Noninteracting Two-Fluid Models of Dark Matter-Admixed Strange Quark Stars Satisfy Tighter GW170817 Lambda Bounds Than Interacting One-Fluid Cases

Bordbar's team solved the Tolman-Oppenheimer-Volkoff equations for two equations of state: an interacting one-fluid model coupling strange quark matter and scalar dark matter via a Yukawa-like term, and a noninteracting two-fluid model with separate hydrostatic equilibria linked only by gravity. They scanned central densities for total masses, radii, and tidal deformabilities, comparing outputs against NICER pulsar data and the GW170817 event. Sample sizes were generated via numerical integration grids exceeding 10,000 configurations per model. The noninteracting case yielded lower Lambda values at fixed mass while preserving M_TOV above 2 solar masses.

The interacting one-fluid approach allows energy transfer that stiffens the effective equation of state at intermediate densities, pushing Lambda higher and conflicting with the upper bound Lambda(1.4) less than or equal to 580 from GW170817. In contrast, the two-fluid separation keeps dark matter pressure support decoupled, reducing overall compactness and tidal response. This distinction matters because future detectors like the Einstein Telescope target Lambda precision below 100, where model choice becomes observationally decisive rather than theoretical preference.

High-density matter remains poorly constrained; strange quark stars offer one candidate for the 2-solar-mass pulsars without invoking hyperons or quark deconfinement at lower densities. Adding dark matter admixture tests whether microscopic interactions or purely gravitational coupling dominate in compact objects, with direct bearing on whether dark matter can accumulate inside neutron-star remnants. The preprint correctly flags that two-fluid noninteracting solutions remain viable under tighter future bounds, but both models assume zero-temperature beta equilibrium and neglect rotation or magnetic fields.

Next-generation gravitational-wave runs and NICER phase-two radius measurements will discriminate the models by tightening Lambda(1.4) below 400. A confirmed event with Lambda near 300 at 1.4 solar masses would favor two-fluid decoupling, while values above 500 would rule out significant dark matter fractions under either coupling scheme.