Abstract

Observations of structure formation and assembly over a range of physical (Gpc to pc) and temporal (10⁹-10³ yr) scales have been successfully accounted for by the concordance cosmological model comprising cold dark matter (CDM) and a cosmological constant. Recently, however, tensions have emerged in the several key directions, from: the discordant (now at the 5-sigma level) measurements of the Hubble parameter; discrepant observationally determined values of the growth of structure parameter sigma-8 and most recently from the order of magnitude discrepancy in strong lensing cross-sections on small-scales in galaxy clusters. I will elaborate on this third tension, derived from galaxy-galaxy strong lensing. While several other crises on small-scales in CDM reported in the past stand resolved, this new one intriguingly persists. In this talk, I focus on this new gnawing gap between the observed and theoretically predicted galaxy-galaxy strong lensing cross-sections inferred from the small-scale lensing properties of cluster member galaxies. The implications of this mis-match and potential resolutions, both within the standard cold dark matter paradigm and beyond will be discussed.