To be published in the proceedings of IAU Symposium No. 225: The Impact of Gravitational Lensing on Cosmology, Y. Mellier and G. Meylan, eds.
astro-ph/0408338

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Abstract. Present day estimates of the Hubble constant based on Cepheids and on the cosmic microwave background radiation are uncertain by roughly 10% (on the conservative assumption that the universe may not be perfectly flat). Gravitational lens time delay measurements can produce estimates that are less uncertain, but only if a variety of major difficulties are overcome. These include a paucity of constraints on the lensing potential, the degeneracies associated with mass sheets and the central concentration of the lensing galaxy, multiple lenses, microlensing by stars, and the small variability amplitude typical of most quasars. To date only one lens meets all of these challenges. Several suffer only from the central concentration degeneracy, which may be lifted if one is willing to assume that systems with time delays are either like better constrained systems with non-variable sources, or alternatively, like nearby galaxies.

Table of Contents

• CONTEXT
• H₀ FROM TIME DELAYS CIRCA 2003
• LENSING FUNDAMENTALS
• The 3 D's
• Modeling the lens
• PROBLEM LENSES
• Kochanek's time delay formulation
• The mass sheet degeneracy
• The central concentration degeneracy
• Multiple lenses
• Microlensing
• The myth of quasar variability
• A GOLDEN LENS
• SUMMARY AND PROSPECTS
• REFERENCES

¹ Present address: Center for Space Research 37-664G, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Back.