Evidence for the accelerated expansion of the Universe from weak lensing
  tomography with COSMOS
  We present a tomographic cosmological weak lensing analysis of the HST COSMOS
Survey. Applying our lensing-optimized data reduction, principal component
interpolation for the ACS PSF, and improved modelling of charge-transfer
inefficiency, we measure a lensing signal which is consistent with pure
gravitational modes and no significant shape systematics. We carefully estimate
the statistical uncertainty from simulated COSMOS-like fields obtained from
ray-tracing through the Millennium Simulation. We test our pipeline on
simulated space-based data, recalibrate non-linear power spectrum corrections
using the ray-tracing, employ photometric redshifts to reduce potential
contamination by intrinsic galaxy alignments, and marginalize over systematic
uncertainties. We find that the lensing signal scales with redshift as expected
from General Relativity for a concordance LCDM cosmology, including the full
cross-correlations between different redshift bins. For a flat LCDM cosmology,
we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect
agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023,
sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness
and using HST Key Project and BBN priors only, we find a negative deceleration
parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing
independent evidence for the accelerated expansion of the Universe. For a flat
wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark
energy constraints are still relatively weak solely due to the limited area of
COSMOS. However, they provide an important demonstration for the usefulness of
tomographic weak lensing measurements from space. (abridged)
