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Illustration of neutron star layers against nebulae.

A new type of matter discovered inside neutron stars

A group of researchers including Aleksi Kurkela from the University of Stavanger and CERN has found strong evidence for the presence of exotic quark matter inside the cores of the largest neutron stars in existence.

This conclusion was reached by combining recent results from theoretical particle and nuclear physics to measurements of gravitational waves from neutron star collisions.

All normal matter surrounding us is composed of atoms, whose dense nuclei, comprising protons and neutrons, are surrounded by negatively charged electrons. Inside so-called neutron stars, however, atomic matter is known to collapse into immensely dense nuclear matter, in which the neutrons and protons are packed together so tightly that the entire star can be considered one enormous nucleus.

Up until now, it has remained unclear whether inside the cores of the most massive neutron stars nuclear matter collapses into an even more exotic state called quark matter, in which even the protons and neutrons themselves no longer exist. This new study from University of Stavanger (among others) now claims that the answer to this question is yes. The new results were recently published in the prestigious journal Nature Physics.

Aleksi Kurkela.

Important research goal

“Quark matter is really quite exotic. It is stuff that is made from the fundamental building blocks of protons and neutrons—elementary particles called quarks and gluons. We believe that the universe was filled with such quark matter shortly after the Big Bang and it can be recreated in particle colliders on Earth, such as the Large Hadron Collider at CERN. Answering the question of whether or not quark matter also exists 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 Kurkela.

Cosmology scientist

An associate professor at the University of Stavanger, Kurkela has been doing research at CERN for the last five years, and will be returning to his post at the University of Stavanger in August. An award-winning theoretical physicist (link to article in Norwegian), Kurkela is part of the Cosmology research group in the Department of Mathematics and Physics at UiS.

The recent study was done by an international the research group including himself, Eemeli Annala and Aleksi Vuorinen from University of Helsinki, Tyler Gorda from the University of Virginia, and Joonas Nättilä from Columbia University.

Existence very likely

With even large-scale simulations run on supercomputers unable to determine the fate of nuclear matter inside neutron stars, the research group proposed a new approach to the problem. They realised that by combining recent findings from theoretical particle and nuclear physics with astrophysical measurements, it might be possible to deduce the characteristics and identity of matter inside neutron stars.

According to the study, matter residing inside the cores of the most massive stable neutron stars bears a much closer resemblance to quark matter than to ordinary nuclear matter. The calculations indicate that in these stars the diameter of the core identified as quark matter can exceed half of that of the entire neutron star.

Detective work

“We cannot just peek into the cores. Instead we have to do a bit more detective work. It turns out that some of the properties we can observe, such as radii or masses of the stars, can inform us about the material properties of the stuff the stars is made of, and there has been an explosion of new data available in this field. The most notable example is the detection of gravitational waves originating from a collision of two neutron stars by the LIGO/Virgo collaboration“ Kurkela explains.

For example, if the matter is soft the gravity can squeeze the star to have a small radius. A star with small radius can easily collapse to a black hole so it is difficult to build very massive star from soft matter. A very compact object is also difficult to deform by tidal forces (like the moon deforms the oceans of the Earth), a property that was constrained by the data from the collision.

“Now, quark matter is soft —at least if you compare it to nuclear matter. Our results show a rapid softening of the matter in cores of large stars suggesting that hidden under the nuclear-matter curst there are cores made of quark matter,“ he says.

New data is coming

However, Kurkela points out that there are still many uncertainties associated with the exact structure of neutron stars. “Our analysis does not completely rule out the existence of massive stars with neutron cores but it demonstrates that quark-matter cores are not just a theoretical daydream but are quite likely to appear in nature,” says Kurkela.

“We can’t wait to incorporate new neutron-star data into our analysis and see how they will affect this conclusion.”

And new data is coming. Since autumn 2017, a number of new neutron star mergers have been observed, and LIGO and Virgo have quickly become an integral part of neutron star research. With further observations expected in the near future, the research group expects that their conclusions will be strengthened.

“We live in extraordinary times. With particle colliders on Earth and neutron stars in the heavens we have an unprecedented window into the fundamental building blocks of nature,” Kurkela rejoices.


Annala, Eemeli et al, Evidence for quark-matter cores in massive neutron stars, Nature Physics, June 2020

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