Studying neutron stars is a challenge for modern science. The nearest neutron star is 400 light-years away. With current technology, sending a probe to it would take hundreds of thousands of years. Telescopes cannot study these stars in detail because they are the size of a small city and appear only as bright dots in the sky. Laboratory experiments are also powerless: the density of neutron stars is several times that of atomic nuclei, making it impossible to reproduce.
The quantum chromodynamics (QCD) equations that describe the internal processes of neutron stars cannot be solved by standard methods due to extreme complexity. However, Ryan Abbott’s team at the Massachusetts Institute of Technology has made a significant breakthrough in understanding these objects. They set new constraints on the properties of the interior of neutron stars, in particular showing that the speed of sound inside these stars may be higher than expected. This means that neutron stars may have more mass than previously thought.
Why is it so hard to study neutron stars?
The internal properties of a neutron star – pressure and density – depend on the QCD equations. The problem is that standard methods, in particular perturbation theory, work only under certain conditions: in the outer atmosphere, where the density is low, and in the core of the most massive stars, where the binding parameter is small. But in the main part of a neutron star, these methods do not work.
Physicists use a numerical method, the lattice QCD, which considers the interaction of quarks and gluons in discretized spacetime. At low densities this method gives accurate results, but at densities characteristic of neutron stars it becomes unusable. To overcome this problem, the researchers applied the concept of isospin density – the difference between the number of protons and neutrons. Previous studies have shown that isospin density provides useful constraints on the pressure of nuclear matter.
Breakthrough in research
Abbott and his team performed large-scale calculations using supercomputers, modeling matter with a non-zero isospin density. They extrapolated the results to the “continuum boundary”, that is, to the vanishingly small distance between the lattices. This has provided new data on the properties of extreme density matter.
In particular, the researchers found that matter with high isospin density is superconducting and determined its superconducting gap, a parameter that characterizes the energy state of the system. They also found that the speed of sound in such matter exceeds the conformal limit, but remains below the newly proposed speed limit. This has implications for determining the maximum mass of neutron stars before they collapse into black holes.
Future prospects
The results open new possibilities for the study of neutron stars. They place constraints on the properties of matter within these objects, which is important for testing models and theories. For example, scientists can now estimate the viscosity and conductivity of matter, which affects the spin decay and cooling of stars.
Lattice QCD allows predictions to be made for astrophysical observations. In the future, this approach may become key to unlocking the mysteries of neutron stars and their role in the evolution of the Universe.
We previously reported on how the heaviest neutron star in the Universe was found.
According to physics.aps.org
