COSMOLOGICAL CONSTRAINTS FROM GRAVITATIONAL LENS TIME DELAYS

Future large ensembles of time delay (TD) lenses have the potential to provide interesting cosmological constraints complementary to those of other methods. In a flat universe with constant w including a Planck prior, The Large Synoptic Survey Telescope TD measurements for similar to 4000 lenses should constrain the local Hubble constant h to similar to 0.007 (similar to 1%), Omega(de) to similar to 0.005, and w to similar to 0.026 (all 1 sigma precisions). Similar constraints could be obtained by a dedicated gravitational lens observatory (OMEGA) which would obtain precise TD and mass model measurements for similar to 100 well-studied lenses. We compare these constraints (as well as those for a more general cosmology) to the "optimistic Stage IV" constraints expected from weak lensing, supernovae, baryon acoustic oscillations, and cluster counts, as calculated by the Dark Energy Task Force. TDs yield a modest constraint on a time-varying w(z), with the best constraint on w(z) at the "pivot redshift" of z approximate to 0.31. Our Fisher matrix calculation is provided to allow TD constraints to be easily compared to and combined with constraints from other experiments. We also show how cosmological constraining power varies as a function of numbers of lenses, lens model uncertainty, TD precision, redshift precision, and the ratio of four-image to two-image lenses.