8. FUTURE PROSPECTS

Since the discovery of giant arcs in the late 80's gravitational lensing by clusters of galaxies has now become a powerful cosmological tool.

We list below some possible new avenues for the next exciting discoveries in the coming years using gravitational lensing in clusters, assuming improvements in the data quality and data volume:

• Dedicated lensing surveys of well defined massive cluster samples can probe in a systematic way the cluster mass distribution at high-resolution from galaxy-scales out to the virial radius using both weak and strong lensing. In particular, we can hope to study the build-up of the mass in clusters as a function of time and disentangle the time scales on which the segregation between the different mass components occurs during the assembly process. One could also envision being able to systematically trace the filamentary structures linking massive clusters that are simply the nodes of the cosmic web seen in numerical simulations of the formation and evolution of structure in the Universe.
• From wide field imaging surveys: mass selected clusters can be identified, but likely the more interesting prospect is the ability to investigate the dark matter mass versus stellar-mass relation and its evolution with time, thus providing useful cosmological constraints on the growth of structure in the Universe, as well as on the underlying geometric cosmological parameters.
• With larger cluster samples, better constraints will be obtained on density profile slopes in the inner and outer regions of clusters thus permitting to robustly test theoretical predictions of the ΛCDM model.
• Using numerous multiple-images with measured redshifts in a number of massive clusters, we should be able to provide geometrical constraints of Dark Energy in a complementary way to other cosmological probes.
• Measuring the time-delays of temporally variable phenomena such as Supernovae or AGN when observed behind well-known massive lensing clusters, will lead to measurement of the Hubble parameter H(z), in a similar way as multiple quasars behind galaxies, but with much improved accuracy. However, in order to have a time-delay of a limited number of years, these transients event must be located close to the critical lines. While, the likelihood of detecting multiple-images of such transient phenomena is extremely low, it is not insignificant provided there is a steady increase in the volume of the Universe probed with time.
• Finally, massive cluster lenses will always be the unique places to probe the high-redshift Universe, as they offer enhanced sensitivities at all wavelengths and enable mapping the detailed morphology and physical properties of the most distant galaxies in the Universe.

In the near future lensing observations will likely be geared towards optical and near-infrared imaging exploiting the next generation of ground based experiments (DES, LSST, TMT, E-ELT), and spaced based observatories (JWST, EUCLID/WFIRST).

However, in the long run, it is not too unreasonable to think that cluster lensing observations may be well conducted in the radio domain. In such an event we foresee up to an order of magnitude improvement in lensing measurement that will result from combining information on galaxy shapes with velocity field data (Blain 2002; Morales 2006). Such preliminary developments will certainly come first with ALMA, but only centimeter radio interferometers such as SKA (Square Kilometer Array) will allow the exploration of these techniques on cosmological scales.

Acknowledgments

The phenomenon of gravitational lensing is one of the most beautiful predictions of Einstein's Theory of General Relativity. We feel very privileged to have had the opportunity to work and contribute to this area. In particular, during and since the start of our scientific collaboration, our goal has been to inter-weave the observational and theoretical aspects of lensing by clusters to enable the exploitation of the potency of this technique. Much of the work reviewed here is heavily based and biased toward recent results, those obtained since JPK's PhD thesis on cluster lenses in 1993; and PN's work on various theoretical aspects of cluster lenses during and since her PhD in 1999. This review consolidates the rapid progress made thus far and provides a snap shot of this exciting field as it stands today. We would like to thank our many colleagues for the interesting work and fruitful discussions at various conferences, workshops and meetings over the past decade that have provided us a better understanding of our Universe and have transformed cluster lenses into a powerful cosmological tool. JPK acknowledges support from CNRS. PN acknowledges the John Simon Guggenheim Foundation for a Guggenheim Fellowship and the Rockefeller Foundation for a residency at the Bellagio Rockefeller Center during the tenure of which this review was completed. She would also like to thank the Institute for Theory and Computation at Harvard University for hosting her during her fellowship year.