Tensions on small scales: too many galaxy-galaxy strong lenses in galaxy clusters?

In the standard cosmological model, the matter content of the Universe is dominated by cold dark matter (CDM), collisionless particles that interact with ordinary matter (baryons) only through gravity. Gravitationally bound dark-matter halos form hierarchically, with the most massive systems growing through mergers of smaller ones. As structure assembles in this fashion, large dark-matter halos contain smaller-scale substructures in the form of embedded subhalos. I will show that observations of gravitational lensing can be used to map the inner mass distribution of cosmic structures such as galaxy clusters to test these predictions of the CDM paradigm. Interestingly, the reconstructed granularity of cluster cores implies an excess of galaxy-galaxy strong lensing (GGSL) probability compared to expectations in the ?CDM cosmological model. The theoretical estimates on the GGSL probability are based on the analysis of hydrodynamical simulations, while the observational measurements are derived from parametric strong lensing reconstructions combining inputs from HST and JWST imaging and MUSE/VLT spectroscopy. In an attempt to understand this issue, cluster-size halos simulated with different mass and force resolutions and implementing several AGN energy feedback schemes have been analyzed. The feedback model has a significant impact on the properties of subhalos and on their ability to produce GGSL effects. However, none of the hydrodynamical simulations studied so far are in agreement with observations. They persistently have difficulty reproducing the stellar mass and the internal structure of cluster galaxies simultaneously. The reported mismatch may indicate an unidentified problem with either prevailing simulation methods or standard cosmology.