Dark Energy Survey Year 1 Results: Cosmological Constraints from Cosmic
  Shear
  We use 26 million galaxies from the Dark Energy Survey (DES) Year 1 shape
catalogs over 1321 deg$^2$ of the sky to produce the most significant
measurement of cosmic shear in a galaxy survey to date. We constrain
cosmological parameters in both the flat $\Lambda$CDM and $w$CDM models, while
also varying the neutrino mass density. These results are shown to be robust
using two independent shape catalogs, two independent \photoz\ calibration
methods, and two independent analysis pipelines in a blind analysis. We find a
3.5\% fractional uncertainty on $\sigma_8(\Omega_m/0.3)^{0.5} =
0.782^{+0.027}_{-0.027}$ at 68\% CL, which is a factor of 2.5 improvement over
the fractional constraining power of our DES Science Verification results. In
$w$CDM, we find a 4.8\% fractional uncertainty on $\sigma_8(\Omega_m/0.3)^{0.5}
= 0.777^{+0.036}_{-0.038}$ and a dark energy equation-of-state
$w=-0.95^{+0.33}_{-0.39}$. We find results that are consistent with previous
cosmic shear constraints in $\sigma_8$ -- $\Omega_m$, and see no evidence for
disagreement of our weak lensing data with data from the CMB. Finally, we find
no evidence preferring a $w$CDM model allowing $w\ne -1$. We expect further
significant improvements with subsequent years of DES data, which will more
than triple the sky coverage of our shape catalogs and double the effective
integrated exposure time per galaxy.
