The origins of merging black holes in galaxies like our own are a fascinating issue in astronomy and cosmology. To comprehend this phenomenon, we must dissect it into several crucial components: the genesis of black holes, their evolution, and the processes that lead to their final merging.
Black holes, among of the most fascinating things in the universe, with enormous gravitational pulls so intense that not even light can escape. The unprecedented observation of gravitational waves in 2015, created by the merger of two black holes, offered a new window into the universe. Since then, scores of similar sightings have fueled astrophysicists’ search to comprehend their cosmic 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 astronomical objects formed by the collapse of stars with masses ranging from a few hundred to thousands of times that of our sun. Because their gravitational field is so strong, neither matter nor radiation can escape, making detection extremely challenging. As a result, when the Laser Interferometer Gravitational-wave Observatory (LIGO) observed minuscule ripples in spacetime caused by the merger of two black holes in 2015, it was heralded as a watershed event. The two merging black holes at the source of the signal, according to astrophysicists, were around 30 times the mass of the sun and 1.5 billion light-years apart.
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.
Simone Bavera
Bridging Theory and Observation
What mechanisms are responsible for the formation of 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 are they the consequence of black holes in densely populated star clusters colliding by chance? Or may a more bizarre mechanism be at work? All of these issues are still being argued today.
The POSYDON partnership, which includes scientists from the University of Geneva (UNIGE), Northwestern University, and the University of Florida (UF), has made substantial progress in modelling binary-star populations. This research is assisting in providing more precise answers and reconciling theoretical predictions with empirical facts.
“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
To interpret the origins of merging binary black holes, such as those detected in 2015, theoretical model predictions must be compared to real observations. The method for modeling these systems is known as “binary population synthesis.” The statistical features of the resulting gravitational-wave source population are estimated by simulating the evolution of tens of millions of binary star systems. To do so in a realistic time frame, researchers have hitherto relied on models that use approximation approaches to predict the evolution of stars and their binary interactions. As a result, oversimplification of single and binary stellar physics leads to less accurate predictions,” argues Anastasios Fragkos, assistant professor at the UNIGE Faculty of Science’s Department of Astronomy.
POSYDON has overcome these constraints. It is open-source program that uses a wide library of pre-computed comprehensive single- and binary-star simulations to predict the evolution of isolated binary systems. Each of these complex simulations could take up to 100 CPU hours to complete on a supercomputer, limiting the applicability of this simulation technique to binary population synthesis. “However, by precompiling a library of simulations that cover the entire parameter space of initial conditions, POSYDON can use this massive dataset in conjunction with machine learning methods to predict the entire evolution of binary systems in less than a second. This speed is comparable to previous-generation rapid population synthesis codes, but with improved accuracy,” says Jeffrey Andrews, assistant professor 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 exaggerated key factors, such as the expansion of big stars, which affects their mass loss and binary interactions. These factors are critical in determining the properties of merging black holes. POSYDON achieves more accurate predictions of merging binary black hole features including masses and spins thanks to fully self-consistent detailed stellar-structure and binary-interaction simulations.
This is the first study to use the recently published open-source POSYDON software to investigate merging binary black holes. It sheds new light on the mechanisms that lead to the development 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.
