21 September 2009
Dan Coe (Caltech / JPL)
'Cosmological Constraints from Gravitational Lens Time Delays'
Future large ensembles of time delay lenses have the potential to provide interesting
cosmological constraints complementary to those of other methods. Current constraints
from 10-16 time delay lenses already yield the Hubble constant (h) in agreement with
and to roughly the same level of precision (10%) as the HST Key Project which analyzed
40 Cepheids. Future surveys (Pan-STARRS, LSST, JDEM / IDECS, SKA, OMEGA) will yield
hundreds or even thousands of lenses with well-measured time delays. We find in a
flat universe with constant w including a Planck prior, LSST time delay measurements
for ~4,000 lenses should constrain 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 lenses with spectroscopic redshifts. Constraints for a
general cosmology are presented as well. We compare these 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. As with any method,
there are systematics we must learn to control, and we discuss these issues.