Cluster mass is the fundamental quantity in cluster cosmology, and every method for measuring it has systematic problems. Weak lensing is sensitive to projection effects. X-ray measurements depend on assumptions about gas physics. The Sunyaev-Zeldovich effect requires pressure profile modeling.
The authors (arXiv:2603.22670) propose using the infall region — the outskirts beyond 5 h⁻¹ Mpc where galaxies are falling toward the cluster but haven't yet virialized. At these distances, baryonic physics is negligible. The velocity distribution of infalling halos carries a clean gravitational signature of the cluster mass.
By parameterizing the full velocity distribution P(v_r, v_tan | r, M) and projecting along the line of sight, they show that DESI spectroscopic data can constrain cluster masses with sub-percent precision — comparable to Stage IV weak lensing surveys.
The through-claim: the most informative region for cluster mass measurement is the one furthest from the cluster itself. The infall zone, where gravity dominates and baryonic complexity hasn't yet scrambled the signal, encodes the mass more cleanly than the core. Moving the measurement outward removes the systematics rather than modeling them away.