Dark matter accumulation or effects within neutron stars, detectable through gravitational waves or other signatures.Neutron stars are the collapsed cores of massive stars, packing more mass than the Sun into a sphere the size of a city. Their extreme density and strong gravity make them natural gravitational traps for dark matter particles, which might accumulate in their interiors over cosmic time. This accumulation could subtly alter the star's properties — its temperature, rotation, or structure — and even trigger collapse if enough dark matter gathers in the core. Because neutron stars are so compact, they amplify the effects of even a small amount of captured dark matter, making them sensitive probes of a substance that otherwise passes through ordinary matter almost unnoticed.
The idea that compact stars could capture dark matter dates back to the 1980s, with early work by Press and Spergel on capture in the Sun. The specific application to neutron stars as dark-matter detectors gained traction in the 1990s and 2000s, particularly with the realization that gravitational-wave observatories could probe the resulting black-hole formation.
Dark matter particles passing through a neutron star can lose energy through scattering with nucleons and become gravitationally bound. Over millions of years, repeated interactions cause them to sink toward the center, forming a dense dark-matter core. If the particles are self-annihilating, this core heats the star, potentially observable as an anomalous surface temperature. If they do not annihilate, the core may grow until it exceeds a critical mass and collapses into a small black hole, which would then swallow the neutron star from inside — a process that could be detected through gravitational waves or the sudden disappearance of the star.
Neutron stars serve as natural dark-matter detectors: by monitoring their temperatures, rotation rates, and gravitational-wave emissions, astronomers can constrain dark-matter properties such as its scattering cross-section and self-annihilation rate.
If dark matter accumulates enough in a neutron star, it can trigger a collapse into a black hole, and the resulting gravitational-wave signal would be a unique fingerprint of dark matter's particle nature.
| C | variable | capture rate of dark matter particles by the star — capture rate per second |
| R | variable | radius of the star — radius metre |
| ρ_χ | variable | mass density of dark matter in the local environment — mass density kilogram per cubic metre |
| m_χ | variable | mass of a single dark matter particle — mass kilogram |
| v | variable | typical relative speed of dark matter particles with respect to the star — velocity metre per second |
| σ | variable | scattering cross-section between dark matter and stellar nucleons — cross section square metre |
| v_min | variable | Minimum dark matter speed — velocity metre per second |
| m_N | variable | Mass of the target nucleus — mass kilogram |
| E_R | variable | Recoil energy of the nucleus — energy joule |
| μ | variable | Reduced mass of the dark matter-nucleus system — reduced mass kilogram |
| δ | variable | Mass splitting between dark matter states (energy gap) — energy joule |
| m_crit | variable | critical dark matter mass — mass kilogram |
| k_B | variable | Boltzmann constant — boltzmann constant joule per kelvin |
| T | variable | temperature — temperature kelvin |
| v | variable | velocity — velocity metre per second |
| Φ_X | variable | dark matter particle flux — particle flux per square metre per second |
| f_X | variable | fraction of dark matter in the form of particle X — dimensionless |
| ρ_DM | variable | dark matter mass density — mass density kilogram per cubic metre |
| m_X | variable | mass of the dark matter particle — mass kilogram |
| v_DM | variable | typical speed of dark matter particles — velocity metre per second |
