JWST Lensing Tightens Dark Matter Free-Streaming Bounds, Bridging Particle Physics and Cosmology

A new analysis of 28 quadruple-image strong gravitational lenses observed by JWST delivers direct constraints on the dark matter free-streaming length, a key parameter distinguishing cold from warm dark matter scenarios. Using simultaneous reconstruction of extended lensed arcs alongside image positions and magnifications, the study improves sensitivity to low-mass halo perturbations compared to prior lensing work. This preprint (arXiv:2606.05277) rules out deviations from cold dark matter predictions above halo masses of 10^7.2-10^7.4 solar masses, translating to free-streaming length upper limits of 6-7 kpc and thermal relic particle mass lower bounds of 6.5-7.4 keV. Methodology relies on cosmological N-body simulations and semi-analytic models for subhalo abundance, with a measured subhalo mass density of 1.7^{+2.6}_{-1.2} x 10^7 solar masses per kpc^2 under CDM assumptions. Limitations include the modest sample of 28 systems, model-dependent assumptions about halo mass functions, and lack of peer review. The work connects high-redshift lensing to longstanding tensions in Milky Way satellite counts and Lyman-alpha forest constraints (e.g., Viel et al. 2013 MNRAS), which mainstream coverage often separates from JWST galaxy-formation puzzles. It highlights how free-streaming limits could ease or exacerbate small-scale structure issues without invoking baryonic feedback alone.