Researchers A. C. Khunt, K. Yavuz Ekşi, and P. C. Vinodkumar, affiliated with various institutions including the University of Tübingen and the University of Maryland, have recently published a study in the journal Physical Review D, exploring the impact of pressure anisotropy on the structure and geometry of neutron stars within the framework of general relativity. Their work focuses primarily on the Bowers-Liang (BL) model, a phenomenological approach to describing anisotropic stresses in neutron stars, and compares these findings with a quasi-local prescription.
Neutron stars, the ultra-dense remnants of massive stars that have exploded as supernovae, are natural laboratories for studying the behavior of matter under extreme conditions. In this study, the researchers used the SLy equation of state, a theoretical model that describes the properties of neutron star matter, to explore how anisotropic stresses—pressures that differ in different directions—modify global observables such as the mass-radius relation, moment of inertia, compactness, and tidal deformability. They found that moderate positive anisotropy can increase the maximum supported mass of a neutron star up to approximately 2.4 times the mass of the Sun and enhance stellar compactness by up to 20% relative to isotropic configurations, which assume uniform pressure in all directions. Importantly, these findings remain broadly consistent with current observational constraints from the Neutron star Interior Composition Explorer (NICER) and gravitational-wave observations.
To probe the internal gravitational field of neutron stars, the researchers computed various curvature invariants, including the Ricci scalar, the Ricci tensor contraction, the Kretschmann scalar, and the Weyl scalar. These invariants provide insights into the geometric properties of spacetime within and around the neutron star. The study showed that curvature measures directly tied to the matter distribution exhibit a strong sensitivity to anisotropy, while the Weyl curvature, which reflects the free gravitational field, remains comparatively insensitive. Within the BL framework, the maximum compactness of neutron stars increases with anisotropy and can reach values as high as 0.25 to 0.38 for a range of anisotropy parameters. However, the physical realizability of such highly compact configurations depends sensitively on the underlying anisotropy mechanism.
The comparison with the quasi-local model highlights the strong model dependence of anisotropic effects, underscoring both the potential significance and the limitations of phenomenological anisotropy prescriptions in modeling the strong-field interiors of neutron stars. This research contributes to our understanding of neutron star structure and the role of anisotropic stresses in these extreme environments, with implications for interpreting observational data and developing more accurate theoretical models.
Source: Physical Review D
This article is based on research available at arXiv.