Neutron stars could be more compact than black holes express
In the standard picture, black holes are the undisputed density champions: all their matter is squeezed into a single point. Imagine the entire mass of Mount Everest packed into a grain of salt. But physicists have found a theoretical trick. If you add tiny corrections to the ordinary law of gravity, corrections that only activate at mind-boggling densities, then neutron stars can crunch down even further, snatching the record. These corrections, born from spacetime curvature, act like extra support: the star doesn’t collapse into a black hole but stays as a sphere, just compressed to the absolute limit.
Researchers tested different models, varying the composition and tweaking the theory’s parameters. It turns out the behavior of these stars is universal — what they’re made of doesn’t matter. They remain stable even where, according to Einstein, they’d be doomed to collapse. On top of that, these objects can churn out gravitational waves — ripples in spacetime picked up by detectors like LIGO. So, lurking in the depths of space, there might be “almost-black-holes,” and we can actually hear them.
Original abstract
as written by the authors arXiv:2605.19731
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 -- quasi-topological gravity (QTG) -- whose compactness can exceed the black-hole limit~ [S. Li, H. L\"u, 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 ultra-compact neutron stars in QTG as theoretically viable strong-field configurations and provide a foundation for further investigations of their dynamical and phenomenological implications.
If a sugar cube were made of neutron star stuff, it would weigh as much as Mount Everest.
In science fiction, super-dense “neutronium” is a favorite material for impenetrable armor, like in Star Trek.