Neutron stars more compact than black holes as a probe of strong-field gravity

TL;DR

This paper demonstrates the theoretical possibility of stable, ultra-compact neutron star configurations exceeding black hole compactness within a modified gravity framework, with potential observable signatures like gravitational-wave echoes.

Contribution

It introduces stable, ultra-compact neutron star models in quasi-topological gravity, extending the understanding of strong-field gravity beyond standard general relativity.

Findings

Stable ultra-compact stars can exist in quasi-topological gravity.

These stars can be distinguished from black holes via gravitational-wave echoes.

The models are stable against radial perturbations, confirming their physical plausibility.

Abstract

Probing gravity in its strongest regime is a central goal of modern physics, as the nature of the most compact objects reflects fundamental aspects of Einstein's theory of general relativity (GR). In GR, black holes are regarded as the most compact objects in the Universe. Here, for the first time, we demonstrate that stable stellar configurations more compact than black holes can arise when neutron-star equations of state are embedded in quasi-topological gravity, a class of higher-curvature extensions of GR. We construct such ultra-compact stars, analyze their macroscopic properties, and establish their stability against radial perturbations, confirming their physical plausibility. We further identify potential observational signatures to distinguish these stars from black holes, most notably gravitational-wave echoes whose detectability could provide direct evidence of physics beyond Einstein's GR in the strong-field regime.