  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.
