Galaxy clusters as laboratories and telescopes: from the Dragon to VENUS

Since the discovery of the giant Dragon arc in Abell 370, massive galaxy clusters have been identified as the Universe's most powerful natural telescopes, as well as the most massive laboratories for studying dark matter. After a brief introduction to cluster strong-lensing formalism and history, from the first arcs to HST, CLASH, the Frontier Fields and JWST, we show how a lens model can serve as a probe of the cluster itself. Combining the strong-lensing potential with X-ray and/or SZ observations, we establish a general bijective relationship between them for relaxed systems, whose validity allows us to probe the baryon–dark matter relationship. We then turn the telescope around. Our analysis of SPT-CL J0546-5345 at z=1.07, based on JWST SLICE data, shows what a very massive, very distant cluster magnifies, and we discuss what such rare objects mean for ΛCDM. We then present the impact of the JWST VENUS survey of 60 lensing clusters: SN Eos (z=5.13), the farthest multiply imaged supernova, and its ultra-faint host galaxy; SN Helios, a Type II supernova at z=3.34 with a predicted reappearance; the long-delay SN Ares; a massive quiescent galaxy at z~5.2 resolved to ~70 pc; and a growing zoo of lensed Little Red Dots. In each case the result depends on the quality of the lens model. We close on the first statistics of the 60 VENUS lens models, which bring us back to the cluster as an object: what do ~60 well-constrained mass distributions say about dark matter and ΛCDM?