The core of a neutron star contains the highest density of matter in the universe. This highly compressed matter can undergo a phase transition in which nuclear matter dissolves into unconfined quark matter, releasing its constituent quarks and gluons. However, it is currently unknown whether this transition occurs inside at least some physical neutron stars. In a new study, physicists from the University of Helsinki, the University of Stavanger, the Flatiron Institute, and Columbia University quantified this possibility by combining information from astrophysical observations and theoretical calculations.

Neutron stars are extreme astrophysical objects containing the densest matter found in the modern universe.

It has a radius of about 10 km (6 miles) and a mass of about 1.4 solar masses.

“A long-standing unresolved question concerns whether the enormous central pressure of a neutron star can compress protons and neutrons into a phase called cold quark matter. In this exotic state, individual protons and neutrons no longer exist. We don’t,” said Professor Aleksi Vuorinen of the University of Helsinki.

“The quarks and gluons that make them up are instead freed from typical color confinement and can move almost freely.”

In a new paper, Professor Vuorinen and colleagues provide the first quantitative estimate of the possibility of a core of quark matter existing inside a massive neutron star.

They showed that quark matter is almost inevitable in the most massive neutron stars, based on current astrophysical observations. The quantitative estimates they extracted put the likelihood in the 80-90% range.

For there to be a small chance that all neutron stars are composed only of nuclear matter, the change from nuclear matter to quark matter must occur through a strong primary phase similar to the phenomenon in which liquid water turns to ice. Must be a metastasis.

This type of rapid change in the properties of neutron star matter could destabilize the star in such a way that even the formation of a tiny quark matter core could cause the star to collapse into a black hole.

An artist's impression of the various layers inside a giant neutron star. The red circle represents a significant amount of quark matter core. Image credit: Jyrki Hokkanen, CSC.

“A key element in deriving the new results is a series of large-scale supercomputer calculations that utilize Bayesian inference, a branch of statistical deduction that estimates the likelihood of various model parameters through direct comparison with observed data. “, the authors explained.

“We demonstrate that the Bayesian component allows us to derive new limits on the properties of neutron star matter, approaching the so-called conformal behavior near the center of the most massive and stable neutron stars.”

Dr. Joonas Nettila from the University of Helsinki added: “It is interesting to see specifically how each new neutron star observation improves the ability to estimate the properties of the neutron star material.” .

“Being able to compare theoretical predictions with observations and constrain the possibility of quark-matter nuclei requires hundreds of supercomputers,” said Jonas Hirvonen, a doctoral student at the Flatiron Institute and Columbia University. “We had to spend tens of thousands of CPU hours.”

“We are very grateful to the Finnish Supercomputer Center CSC for providing us with all the necessary resources.”

of paper It was published in the magazine nature communications.

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E.Annara other. 2023. Strongly interacting matter exhibits unconfined behavior in massive neutron stars. Nat Commune 14, 8451; doi: 10.1038/s41467-023-44051-y