7. SUMMARY AND CONCLUSION

The future uses of quasar lenses, as in the past, divide into three natural applications: study of the lenses, study of the sources, and study of cosmology and galaxy evolution. I briefly summarise the results from the main body of the review, and the prospects for the next ten years.

• We already know basic facts about lens mass distributions; elliptical galaxies have isothermal mass distributions at low redshift, and there is some indication of steepening with redshift. In prospect is a vast increase in parameter space, with the ability to study the evolution of galaxy mass profiles over a much wider range of redshift, and over different masses of lens galaxies and Hubble types. Much of this work will be done with lens-selected surveys, like the existing SLACS galaxy-galaxy lenses; however, quasar lens systems give the opportunity to make large, source-selected surveys and examine the statistical properties of lenses independently of their selection. The major impact will be in the study of substructure in lens galaxies, however. The existing sample of substructure-friendly quasar lenses is very small and has already yielded a large and surprising body of information about the small-scale features of lensing galaxies. Expansion of these samples will provide critical tests for galaxy formation models.
• Microlensing studies have yielded unprecedented insights into the nature of quasars, particularly the proportions of stellar and smooth matter within the mass budget and the properties – size and physics – of the central engine and surrounding emission line regions. These studies require multi-wavelength observations and patient monitoring, and have concentrated on relatively few objects. With future telescopes and high-cadence monitoring we can expect that studies of quasar physics at huge effective resolution will become routine.
• Perhaps the most important future application of quasar lensing lies in a promise which has taken some time to fulfil, that of cosmography. Many years passed between the discovery of the first quasar lens and the first reliable determinations of the Hubble constant. The process is now accelerating, thanks to coordinated and long-term monitoring campaigns, allied to advances in lens modelling and observation of individual lens systems. Such investigations are already closing in on estimates of the Hubble constant with stringent enough error constraints to contribute to the overall cosmological world model. Future measurements on a large sample of quasar lenses will give cosmological parameter estimates with error circles orthogonal to many others, and factors of several increase in figures of merit for dark energy searches.
• Finally, the prospect of huge lens samples carries with it the probability of finding something totally new. We can speculate about the likelihood of possible candidates - completely dark lenses, cosmic strings - but with the expectation that the unexpected is likely to prove more surprising still.

Acknowledgements

I thank Ian Browne for a careful reading of, and comments on, the manuscript.