Dark Energy Survey Year 3 Results: Cosmology from Cosmic Shear and
  Robustness to Data Calibration
  This work, together with its companion paper, Secco and Samuroff et al.
(2021), presents the Dark Energy Survey Year 3 cosmic shear measurements and
cosmological constraints based on an analysis of over 100 million source
galaxies. With the data spanning 4143 deg$^2$ on the sky, divided into four
redshift bins, we produce the highest significance measurement of cosmic shear
to date, with a signal-to-noise of 40. We conduct a blind analysis in the
context of the $\Lambda$CDM model and find a 3% constraint of the clustering
amplitude, $S_8\equiv \sigma_8 (\Omega_{\rm m}/0.3)^{0.5} =
0.759^{+0.025}_{-0.023}$. A $\Lambda$CDM-Optimized analysis, which safely
includes smaller scale information, yields a 2% precision measurement of $S_8=
0.772^{+0.018}_{-0.017}$ that is consistent with the fiducial case. The two
low-redshift measurements are statistically consistent with the Planck Cosmic
Microwave Background result, however, both recovered $S_8$ values are lower
than the high-redshift prediction by $2.3\sigma$ and $2.1\sigma$ ($p$-values of
0.02 and 0.05), respectively. The measurements are shown to be internally
consistent across redshift bins, angular scales and correlation functions. The
analysis is demonstrated to be robust to calibration systematics, with the
$S_8$ posterior consistent when varying the choice of redshift calibration
sample, the modeling of redshift uncertainty and methodology. Similarly, we
find that the corrections included to account for the blending of galaxies
shifts our best-fit $S_8$ by $0.5\sigma$ without incurring a substantial
increase in uncertainty. We examine the limiting factors for the precision of
the cosmological constraints and find observational systematics to be
subdominant to the modeling of astrophysics. Specifically, we identify the
uncertainties in modeling baryonic effects and intrinsic alignments as the
limiting systematics.
