Gravitational lensing of background objects by foreground galaxies and galaxy clusters (thereafter the lenses) has unique importance in Cosmology as it allows us to measure the total mass distribution (and particular Dark Matter) of the lenses, independent of their light emission and dynamical state. Lens systems are also unique cosmological probes, allowing measurements of angular diameter distance or ratios of these distances, thus ultimately sensitive to the properties of Dark Energy. The wide range of lensing science projects will be further enhanced in the future by the hundred of thousands strong-lensing systems (compared with the current few hundreds) that should be discovered and observed with the next generation of wide-field optical and near IR imaging space missions (e.g. Euclid, WFIRST). With the expected larger statistical sample, one can expect 30x higher statistical precision for many of the physical measurements, which then will likely be limited by systematic uncertainty. In order to investigate these various systematics and derive new science, we have gathered an excellent team to focus on two strong lensing key problems:

1) Finding galaxy scale lenses in future wide field imaging space surveys (Euclid, WFIRST), recovering their mass distributions and constraining Dark Energy models.

2) Finding multiple image systems in massive clusters, modeling the cluster mass distribution to better than the per cent level (which can be tested today using the current Hubble Frontier Fields1 observations), and possibly probing the nature of the Dark Matter particles.

To evaluate our algorithms, techniques, softwares and the quality of the derived results, we have created tools to produce realistic image simulations that closely mimic the observations from various space telescopes (Hubble and the future Euclid, WFIRST and James Webb Space Telescopes). These image simulations include all lensing effects produced by foreground and line of sight mass distributions. The goal of this ISSI international team is to get organized and have these simulated data to be analyzed blindly by different groups as a form of a public challenge, open to anyone in the scientific community. We foresee two different challenges addressing the two key problems above. This will allow us to train and evaluate softwares for the automatic detection of gravitational arcs and multiple images, as well as for the determination of the mass distribution of the lenses and ultimately to recover cosmological parameters such as the equation of state of Dark Energy through statistical and geometrical tests (which can be tested in the next years with the current Hubble Frontier Fields observations).

Our team includes some of the best expert scientists in the field of (strong) lensing. We have already developed the required simulation tools and have made great progress on the lens finding algorithms and modeling techniques to be exploited in this project. Our goal is to make all of this work (in particular the simulated data) publicly available to the scientific community. These standard sets of simulated images will enable many investigations by any scientists interested in strong lensing science beyond our expert group of scientists, which will certainly leads to better science results. We also plan to write-up our findings in a set of review papers that will become references on this topic for the coming years.

Research keywords: extragalactic astronomy, cosmology, dark matter, dark energy, gravitation (strong) gravitational lensing, galaxies and galaxy clusters.