To be published in the proceedings of IAU Symposium
No. 225: The Impact of Gravitational Lensing on Cosmology, Y. Mellier
and G. Meylan, eds.
For a PDF version of the article, click
For a PDF version of the article, click here.
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.
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1 Present address: Center for Space Research 37-664G, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Back.