Universal relationships for neutron stars from perturbative approach

The universal relationships for compact stars have been investigated employing perturbative approach using canonical (APR) and Brussels–Montreal Skyrme (BSk22, BSk24, BSk26) equations of state describing hadronic matter of neutron stars. The neutron star matter has been considered to be $\beta$-equilibrated neutron–proton–electron–muon matter at the core with a rigid crust. The multipole moments of a slowly rotating neutron star characterize its external gravitational field. These variables are dependent on the interior structure of the neutron star described by the equation of state of the neutron star matter. The properties of neutron stars, such as the mass, the radius, the dimensionless moment of inertia, the compactness, the Love number, the dimensionless tidal deformability and the dimensionless quadrupole moment have been calculated and relations among these quantities have been explored. It is found that most of these relations do not depend sensitively on the details of the internal structure of neutron stars. Such universality implies that the measurement of a single quantity appearing in a universal relation would automatically provide information about the others, notwithstanding the fact that those may not be accessible observationally. Such relations can be utilized to assess the deformability of compact stars by measuring their moment of inertia, to evaluate spin in binary inspirals by resolving degeneracies in gravitational wave detection or to examine General Relativity in a manner that is not reliant on nuclear structure.