New theoretical analysis shows that the probability that massive neutron stars hide a core of quark matter is between 80% and 90%. The results were obtained through large-scale supercomputer runs using Bayesian statistical inference methods. The core of a neutron star contains the highest density of matter in the universe today, compressing up to two solar masses into a sphere 25 kilometers in diameter. These objects can indeed be thought of as giant atomic nuclei, with gravity compressing their cores to densities many times greater than those of individual protons and neutrons.
These densities make neutron stars interesting astrophysical objects from the perspective of particle physics and nuclear physics. A long-standing question is whether the massive central pressure of a neutron star can compress protons and neutrons into a new substance called cold quark matter. In this bizarre state of matter, individual protons and neutrons no longer exist.
"Their constituent quarks and gluons are freed from their typical color constraints and can move almost freely," explains Aleksi Vuorinen, professor of theoretical particle physics at the University of Helsinki.
In a new article just published in the journal Nature Communications, a research team centered on the University of Helsinki made the first quantitative estimate of the possibility of a quark matter core appearing inside a massive neutron star. Their study shows that, based on current astrophysical observations, quark matter is almost inevitable in the most massive neutron stars: quantitative estimates extracted by the team put this possibility at between 80%-90%.
The remaining probability that all neutron stars are composed of only nuclear matter is small, which requires that the change from nuclear matter to quark matter is a strong first-order phase transition, somewhat similar to the process of liquid water turning into ice. Such rapid changes in the properties of the neutron star's matter have the potential to destabilize the neutron star to the point where the formation of even a minuscule core of quark matter would cause the neutron star to collapse into a black hole.
An international collaboration between scientists from Finland, Norway, Germany and the United States further suggests that the existence of a core of quark matter may one day be fully confirmed or ruled out. The key is to be able to control the strength of the phase transition between nuclear matter and quark matter, which is expected to be possible once the gravitational wave signal produced by the final part of the merger of binary neutron stars is one day recorded.
Leveraging observational data for large-scale supercomputer operations
A key factor in arriving at the new results was a set of large-scale supercomputer calculations using Bayesian inference, a branch of statistical inference that infers the likelihood of different model parameters through direct comparison with observed data. The Bayesian inference part of the study allowed the researchers to derive new bounds on the material properties of neutron stars, demonstrating that they approach so-called conformal behavior near the cores of maximally stable neutron stars.
Dr. Joonas Nättilä, one of the lead authors of the paper, believes that this work is an interdisciplinary one that requires expertise in astrophysics, particle physics, nuclear physics, and computer science. He will start working as an associate professor at the University of Helsinki in May 2024.
"It's fascinating that each new neutron star observation allows us to infer the properties of the neutron star's material with increasing precision."
On the other hand, Joonas Hirvonen, a doctoral student working under the supervision of Neytilai and Wurinen, emphasized the importance of high-performance computing:
"We had to use millions of hours of supercomputer CPU time to compare our theoretical predictions with observations and determine the possibility of a quark matter core. We are very grateful to the Finnish Supercomputer Center CSC for providing us with all the resources we need!"
Reference: "Strongly interacting matter exhibits deconfining behavior in massive neutron stars", author: Eemeli Annala, Tyler Gorda, Joonas Hirvonen, Oleg Komoltsev, Aleksi Kurkela, Joonas Nättilä and Aleksi Vuorinen, December 19, 2023, "Nature - Communications".
DOI:10.1038/s41467-023-44051-y
Compiled source: ScitechDaily