A theoretical model suggests that a black hole could contain a structured neutron star at its center, with the event horizon forming around the star rather than the star collapsing into a singularity.
The idea comes from Chen Tan and Yong-Qiang Wang of Lanzhou University, who explored what happens when a neutron star is placed inside a particular kind of dark matter halo. Their work describes a possible configuration in which a black hole can have an interior that remains regular rather than ending in a singularity.
The model depends on an unusual form of dark matter that can produce what are known as regular black holes. Unlike the standard picture, these objects do not require a singularity at their centers. Tan and Wang examined whether an ordinary neutron star could exist within such an environment.
The model allows three different outcomes
Using standard equations for stellar structure, the researchers found that the result depends on the density of the dark matter halo.
At lower densities, the neutron star remains intact. It is surrounded by a cloud of dark matter and becomes slightly squashed.
At sufficiently high densities, however, the configuration collapses and no stable solution exists.
Between those two regimes, the calculations produce a very different structure.
A shell develops outside the neutron star, and the geometry in this region takes on the form associated with the interior of a black hole. In one model, the shell extends from roughly 10 to 12 kilometers, or about 6.2 to 7.5 miles, while the neutron star itself has a surface radius of 8.5 kilometers, or about 5.3 miles.
An event horizon therefore surrounds the star.
But the star itself does not become a singularity.
The horizon forms around an intact star
This is the central feature of the proposed configuration. The neutron star remains structured and nonsingular inside the region bounded by the horizon.
The horizon is produced by the combined gravitational effect of the neutron star and the surrounding dark matter. In the model, the dark matter contributes roughly as much mass as the neutron star itself.
The researchers also tested the configuration using two different descriptions of neutron star matter and obtained the same general result. This indicates that the unusual solution does not depend on just one particular choice for describing the star’s matter.
The picture is therefore different from a scenario in which a star collapses and creates a conventional black hole. Here, the calculations allow the horizon to appear around an already existing neutron star while the star remains regular inside it.
The required dark matter is extremely dense
There is a major limitation to the model.
The dark matter densities needed to produce this configuration are far greater than densities astronomers actually expect dark matter to reach. That makes the proposed object a mathematical possibility rather than a prediction that such objects currently exist in the universe.
The model does, however, provide a solution in which the interior of a black hole is not necessarily a singular point where the equations cease to give a meaningful description. Instead, under the conditions explored by Tan and Wang, the region inside the horizon can contain a structured neutron star.
Whether anything resembling this theoretical configuration exists in nature remains unresolved. Detecting such an object would be required to establish that these unusual solutions correspond to real astronomical objects.
The study was published on the arXiv preprint server.
