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Merging neutron stars provides insight into the fundamental properties of matter



  Association of neutron stars

Simulation of fusing neutron stars, calculated with supercomputers. Different colors show the mass density and the temperature some time after the fusion and just before the object collapses into a black hole. Quarks are expected to form where temperature and density are higher.

The option to measure the gravitational waves of two merging neutron stars offered the opportunity to answer some of the fundamental questions about the structure of matter. At the extremely high temperatures and densities in fusion, scientists suspect a phase transition in which neutrons dissolve into their constituents: quarks and gluons. In the current issue of Physical Review Letters, two international research groups report their calculations on what the signature of such a phase transition would look like in a gravitational wave.

Quarks, the smallest building blocks of matter, never occur alone in nature. They are always tightly connected to the protons and neutrons. However, neutron stars, which weigh just like the sun, but only the size of a city like Frankfurt, have such a dense core that a transition from neutron matter to quark matter can take place. Physicists refer to this process as a phase transition, similar to the transition from liquid to vapor in water. Such a phase transition is particularly possible when neutron stars are combined to form a very massive metastable object with densities in excess of that of the atomic nuclei and at temperatures 10,000 times higher than in the solar nucleus.

Measurement of gravitational waves emitted neutron stars could serve as ambassadors for possible phase transitions in space. The phase transition should leave a characteristic signature in the gravitational wave signal. The research groups from Frankfurt, Darmstadt and Ohio (Goethe University / FIAS / GSI / Kent University) as well as from Darmstadt and Wroclaw (GSI / Wroclaw University) used modern supercomputers to calculate what this signature might look like. For this purpose, they used different theoretical models of phase transition.

If a phase transition occurs after the actual fusion, small amounts of quarks gradually appear throughout the object. "Using the Einstein equations, we were able to show for the first time that this slight change in structure causes a deviation in the gravitational wave signal until the newly formed massive neutron star breaks its own weight under its collapse to form a black hole [19659008]"explains Luciano Rezzolla, Professor of Theoretical Astrophysics at the Goethe University.

Computer models by Dr. Andreas Bauswein of the GSI Helmholtz Center for Heavy Ion Research in Darmstadt A phase transition takes place immediately after the fusion – inside "We were able to show that in this case the frequency of the gravitational wave signal shifts significantly," says Bauswein. "Therefore, in the future we will have a measurable criterion for a phase transition in gravitational waves of Neutron star fusion identified. "

N Not all details of the gravitational wave signal can be measured with current detectors. However, they are observable with both the next generation of detectors and with a fusion event that is relatively close to us. A complementary approach to answer quark matter questions is provided by two experiments: collision with heavy ions at the existing HADES facility at the GSI and the future CBM detector at the under construction Antiproton and Ion Research Facility (FAIR) at GSI produced compressed nuclear material. The collisions may produce temperatures and densities that are similar to those of a neutron star fusion. Both methods provide new insights into the occurrence of phase transitions in nuclear matter and thus their fundamental properties.

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