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

Abstract

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 H0=48-4+7 km/s Mpc (95% confidence) if the lenses have flat rotation curves and H0=71+/-6 km/s Mpc (95% confidence) if they have constant mass-to-light ratios, as compared to H0=72+/-8 km/s Mpc (68% confidence) for local estimates by the HST Key Project. Either all five H0 estimates based on the lenses are wrong, local estimates of H0 are too high, or dark matter distributions are more concentrated than expected. The average value for H0 including the uncertainties in the mass distribution, H0=62+/-7 km/s Mpc, has uncertainties that are competitive with local estimates. However, by selecting a value for H0 you also determine the dark matter distribution of galaxies.