Quark Matter: Neutron Stars Reveal New Type of Matter Inside Them

Meet Quark Matter. Confirming the existence of quark cores inside neutron stars has been one of the most important goals of neutron star physics for the last 40 years.

A group of Finnish researchers has discovered what they believe is a new state of matter inside Neutron Stars, a new study has revealed.

A Finnish research group has found strong evidence for the presence of exotic quark matter within the nuclei of the largest neutron stars in existence. This conclusion was reached by experts after combining recent results from theoretical nuclear and theoretical particle physics with gravitational-wave measurements of neutron star collisions.

All normal matter around us is made up of atoms, the dense nuclei of which comprise protons and neutrons are surrounded by negatively charged electrons. However, within neutron stars, the atomic matter is known to collapse into the immensely dense nuclear matter in which neutrons and protons are so close together that the entire star can be considered a single huge nucleus.

Until now, it has not been clear whether the nuclear matter in the nuclei of the most massive neutron stars collapses into an even more exotic state called quark matter, in which the nuclei no longer exist.

Researchers from the University of Helsinki now claim that the answer to this question is yes. The new results were published in Nature Physics.

“Confirming the existence of quark cores inside neutron stars has been one of the most important goals of neutron star physics ever since this possibility was first entertained roughly 40 years ago,” says Associate Professor Aleksi Vuorinen from the University of Helsinki’s Department of Physics and Helsinki Institute of Physics.

Even with large-scale simulations performed on supercomputers unable to determine the fate of nuclear matter within neutron stars, the Finnish research group proposed a new approach to the problem.

They realized that by combining recent findings from theoretical particles and nuclear physics with astrophysical measurements, it might be possible to deduce the characteristics and identity of the matter that resides within neutron stars.

According to the study, the matter residing within the nuclei of the most massive stable neutron stars is much more like a quark matter than ordinary nuclear matter.

Calculations indicate that in these stars, the diameter of the nucleus identified as quark matter may exceed half the diameter of the entire neutron star.

However, Vuorinen notes that there are still many uncertainties associated with the exact structure of neutron stars. What does it mean to say that quark matter has almost certainly been discovered?

“There is still a small but nonzero chance that all neutron stars are composed of nuclear matter alone. What we have been able to do, however, is to quantify what this scenario would require. In short, the behavior of dense nuclear matter would then need to be truly peculiar. For instance, the speed of sound would need to reach almost that of light,” Vuorinen explains.

A key factor contributing to the new findings was the emergence of two recent results in observational astrophysics: the measurement of gravitational waves from a neutron star fusion and the detection of very massive neutron stars, with masses close to two solar masses.

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