2 00 2 Gravitational Lenses , the Distance Ladder and the Hubble Constant : A New Dark Matter Problem

In cold dark matter models, a galaxy's dark matter halo is more spatially extended than its stars. However, even though the five well-constrained gravitational lenses with time delay measurements must have similar dark matter distributions, reconciling the Hubble constant estimated from their time delays with local estimates is possible only if that dark matter distribution is as compact as the luminous galaxy. The Hubble constant is H 0 = 48 +7 −4 km/s Mpc (95% confidence) if the lenses have flat rotation curves and H 0 = 71 ± 6 km/s Mpc (95% confidence) if they have constant mass-to-light ratios, as compared to H 0 = 72 ± 8 km/s Mpc (68% confidence) for local estimates by the HST Key Project. Either all five H 0 estimates based on the lenses are wrong, local estimates of H 0 are too high, or dark matter distributions are more concentrated than expected. The average value for H 0 including the uncertainties in the mass distribution, H 0 = 62 ± 7 km/s Mpc, has uncertainties that are competitive with local estimates. However, by selecting a value for H 0 you also determine the dark matter distribution of galaxies.