Neutron-star cores stand as cosmic vaults harboring matter at densities unmatched in our present universe—up to two solar masses squeezed into a mere 25 km diameter. Picture these astrophysical marvels as colossal atomic nuclei; gravity wields its force, compressing their cores to densities surpassing individual protons and neutrons by magnitudes.
These extreme densities spark intrigue in the realms of particle and nuclear physics. A persistent enigma lies in the colossal central pressure of neutron stars, questioning whether it can force protons and neutrons into an uncharted phase called cold quark matter. Within this exotic realm, the conventional entities of protons and neutrons fade away.
Aleksi Vuorinen, professor of theoretical particle physics at the University of Helsinki, sheds light on this mysterious transformation. In cold quark matter, constituent quarks and gluons break free from their customary color confinement, navigating a nearly liberated existence. It’s a captivating exploration into the transformative forces at play within these cosmic giants.
A strong phase transition may still ruin the day
In a groundbreaking article featured in Nature Communications, a research team led by the University of Helsinki has presented a pioneering quantitative assessment of the likelihood of quark-matter cores within massive neutron stars. Astonishingly, their findings suggest that, given current astrophysical observations, quark matter is nearly unavoidable in the most colossal neutron stars, with the team’s estimate falling within the compelling range of 80–90%.
The slender probability of all neutron stars exclusively comprising nuclear matter implies that the transition from nuclear to quark matter must be a robust first-order phase shift, akin to the abrupt transformation of liquid water into ice. Such a swift alteration in the properties of neutron-star material bears the potential to destabilize the star, paving the way for a precarious scenario where even a minuscule quark-matter core could trigger the star’s collapse into a black hole.
This international collaboration, bringing together scientists from Finland, Norway, Germany, and the US, not only offers a glimpse into the high probability of quark-matter cores but also outlines a pathway for the future verification or dismissal of their existence. The crucial determinant lies in the ability to constrain the strength of the phase transition between nuclear and quark matter, a feat anticipated to become achievable upon the eventual detection of a gravitational-wave signal emanating from the concluding stages of a binary neutron-star merger.
Massive supercomputer runs using observational data
At the heart of these groundbreaking findings were intricate supercomputer calculations employing Bayesian inference—a statistical deduction branch illuminating the likelihoods of various model parameters through direct alignment with observational data.
The Bayesian facet of this study empowered researchers to establish fresh boundaries for the characteristics of neutron-star matter, revealing an intriguing convergence toward what is termed conformal behavior near the cores of the most massive, stable neutron stars.
Lead author Dr. Joonas Nättilä underscores the interdisciplinary nature of this endeavor, where expertise from astrophysics, particle and nuclear physics, and computer science converged. As he prepares to assume the role of Associate Professor at the University of Helsinki in May 2024, he expresses fascination at how each new neutron-star observation intricately refines our understanding of neutron-star matter.
Joonas Hirvonen, a Ph.D. student under the mentorship of Nättilä and Vuorinen, highlights the pivotal role of high-performance computing in this exploration. With millions of CPU hours on supercomputers, the team meticulously compared theoretical predictions to observations, intricately constraining the likelihood of quark-matter cores. Gratitude extends to the Finnish supercomputer center CSC for furnishing the essential resources, underscoring the indispensable role of technology in pushing the frontiers of astrophysical understanding.
Source: University of Helsinki