Strongly Interacting Dark Matter admixed Neutron Stars

Dark matter may accumulate in neutron stars given its gravitational interaction and abundance. We investigate the influence of strongly-interacting dark matter, described by a QCD-like one-flavor $G_2$ gauge theory, on neutron stars. This choice allows to test, for the first time, a first-principles-determined non-Abelian dark matter equation of state, which supports composite fermionic dark matter and thus a Fermi-pressure-stabilized dark matter component. The ordinary matter part of the mixed star is described by available model-agnostic equations of state that interpolate between the low-density regime and high-density regime. We find that strongly-interacting dark matter has a similar impact on neutron stars as other model equation of states and confirm that strongly-interacting dark matter can be accommodated by constraints from neutron star observations within our uncertainties.

💡 Research Summary

This paper investigates the impact of a strongly interacting dark‑matter component on neutron‑star structure by employing a first‑principles equation of state (EoS) for the dark sector. The dark sector is modeled as a one‑flavor G₂‑QCD gauge theory, an exceptional Lie‑group analogue of QCD that can be simulated on the lattice without a sign problem. In this theory the lightest stable particle is a three‑dark‑quark fermionic baryon, providing a fermionic dark‑matter candidate whose pressure is supported by Fermi degeneracy. Lattice simulations supply a set of discrete pressure–energy‑density points for this dark‑matter EoS; the authors interpolate these points with polytropes while enforcing causality (sound speed < c).

For the ordinary (baryonic) component they adopt three model‑agnostic EoS (labeled I, II, III) from recent literature, which interpolate between low‑density nuclear chiral perturbation theory and high‑density perturbative QCD, respecting the speed‑of‑sound bound and extending to zero density with a Γ = 5/3 polytrope to mimic a free Fermi gas in the crust.

The two fluids are coupled through the Tolman–Oppenheimer–Volkoff (TOV) equations generalized to a two‑fluid system. Each fluid’s pressure and energy density follow its own EoS, while gravity couples them via the total stress‑energy tensor. Stability is assessed by the usual dM/dp_c = 0 criterion for the mass‑radius (M‑R) curve, and by the tidal deformability Λ, which is compared to the LIGO/Virgo measurement from GW170817 (Λ≈190–580). Direct interactions between dark and ordinary matter are neglected, consistent with the expectation that portal couplings are highly suppressed in strongly interacting dark‑matter models.

The authors explore a range of dark‑matter masses (equal to the neutron mass, 2 m_n, 5 m_n) and dark‑matter fractions (1 %–10 % of the total stellar mass). Key findings include:

1. With a dark‑matter mass comparable to the neutron, even a 10 % dark‑matter fraction produces only modest shifts in the M‑R relation; the maximum mass remains above the observed ≈2 M⊙ limit for all three ordinary‑matter EoS.

2. Increasing the dark‑matter particle mass reduces the maximum mass for a given fraction, but the softest ordinary‑matter EoS (I) still yields stable configurations exceeding 2 M⊙ when the dark‑matter fraction is ≤5 %.

3. The tidal deformability Λ decreases as the dark‑matter content grows, moving the predicted Λ values deeper into the GW170817 allowed band. This demonstrates that current gravitational‑wave constraints do not rule out a sizable strongly interacting dark‑matter component.

The study therefore shows that a G₂‑QCD‑based dark‑matter EoS, derived from lattice QCD, can be consistently incorporated into neutron‑star models without violating existing astrophysical observations. The work highlights the importance of using first‑principles dark‑sector physics to reduce uncertainties in mixed‑star calculations. It also points to future extensions: inclusion of portal interactions, a more realistic crust EoS, and exploration of multi‑flavor G₂‑QCD or other QCD‑like hidden sectors. Overall, the paper provides a proof‑of‑principle that strongly interacting, fermionic dark matter can coexist with ordinary nuclear matter inside neutron stars while remaining compatible with mass, radius, and tidal‑deformability measurements.