Posted on February 15, 2019
Quarks, the smallest building blocks of matter, are never found 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, a fancy term, to describe a holistic change in the overall arrangement of the system structure and again its function.
Such a phase transition is in principle When merging neutron stars a very massive metastable object is possible whose densities exceed those of atomic nuclei and whose temperatures are 10,000 times higher than in the solar nucleus.
The option of measuring the gravitational waves of two confluent neutron stars has led us to answer some 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 latest 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.
Simulation of merging 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. (C. Breu, L. Rezzolla)
The measurement of gravitational waves emitted by the fusion of neutron stars could serve as carriers for possible phase transitions serve 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.
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If a phase transition occurs after the actual fusion, small amounts of quarks will gradually appear throughout the object. "Using the Einstein equations, we were able to show for the first time that this subtle change in structure causes a deviation in the gravitational wave signal until the newly formed massive neutron star collapses to a black under its own weight," explains Luciano Rezzolla, professor of theoretical astrophysics the Goethe University.
In the computer models of Dr. med. Andreas Bauswein from the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, a phase transition takes place immediately after the fusion – a core of quark matter forms inside the central object.
"We were able to show that the frequency of the gravitational wave signal shifts significantly in this case," says Bauswein. "Thus, in the future we have identified a measurable criterion for a phase transition in gravitational waves of neutron star fusions."
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Not all details of the gravitational wave signal with current detectors are measurable yet. 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 offered by two experiments: By collision with heavy ions in The existing HADES setup at GSI and the future CBM detector at GSI's Antiproton and Ion Research Facility under construction (FAIR) will produce compressed nuclear matter. 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.
The Daily Galaxy over the Helmholtz Association
Credit: Merged neutron stars Caltech