. Although the city of Trieste itself did not suffer any damage, physicist Paolo Budinich asked and obtained from the Italian government to include in the interventions the institution of a new, post-graduate teaching and research institute, modeled on the Scuola Normale Superiore di Pisa(IT). The school became operative with a PhD course in theoretical physics, and Budinich himself was appointed as general director. In 1986, Budinich left his position to Daniele Amati, who at the time was at the head of the theoretical division at The European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire](CH)[CERN]. Under his leadership, SISSA expanded its teaching and research activity towards the field of neuroscience, and instituted a new interdisciplinary laboratory aiming at connecting humanities and scientific studies. From 2001 to 2004, the director was the Italian geneticist Edoardo Boncinelli, who fostered the development of the existing research areas. Other directors were appointed in the following years, which saw the strengthening of SISSA collaboration with other Italian and European universities in offering master’s degree programs in the three areas of the School (mathematics, physics and neuroscience). The physicist Stefano Ruffo, the current director, was appointed in 2015. He signed a partnership with the International Centre for Genetic Engineering and Biotechnology to set up a new PhD program in Molecular Biology, with teaching activity organized by both institutions.


SISSA houses the following research groups:

Astroparticle Physics
Condensed Matter
Molecular and Statistical Biophysics
Statistical Physics
Theoretical Particle Physics
Cognitive Neuroscience
Molecular Biology
Applied Mathematics
Mathematical Analysis
Mathematical Physics

In addition, there is the Interdisciplinary Laboratory for Natural and Humanistic Sciences (now LISNU – Laboratorio Interdisciplinare Scienze Naturali e Umanistiche), which is endowed with the task of making connections between science, humanities, and the public. It currently offers a course in Scientific Communication and Scientific journalism.

SISSA also enjoys special teaching and scientific links with the International Centre for Theoretical Physics, the International Centre for Genetic Engineering and Biotechnology and the Elettra Synchrotron Light Laboratory.