As dense as it gets: New model for matter in neutron star collisions

With the exception of black holes, neutron stars are the most dense objects in the universe. As their name suggests, neutron stars are composed mostly of neutrons. However, our knowledge of the matter produced in the collision of two neutron stars is still limited. Scientists from Goethe University Frankfurt and the Asia-Pacific Center for Theoretical Physics in Pohang have developed a model that gives information about matter under such extreme conditions.

After a massive star has burned through its fuel and exploded as a supernova, an extremely compact object, called a neutron star, can form. Neutron stars are extremely dense: To reach the density inside them, one would need to compress a massive body like our sun into the size of a city like Frankfurt. In 2017, gravitational waves, the tiny ripples in spacetime produced during a collision between two neutron stars, could be measured directly here on earth for the first time.

However, the composition of the resulting hot and dense fusion product is not precisely known. It is still an open question, for example, whether quarks, which are otherwise trapped in neutrons, can appear in free form after the collision. Dr. Christian Ecker from the Institute for Theoretical Physics of the Goethe University in Frankfurt, Germany, and Dr. Matti Järvinen and Dr. Tuna Demircik from the Asia-Pacific Center for Theoretical Physics in Pohang, South Korea, developed a new model that allows them to get one step closer to answering this question.

In their paper published in Physical Review X, extend models from nuclear physics, which are not applicable to high densities, with a method used in string theory to describe the transition to dense and hot quark matter. “Our method uses a mathematical relationship found in string theory, namely the correspondence between five-dimensional black holes and strongly interacting matter, to describe the phase transition between dense nuclear and quark matter,” explain Dr. Demircik and Dr. Järvinen.

“We have already used the new model in computer simulations to calculate the gravitational wave signal from these collisions and show that both hot and cold quark matter can be produced,” adds Dr. Ecker, who carried out these simulations in collaboration with Samuel Tootle and Konrad Topolski from Professor Luciano Rezzolla’s working group at Goethe University Frankfurt.

The researchers then hope to be able to compare their simulations with future gravitational waves measured from space, in order to gain further insights into the quark matter in neutron star collisions.