Recent research into neutron star mergers is shedding new light on one of the most elusive states of matter in the universe: quark matter. Neutron stars, the dense remnants of exploded massive stars, occasionally collide in violent events known as binary mergers. These mergers produce gravitational waves detectable on Earth and create conditions so extreme that they may allow quark matter, the building blocks of protons and neutrons, to form and flow freely.
When neutron stars merge, the intense gravitational forces rapidly distort their shapes and raise their internal temperatures, potentially transforming their internal structure. During these mergers, quark matter may form—an exotic state where quarks and gluons, usually confined within protons and neutrons, are liberated and able to move independently. This phenomenon, though theoretical, has been a focus of astrophysical research for years, and now, new studies are beginning to confirm these theories.
Scientists from the University of Helsinki and their colleagues have advanced our understanding of neutron star mergers by investigating a critical property known as bulk viscosity. Bulk viscosity measures how a system’s internal particle interactions resist changes in flow—essentially describing how “sticky” the system is during oscillations. This property becomes crucial in neutron star mergers, where radial oscillations occur, and can influence the dynamics of the collision.
The research, published in Physical Review Letters, combined two advanced theoretical approaches—string theory-based holography and perturbation theory. Holography, in this case, uses a higher-dimensional curved space to describe the behavior of quantum chromodynamics (QCD), the theory of strong interactions at the heart of neutron stars. Perturbation theory, widely used in particle physics, calculates physical quantities in power series, though it is only applicable at very high densities. By integrating both methods, the researchers were able to estimate the bulk viscosity of quark matter, finding that it peaks at much lower temperatures than previously thought.
If future observations confirm the presence of quark matter during mergers, this could fundamentally change our understanding of the phases of matter in the universe. For example, gravitational wave data from observatories like LIGO and Virgo could be analyzed for signs of viscous effects, potentially indicating the presence of quark matter. The absence or presence of such effects would provide critical insights into the creation and properties of quark matter during these extreme events.
