A neutron star is a city the size of Manhattan that weighs as much as half a million Earths. A teaspoon of its material would outweigh Mount Everest. Now, into this superdense sphere, something invisible and elusive has been mixed — dark matter particles.
That's exactly what physicists did in a new study. They took three different recipes (equations of state) for ordinary neutron star matter and added a bit of dark matter to each — light neutralinos that interact with ordinary matter via Higgs boson exchange. The amount of dark matter was set by a single parameter: its Fermi momentum (the maximum energy of the particles). The larger it is, the more dark matter in the star.
Then the scientists solved the equations linking pressure and density and obtained how the mass, radius, and tidal deformability of the star change. Tidal deformability shows how easily a star changes shape under the gravity of a neighbor, like a ball that squeezes in your hand.
The result was clear: dark matter softens the star. As its fraction increases, the maximum mass drops, the radius at 1.4 solar masses decreases, and the tidal deformability sharply declines. For example, for one of the recipes, at the maximum dark matter fraction, the deformability dropped 8 times more than the typical scatter from uncertainties in the composition of ordinary matter.
This means dark matter can be searched for simply by observing neutron stars. If the measured deformability turns out to be significantly lower than models without dark matter predict, that would be its signature. Moreover, the effect is so strong that it cannot be confused with ordinary uncertainties in our knowledge of stellar matter. Thus, neutron stars become a laboratory for searching the invisible.
as written by the authors arXiv:2607.17034
We investigate the structure of neutron stars (NSs) admixed with fermionic dark matter (DM) using three density-dependent relativistic mean-field (DDRMF) functionals (DDME, DDB, and GDFM) for $$\beta$$-equilibrated nucleonic matter. Modeling DM as the lightest neutralino interacting via Higgs exchange, we treat the DM Fermi momentum $$k_F^{\rm DM}$$ as a control parameter in the range $$0.02$$-$$0.06$$ GeV. We solve the coupled mean-field and Tolman-Oppenheimer-Volkoff equations to obtain the mass-radius relation, maximum mass $$M_{\rm max}$$, radial sound speed profile $$c_s^2$$, and tidal deformability $$\Lambda$$. In all models, DM softens the equation of state, systematically reducing $$M_{\rm max}$$, the radius $$R_{1.4}$$ (at $$1.4M_{\odot}$$), and the tidal deformability $$\Lambda_{1.4}$$ (at $$1.4M_{\odot}$$) as $$k_F^{\rm DM}$$ increases. Consequently, the $$2 M_\odot$$ pulsar limit and NICER data place a model-dependent upper limit on the DM content, while the GW170817 tidal bound requires a minimal DM content for the stiffest functional. Using a recent Bayesian inference of the DDRMF equation of state as the nucleonic reference band, we evaluate if this DM imprint can be distinguished from nucleonic uncertainties using only measureable quantities. Analyzing the tidal deformability $$\Lambda$$ and mass-radius slope $$dR/dM$$ at fixed mass, we find that $$\Lambda$$ is a sharp discriminator: at $$1.4 M_\odot$$ and $$k_F^{\rm DM}=0.06$$ GeV, the DM-induced reduction of $$\Lambda$$ reaches $$\simeq 8$$ times the nucleonic $$1\sigma$$ width (model-independently $$7.5$$-$$7.8\sigma$$). The DM track leaves the nucleonic $$1\sigma$$ band for $$k_F^{\rm DM}\gtrsim0.03$$-$$0.04$$ GeV, whereas $$dR/dM$$ becomes diagnostic only for the heavier ($$1.8$$-$$2.0 M_\odot$$) branch.
If a neutron star the size of a city were squeezed a bit more, it would turn into a black hole, and the dark matter inside would disappear behind the event horizon.