A neutron star is the ultra-dense remnant of a supernova: the mass of the Sun packed into a sphere the size of a city. For billions of years, it travels through space, gathering around itself invisible matter, dark matter, which emits no light but pulls with gravity. A new model shows that if enough dark matter accumulates, it can envelop the star in an invisible shell from which even light cannot escape.
Here, things are different. Dark matter is distributed around the star according to the Einasto profile — a model proposed by Estonian astronomer Jaan Einasto: density falls off smoothly from the center, like a mist dispersing from a streetlamp. When the halo density reaches a critical value, a horizon is born at a distance of several stellar radii. But inside, there is no abyss, just an ordinary neutron star, dense but finite.
Previously, it was unclear whether a neutron star could exist inside a horizon formed by dark matter, and how the halo would affect its structure. Now it has been shown that for certain halo parameters, a horizon forms around the star, and the star remains regular inside it. This works for different equations of state — relations between pressure and density of the stellar matter. The scientists solved modified Tolman–Oppenheimer–Volkoff equations, adding the contribution of dark matter, and found a parameter region where the function $g_{rr}^{-1}$ changes sign outside the star's surface: this is the birth of the horizon.
The discovery provides a new theoretical tool for studying the interiors of black holes. It shows that an event horizon does not necessarily mean the end of structure: inside, a regular configuration of ordinary matter can exist. This guides the search for observational differences between such objects and ordinary black holes — for example, in gravitational waves or the radiation from accretion disks. No applications are visible yet, but the result is a concrete, computable example that can be tested.
A pearl in a dark matter shell is not just a metaphor but a possible reality. A neutron star wrapped in an invisible halo becomes a black hole without losing its structure. The shell hides it from the outside world, but inside, a world of dense matter continues to exist.
The dark matter density required to form the horizon reaches $10^{19}$ kg/m³ — billions of times denser than the center of the Sun and comparable to the density of an atomic nucleus.
The idea of massive objects inside black holes echoes Dan Simmons' novel 'Hyperion', where structures inside a singularity influence the outside world.