Princeton University Dark Cosmos Seminar

Abstract: Gravitational lensing by galaxy clusters has emerged as a powerful tool to probe the standard CDM paradigm of structure formation in the Universe. Despite the remarkable explanatory power of CDM on large scales, tensions with observations on small scales have been reported. Recent studies show that the observed galaxy-galaxy strong lensing (GGSL) cross-section in clusters exceeds CDM predictions by more than an order of magnitude, persisting even after rigorous examination of various possible systematics. The excess appears incompatible with CDM expectations for subhalo inner structure, even under extreme baryonic contraction. By contrast, the GGSL signal is naturally reproduced if subhalos develop very steep central density slopes, (ρ ∝ r^−γ with γ ≳ 2.5), as expected when self-interacting dark matter (SIDM) enters gravothermal core collapse in dense cluster environments. Such slopes significantly strengthen local tangential critical curves and boost the GGSL signal while respecting the lensing constraints. However, new constraints on the outer structure of cluster subhalos using truncation radii from joint strong+weak lensing remain statistically consistent with collisionless CDM. Together, the inner (GGSL) and outer (truncation) constraints on cluster subhalos imply that neither CDM or SIDM can self-consistently reproduce the observed cluster lensing across scales in its entirety, posing a new challenge for both paradigms.