The existence of exotic quark matter inside neutron stars is proved.

The Finnish research team has found convincing evidence of the presence of exotic quark matter in the nuclei of the largest neutron stars from existing ones. The conclusion was made by combining the latest results of theoretical particle physics and nuclear physics with measurements of gravitational waves from collisions of neutron stars. About it writes Nature Physics.

All normal matter surrounding us consists of atoms, whose dense nuclei, consisting of protons and neutrons, are surrounded by negatively charged electrons. However, it is known that inside the so-called neutron stars, atomic matter collapses into extremely dense nuclear matter, in which neutrons and protons are packed so tightly that the whole star can be considered one huge nucleus.

Until now, it was unclear whether nuclear matter is inside the nuclei of the most massive neutron stars and whether it goes into a more exotic state called quark matter, in which the nuclei themselves no longer exist.

“Confirming the existence of quark nuclei inside neutron stars has been one of the most important goals of neutron star physics ever since it was first used about 40 years ago”.

Associate Professor Alexi Vuorinen, Department of Physics, University of Helsinki.

Even when conducting large-scale simulations on supercomputers, unable to determine the fate of nuclear matter inside neutron stars, the Finnish research team has proposed a new approach to the problem. They realized that by combining the latest results of theoretical particle physics and nuclear physics with astrophysical measurements, it would be possible to determine the characteristics and identity of matter inside neutron stars.

According to the study, the matter located in the nuclei of the most massive stable neutron stars has much greater similarity with quark matter than with ordinary nuclear matter. Calculations show that in these stars the diameter of the nucleus identified as quark matter can exceed half the diameter of the entire neutron star. However, researchers say there are still many uncertainties associated with the exact structure of neutron stars.

“There is still a small, but non-zero probability that all neutron stars are composed of the same nuclear matter. However, we were able to quantify what would be required for this scenario. In short, then the behavior of dense nuclear matter would be truly peculiar. For example, the speed of sound should reach almost the speed of light”.

Associate Professor Alexi Vuorinen, Department of Physics, University of Helsinki.

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

In the fall of 2017, the LIGO and Virgo observatories first discovered gravitational waves generated by two merging neutron stars. This observation established a strict upper limit for a quantity called tidal deformability, which measures the susceptibility of the structure of an orbiting star to its satellite’s gravitational field. This result was subsequently used to obtain the upper limit for the radii of colliding neutron stars, which turned out to be about 13 km.

Similarly, although the first observation of a neutron star dates back to 1967, accurate measurements of the mass of these stars were only possible for the past 20 years or so. Most stars with precisely known masses fall into the window from 1 to 1.7 stellar masses, but in the last decade, three stars have been discovered that have reached or possibly even slightly exceeded the limit of two solar masses.

Since the fall of 2017, a number of new mergers of neutron stars have been observed, and LIGO and Virgo quickly became an integral part of neutron star research. It is this rapid accumulation of new observational information that plays a key role in increasing the accuracy of the new Finnish research group results and in confirming the existence of quark matter inside neutron stars. With further observations expected in the near future, the uncertainties associated with the new results will also automatically decrease.