Neutron-star cores contain matter at the highest densities in our Universe. This highly compressed matter may undergo a phase transition where nuclear matter melts into deconfined quark matter, liberating its constituent quarks and gluons. But it is currently unknown whether the transition takes place inside at least some physical neutron stars. In a new study, physicists from the University of Helsinki, the University of Stavanger, Flatiron Institute and Columbia University quantified this likelihood by combining information from astrophysical observations and theoretical calculations.
Neutron stars are extreme astrophysical objects containing the densest matter found in the present-day Universe.
They have a radius on the order of 10 km (6 miles) and a mass of about 1.4 solar masses.
“A longstanding open problem concerns whether the immense central pressure of neutron stars can compress protons and neutrons into a phase called cold quark matter. In this exotic state, individual protons and neutrons no longer exist,” said University of Helsinki’s Professor Aleksi Vuorinen.
“Their constituent quarks and gluons are instead liberated from their typical color confinement and are allowed to move almost freely.”
In their new paper, Professor Vuorinen and colleagues provided a first-ever quantitative estimate for the likelihood of quark-matter cores inside massive neutron stars.
They showed that, based on current astrophysical observations, quark matter is almost inevitable in the most massive neutron stars: a quantitative estimate that they extracted placed the likelihood in the range of 80-90%.
The remaining small likelihood for all neutron stars to be composed of only nuclear matter requires the change from nuclear to quark matter to be a strong first-order phase transition, somewhat resembling that of liquid water turning to ice.
This kind of rapid change in the properties of neutron-star matter has the potential to destabilize the star in such a way…
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