Neutron stars, remnants of old stars that have exhausted their nuclear fuel and collapsed after a supernova explosion, are incredibly dense. When these stars collide in what are known as binary mergers, they cause ripples in spacetime, generating gravitational waves detectable on Earth, even from hundreds of millions of light years away.

These mergers result in rapid changes in the shape and temperature of the stars, potentially leading to the formation of quark matter. In this state, elementary particles like quarks and gluons, typically confined within protons and neutrons, are liberated and move freely.

Professor Aleksi Vuorinen from the University of Helsinki highlighted that while our understanding of individual neutron stars has significantly advanced, there are still major gaps in our knowledge about the behavior of matter at the highest densities or in dynamic settings.

"Describing neutron-star mergers is particularly challenging for theorists because all conventional theoretical tools seem to break down in one way or another in these time-dependent and truly extreme systems," Vuorinen explained.

Determining Bulk Viscosity Through String Theory and Perturbative QCD
A key concept in studying neutron-star mergers is the bulk viscosity of neutron-star matter, which reflects the resistance of particle interactions to flow in these systems. Researchers from the University of Helsinki, in collaboration with international colleagues, have successfully determined the bulk viscosity of dense quark matter by combining two distinct theoretical approaches: one based on string theory and the other on perturbation theory, a traditional method in quantum field theory.

Viscosity, in general, measures the "stickiness" of a fluid's flow. For example, honey flows slowly due to its high viscosity, while water flows more quickly because it has a lower viscosity. Bulk viscosity, specifically, is related to energy loss in systems undergoing radial oscillations, where density periodically increases and decreases - an effect seen in neutron stars and their mergers, making it a crucial factor in understanding these events.

In a recent study published in 'Physical Review Letters', the bulk viscosity of quark matter was determined using both the AdS/CFT duality, often referred to as holography, and perturbation theory.

Holography involves studying gravity in a higher-dimensional curved space to determine the properties of strongly coupled quantum field theories. This method, while not directly applicable to QCD (quantum chromodynamics - the theory of the strong nuclear force), allows researchers to model similar conditions found in quark matter during neutron-star collisions, where QCD interactions are intense. However, due to technical limitations, the method examines a phenomenological model with properties akin to QCD rather than QCD itself.

Perturbation theory, the other method used, is widely applied in theoretical particle physics and calculates physical quantities as power series in the coupling constant, which describes interaction strength. This approach can directly describe QCD but is only applicable at much higher densities than those present in neutron stars.

The researchers were pleased to find that both methods produced similar results, suggesting that in quark matter, bulk viscosity reaches its peak at much lower temperatures than in nuclear matter.

"This information helps us understand the behavior of neutron-star matter during their binary mergers," stated Academy Research Fellow Risto Paatelainen from Helsinki.

"These results may also aid the interpretation of future observations. We might, for example, look for viscous effects in future gravitational-wave data, and their absence could reveal the creation of quark matter in neutron-star mergers," added University Lecturer Niko Jokela.

The research was part of an international collaboration with significant Finnish involvement, including contributions from Professor Aleksi Kurkela of the University of Stavanger, Group Leader Matti Jarvinen of the Asia Pacific Center for Theoretical Physics in South Korea, and Postdoctoral Researcher Saga Sappi of the Technical University of Munich.

Research Report:Estimate for the Bulk Viscosity of Strongly Coupled Quark Matter Using Perturbative QCD and Holography

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