Researchers have explored a theoretical scenario where a neutron star could persist as a regular configuration within the event horizon of a black hole. This study considers neutron stars with an anisotropic dark matter halo, described by an Einasto density profile. This dark matter model has previously been shown to generate regular, singularity-free black hole solutions, providing a framework for this new hypothesis.

To reach this conclusion, the team solved the modified Tolman-Oppenheimer-Volkoff (TOV) equations, which describe the structure of neutron stars in hydrostatic equilibrium. They used two different equations of state for nuclear matter (BSk19 and SLy4), allowing them to verify the robustness of their findings. The results indicate that the presence of the dark matter halo significantly alters the internal structure of the neutron star.

Most notably, for a specific range of dark matter halo parameters, the $g_{rr}^{-1}$ component of the metric tensor changes sign outside the stellar surface. This change is the indicator of the formation of an event horizon, implying that the neutron star becomes enveloped by a black hole. This configuration, termed "neutron stars in black holes," appears with both equations of state, suggesting it does not depend on the specifics of nuclear matter. This finding opens a new perspective on the nature of what might reside inside black holes, offering a concrete and computable instance for future theoretical investigations.