H₀ from Gravitational Lenses: Recent Results

The 0 th , 1 st and 2 nd derivatives of a “Fermat potential” give the three D's of gravitational lensing: delay, deflection and distortion. Observations of these delays, deflections and distortions for doubly and quadruply imaged quasars give estimates of Hubble's constant, H 0 . The single largest contribution to the uncertainty in H 0 arises from the difficulty in constraining the degree of central concentration of the lensing potential. Fortunately, astronomers have spent a good deal of effort over the past quarter century addressing just this question. If galaxies at z = 0.5 are like nearby galaxies, the associated systematic uncertainty in H 0 is less than 10%. The expected lens-to-lens scatter is 20%. Results from three particularly well constrained systems are reported.