Challenging Previous Theories – Scientists Shed New Light on the Enigmatic Nature of Black Holes

Utilizing sophisticated simulation technology, researchers from UNIGE, Northwestern University, and the University of Florida have shed light on the enigmatic nature of these celestial “beasts.”

Black holes, among the universe’s most mesmerizing phenomena, have a gravitational force so intense that even light cannot break free. The groundbreaking detection of gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>gravitational waves in 2015, caused by the coalescence of two black holes, opened a new window into the universe. This revelation has since ignited a series of findings, driving astrophysicists to delve deeper into their 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 holeA black hole is a place in space where the gravitational field is so strong that not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>black hole binaries in Milky WayThe Milky Way is the galaxy that contains our Solar System and is part of the Local Group of galaxies. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years. The name "Milky Way" comes from the appearance of the galaxy from Earth as a faint band of light that stretches across the night sky, resembling spilled milk.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Milky Way-like galaxies, challenging previous theories. The findings were recently published in the journal 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 (LIGOThe Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory supported by the National Science Foundation and operated by Caltech and MIT. It's designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. It's multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves. It consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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 FloridaEstablished in 1853, the University of Florida (Florida or UF) is a public land-grant, sea-grant, and space-grant research university in Gainesville, Florida. It is home to 16 academic colleges and more than 150 research centers and institutes. University of Florida offers multiple graduate professional programs, including business administration, engineering, law, dentistry, medicine, pharmacy, and veterinary medicine, and administers 123 master's degree programs and 76 doctoral degree programs in eighty-seven schools and departments.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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 learningMachine learning is a subset of artificial intelligence (AI) that deals with the development of algorithms and statistical models that enable computers to learn from data and make predictions or decisions without being explicitly programmed to do so. Machine learning is used to identify patterns in data, classify data into different categories, or make predictions about future events. It can be categorized into three main types of learning: supervised, unsupervised and reinforcement learning.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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 accuracyHow close the measured value conforms to the correct value.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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.

Reference: “The formation of merging black holes with masses beyond 30 M_⊙at solar metallicity” by 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 and Zepei Xing, 29 June 2023, Nature Astronomy.
DOI: 10.1038/s41550-023-02018-5