Cosmological Constraints from Gravitational Lens Time Delays

Abstract: Future large ensembles of time delay 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, LSST time delay measurements for ~4,000 lenses should constrain the local Hubble constant h to ~0.007 (~1%), Omega_DE to ~0.005, and w to ~0.026 (all 1-sigma precisions). Similar constraints could be obtained by a dedicated gravitational lens observatory (OMEGA) which would obtain precise time delay and mass model measurements for ~100 well-studied lenses. We compare these constraints (as well as those for a 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. Time delays yield a modest constraint on a time-varying w(z), with the best constraint on w(z) at the "pivot redshift" of z ~ 0.31. Our Fisher matrix calculation is provided to allow time delay 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, time delay precision, redshift precision, and the ratio of four-image to two-image lenses.