The Hubble Constant from (CLASS) Gravitational Lenses

One of the main objectives of the Cosmic Lens All-Sky Survey (CLASS) collaboration has been to find gravitational lens (GL) systems at radio wavelengths that are suitable for the determination of time delays between image pairs. The survey is now near completion and at least 18 GL systems have been found. Here, I will discuss our efforts to measure time delays from several of these systems with the ultimate aim of constraining the Hubble Constant (H0). Thus far three CLASS GL systems (i.e. B0218+357, B1600+434 and B1608+656) have yielded measurements of time delays, from which values of H0≈60–70 km s−1 Mpc−1 have been estimated. Although most GL systems give similar values of H0, statistical and systematic uncertainties are still considerable. To reduce these uncertainties, I will shortly mention two monitoring programs that we are undertaking to (re)measure time delays in 15 CLASS GL systems and address several important issues for the near future.