Invisible Filling: How Dark Matter Transforms Neutron Stars

Original: "Neutron stars with an agnostic Dark sector: Core and Halo configurations from a two-fluid approach"

· Asit karan, Asim Kumar Saha, Tuhin Malik, Constança Providência, Ritam Mallick

arXiv:2607.03840v1 · 2026-07-04 · CC BY 4.0 · ⏱ 2 min · High Energy HEP Phenomenology

Dark matter can turn neutron stars into dense cores or vast halos, offering a new way to detect it.

Abstract

Dark matter might lurk inside neutron stars. A new flexible model shows: light dark matter makes the star puffier, heavy dark matter makes it denser. Current data cap dark matter's share at about 10% if it's light. Picture two neutron stars with the same mass but one is much squishier—that could betray hidden dark matter.

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When a massive star explodes as a supernova, a super-dense sphere remains—a neutron star. It's the size of a city but weighs more than the Sun. If the remnant were just a bit heavier, it would become a black hole. Around such stars, mysterious dark matter can gather—invisible substance that, according to calculations by astronomer Vera Rubin, emerged after the Big Bang and fills the cosmos.

Imagine a snowball rolling through different powders. Light powder clings to the outside, making it fluffy and large. Heavy metal balls, on the other hand, sink to the center, forming a hard core. That's roughly how dark matter behaves inside a neutron star. Lightweight particles create a diffuse halo, puffing up the star, while heavier ones accumulate into a tiny core, squeezing it.

Some neutron stars spin and pulse, they're called pulsars. They were discovered by Jocelyn Bell Burnell. Their precise signals, traveling to us at the speed of light, help detect the slightest changes in the star's size.

Scientists have studied how these transformations affect observations. A puffed-up halo makes the star more pliable—it compresses and stretches more in a neighbor's gravitational field. This is measured via gravitational waves—ripples in space that were first detected by instruments created by Rainer Weiss. However, a dense dark core shrinks the star, which can be observed in X-rays using spectroscopy (analysis of radiation). It turns out that for light dark matter, its fraction cannot exceed about 11%, otherwise the star would become too large. Heavier particles are allowed in greater amounts.

If in the future we spot two neutron stars with the same mass but behaving differently during a collision—one stretching a lot and the other barely changing—this would indicate that a dark core is hiding inside one of them.

This approach doesn't require precise knowledge of dark matter's nature—it works for almost any theory beyond the Standard Model of physics. Therefore, neutron star observations become a universal detector of invisible matter.

🎯 A teaspoon of neutron star material would weigh about a billion tons—roughly as much as Mount Everest.

\frac{dP_i}{dr} = -\frac{(P_i+\varepsilon_i)(m+4\pi r^3(P_{\text{NM}}+P_{\text{DM}}))}{r(r-2m)}

describes the force balance in a neutron star with dark matter, where each component (nuclear matter and dark matter) has its own pressure P_i and energy density ε_i

\Lambda = \frac{2}{3}k_2\left(\frac{R}{M}\right)^5

characterizes the star's response to an external gravitational tidal field; depends on the Love number k₂, radius R, and mass M

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Invisible Filling: How Dark Matter Transforms Neutron Stars