Neutron stars more compact than black holes in quasitopological gravity: Equilibrium configurations and radial stability
Phys. Rev. D 114, 024049 – Published 21 July, 2026
DOI: https://doi.org/10.1103/zkbc-zwm7
Abstract
Within general relativity, black holes are widely regarded as the ultimate benchmark for compactness in the Universe. Recently, however, neutron star models have been constructed in a higher-curvature theory—quasitopological gravity (QTG)—whose compactness can exceed the black-hole limit [S. Li, H. Lü, Y. Gao, R. Xu, L. Shao, and H. Yu, companion Letter, Neutron stars more compact than black holes as a probe of strong-field gravity, Phys. Rev. D 114, L021504 (2026).]. Here we present a detailed analysis of both the equilibrium structure and radial stability of such configurations in QTG. By examining several representative equations of state and different values of the gravitational coupling constant, we find that in the high-central-density regime the compactness exceeding the black-hole bound exhibits a universal behavior in QTG. We further show that QTG corrections grow increasingly significant at large central densities and can stabilize configurations that are radially unstable in general relativity over a broad parameter range. These results establish ultracompact neutron stars in QTG as theoretically viable strong-field configurations and provide a foundation for further investigations of their dynamical and phenomenological implications.
