Gravitational Lens Time Delays in Cold Dark Matter

In standard cold dark matter (CDM) halo models, the time delay of a gravitational lens is determined by the cold baryon mass fraction, f b = Ω b , cold /Ω 0 , of the visible galaxy relative to the overall halo. The observed time delays in PG 1115+080, SBS 1520+530, B1600+434, and HE 2149-2745 give Hubble constants consistent with the Hubble Space Telescope Key Project value of H 0 = 72 ± 8 km s -1 Mpc -1 only if f b ≳ 0.2 (one-sided 68% confidence), which is larger than the upper bound of f b , max = Ω b /Ω 0 = 0.15 ± 0.05 estimated from the cosmic microwave background. If all available baryons cool and f b = f b , max , then the time delays imply H 0 = 65 ± 6 km s -1 Mpc -1 (95% confidence). If local inventories of cold baryons, f b ≃ 0.013/ h 70 , are correct, then H 0 = 52 ± 6 km s -1 Mpc -1 and the halo parameters closely match isothermal mass models. Isothermal models are also consistent with strong and weak lens studies, stellar dynamics, and X-ray observations on these scales, while significantly more centrally concentrated models are not. There is a conflict between gravitational lens time delays, the local distance scale, and standard CDM halo models.