There is a growing discrepancy between measurements of the Hubble constant, H0, performed with cosmological probes in the early Universe with Cosmic Microwave Background (CMB) and probes in the late universe such as pulsating stars and supernovae. Deciding on whether this discrepancy, called the Hubble tension, is real or not has profound implications for the current cosmological paradigm and can very well imply that new physics beyond the standard model must be considered. However, all cosmological probes currently in use do rely on each other at some level and we lack of truly independent verification. Strong lensing time delays, the topic of COSMICLENS, consists in the only other independent probe that can address the Hubble tension. COSMICLENS implements the whole chain of analysis needed to successfully measure and use time delays in strongly lensed to measure H0 precisely enough. The principle of the method is simple: time delays can be measured using the photometric variations of the lensed images in multiply imaged quasars. As the optical paths to each image are different in length, and as they intersect the lens plane at different impact parameters, the wavefronts along each path reach the observer at different times. As a consequence, the photometric variations in each lensed image of the quasar are seen shifted in time. This time delay, due to both geometry and gravity is related to H0 with only little sensitivity to other cosmological parameters. More precisely, the geometrical part of the time delay contains H0 and can be constrained with a mass model for the lens galaxy(ies). H0 depends on the slope of the lensing potential at the position of the lensing images. This means that models should be as accurate as possible as to turn time delays into reliable values of H0, which is the second goal of COSMICLENS, by designing new modeling tools where the lens and source are described on a basis of wavelets rather than in an analytical way, as done traditionally. A third part of the project is to understand what could possibly go wrong with modeling and to confront the standard and new modeling tools to detailed simulations of lenses and therefore to provide crash-tests and benchmarking for our tools. This uses analytical and well-controlled simulations of lenses. Finally, one has to find as many lenses as possible to measure new time delays. Lenses are rare and not all can be used for time delay cosmography. We have therefore massively exploited public sky surveys to find new objects. New techniques have been devised for this work, which will also benefit lens searches in upcoming space-based and ground-based all sky surveys such as Euclid and Rubin-LSST.
