As dense as possible: new model for matter in neutron star collisions

With the exception of black holes, neutron stars are the densest objects in the universe. As their name suggests, neutron stars are mostly made up of neutrons. However, our knowledge of the material produced when two neutron stars collide is still limited. Scientists from Goethe University Frankfurt and the Asia-Pacific Center for Theoretical Physics in Pohang have developed a model that provides insight into matter under such extreme conditions.

After a massive star burns up its fuel and explodes into a supernova, an extremely compact object, called a neutron star, may form. Neutron stars are extraordinarily dense: to achieve the density inside them, one would have to reduce a massive body like our sun to the size of a city like Frankfurt. In 2017, gravitational waves, the small ripples in spacetime that occur when two neutron stars collide, 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. The question remains open, for example, if quarks, otherwise trapped in neutrons, can appear in free form after the collision. Dr. Christian Ecker from the Institute for Theoretical Physics at 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 take a step closer to answering this question.

In their work published in Physical examination X, they extend nuclear physics models, which are not applicable to high densities, with a method used in string theory to describe the transition to dense, 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 matter 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 of these collisions and show that hot and cold quark matter can be produced,” adds Dr Ecker, who implemented these collisions. simulations in collaboration with Samuel Tootle and Konrad Topolski from the working group of Prof. Luciano Rezzolla at Goethe University Frankfurt.

Next, the researchers hope to compare their simulations with future gravitational waves measured from space to better understand quark matter in neutron star collisions.