Harnessing advanced simulation tools, a team of scientists from UNIGE, Northwestern University and University of Florida shed light on the enigmatic nature of these celestial 'beasts'.

Date:
June 29, 2023
Source:
Universite' de Gene`ve
Summary:
Black holes, some of the most captivating entities in the cosmos,
possess an immense gravitational pull so strong that not even
light can escape.

The groundbreaking detection of gravitational waves in 2015, caused
by the coalescence of two black holes, opened a new window into
the universe. Since then, dozens of such observations have sparked
the quest among astrophysicists to understand their astrophysical
origins. Thanks to the POSYDON code's recent major advancements in
simulating binary-star populations, a team of scientists predicted
the existence of merging massive, 30 solar mass black hole binaries
in Milky Way-like galaxies, challenging previous theories.


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FULL STORY ========================================================================== Black holes, some of the most captivating entities in the cosmos,
possess an immense gravitational pull so strong that not even light
can escape. The groundbreaking detection of gravitational waves in 2015,
caused by the coalescence of two black holes, opened a new window into the universe. Since then, dozens of such observations have sparked the quest
among astrophysicists to understand their astrophysical origins. Thanks
to the POSYDON code's recent major advancements in simulating binary-star populations, a team of scientists, including some from the University
of Geneva (UNIGE), Northwestern University and the University of Florida
(UF) predicted the existence of merging massive, 30 solar mass black hole binaries in Milky Way-like galaxies, challenging previous theories. These results are published in Nature Astronomy.

Stellar-mass black holes are celestial objects born from the collapse of
stars with masses of a few to low hundreds of times that of our sun. Their gravitational field is so intense that neither matter nor radiation can
evade them, making their detection exceedingly difficult. Therefore, when
the tiny ripples in spacetime produced by the merger of two black holes
were detected in 2015, by the Laser Interferometer Gravitational-wave Observatory (LIGO), it was hailed as a watershed moment. According
to astrophysicists, the two merging black holes at the origin of the
signal were about 30 times the mass of the sun and located 1.5 billion light-years away.

Bridging Theory and Observation What mechanisms produce these black
holes? Are they the product of the evolution of two stars, similar to our
sun but significantly more massive, evolving within a binary system? Or
do they result from black holes in densely populated star clusters
running into each other by chance? Or might a more exotic mechanism be involved? All of these questions are still hotly debated today.

The POSYDON collaboration, a team of scientists from institutions
including the University of Geneva (UNIGE), Northwestern and the
University of Florida (UF) has made significant strides in simulating binary-star populations. This work is helping to provide more accurate
answers and reconcile theoretical predictions with observational data. "As
it is impossible to directly observe the formation of merging binary
black holes, it is necessary to rely on simulations that reproduce their observational properties. We do this by simulating the binary-star systems
from their birth to the formation of the binary black hole systems,"
explains Simone Bavera, a post-doctoral researcher at the Department
of Astronomy of the UNIGE's Faculty of Science and leading author of
this study.

Pushing the Limits of Simulation Interpreting the origins of merging
binary black holes, such as those observed in 2015, requires comparing theoretical model predictions with actual observations. The technique used
to model these systems is known as "binary population synthesis." "This technique simulates the evolution of tens of millions of binary star
systems in order to estimate the statistical properties of the resulting gravitational-wave source population. However, to achieve this in a
reasonable time frame, researchers have until now relied on models that
use approximate methods to simulate the evolution of the stars and their
binary interactions. Hence, the oversimplification of single and binary
stellar physics leads to less accurate predictions," explains Anastasios Fragkos, assistant professor in the Department of Astronomy at the UNIGE Faculty of Science.

POSYDON has overcome these limitations. Designed as open-source software,
it leverages a pre-computed large library of detailed single- and
binary-star simulations to predict the evolution of isolated binary
systems. Each of these detailed simulations might take up to 100 CPU
hours to run on a supercomputer, making this simulation technique
not directly applicable for binary population synthesis. "However, by precomputing a library of simulations that cover the entire parameter
space of initial conditions, POSYDON can utilize this extensive dataset
along with machine learning methods to predict the complete evolution
of binary systems in less than a second. This speed is comparable to
that of previous-generation rapid population synthesis codes, but with
improved accuracy," explains Jeffrey Andrews, assistant professor in
the Department of Physics at UF.

Introducing a New Model "Models prior to POSYDON predicted a negligible formation rate of merging binary black holes in galaxies similar to the
Milky Way, and they particularly did not anticipate the existence of
merging black holes as massive as 30 times the mass of our sun. POSYDON
has demonstrated that such massive black holes might exist in Milky
Way-like galaxies," explains Vicky Kalogera, a Daniel I.

Linzer Distinguished University Professor of Physics and Astronomy in
the Department of Physics and Astronomy at Northwestern, director of
the Center of Interdisciplinary Exploration and Research in Astrophysics (CIERA), and co- author of this study.

Previous models overestimated certain aspects, such as the expansion
of massive stars, which impacts their mass loss and the binary
interactions. These elements are key ingredients that determine the
properties of merging black holes. Thanks to fully self-consistent
detailed stellar-structure and binary- interaction simulations, POSYDON achieves more accurate predictions of merging binary black hole properties
such as their masses and spins.

This study is the first to utilize the newly released open-source
POSYDON software to investigate merging binary black holes. It provides
new insights into the formation mechanisms of merging black holes in
galaxies like our own.

The research team is currently developing a new version of POSYDON, which
will include a larger library of detailed stellar and binary simulations, capable of simulating binaries in a wider range of galaxy types.

* RELATED_TOPICS
o Space_&_Time
# Black_Holes # Stars # Astrophysics # Galaxies #
Astronomy # Solar_Flare # Extrasolar_Planets # Sun
* RELATED_TERMS
o Black_hole o Gravitational_wave o Galaxy o General_relativity
o Holographic_Universe o Quasar o Dark_matter o
Spitzer_space_telescope

========================================================================== Story Source: Materials provided by Universite'_de_Gene`ve. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Simone S. Bavera, Tassos Fragos, Emmanouil Zapartas, Jeff
J. Andrews,
Vicky Kalogera, Christopher P. L. Berry, Matthias Kruckow, Aaron
Dotter, Konstantinos Kovlakas, Devina Misra, Kyle A. Rocha,
Philipp M.

Srivastava, Meng Sun, Zepei Xing. The formation of merging
black holes with masses beyond 30 M⊙ at solar
metallicity. Nature Astronomy, 2023; DOI: 10.1038/s41550-023-02018-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230629125715.htm

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