We present cosmological results from a combined analysis of galaxy clustering and weak gravitational lensing, using 1321 deg 2 of griz imaging data from the first year of the Dark Energy Survey (DES Y1). We combine three two-point functions: (i) the cosmic shear correlation function of 26 million source galaxies in four redshift bins, (ii) the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and (iii) the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. To demonstrate the robustness of these results, we use independent pairs of galaxy shape, photometric redshift estimation and validation, and likelihood analysis pipelines. To prevent confirmation bias, the bulk of the analysis was carried out while "blind" to the true results; we describe an extensive suite of systematics checks performed and passed during this blinded phase. The data are modeled in flat ΛCDM and wCDM cosmologies, marginalizing over 20 nuisance parameters, varying 6 (for ΛCDM) or 7 (for wCDM) cosmological parameters including the neutrino mass density and including the 457 × 457 element analytic covariance matrix. We find consistent cosmological results from these three two-point functions, and from their combination obtain S8 ≡ σ8(Ωm/0.3) 0.5 = 0.773 +0.026 −0.020 and Ωm = 0.267 +0.030 −0.017 for ΛCDM; for wCDM, we find S8 = 0.782 +0.036 −0.024 , Ωm = 0.284 +0.033 −0.030 , and w = −0.82 +0.

We use 26 × 10 6 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 ΛCDM and the wCDM models, while also varying the neutrino mass density. These results are shown to be robust using two independent shape catalogs, two independent photo-z calibration methods, and two independent analysis pipelines in a blind analysis. We find a 3.5% fractional uncertainty on σ 8 ðΩ m =0.3Þ 0.5 ¼ 0.782 −0.39 . We find results that are consistent with previous cosmic shear constraints in σ 8 -Ω m , and we see no evidence for disagreement of our weak lensing data with data from the cosmic microwave background. Finally, we find no evidence preferring a wCDM model allowing w ≠ −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.

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We report a detection of the coherent distortion of faint galaxies arising from gravitational lensing by foreground structures. This ‘cosmic shear’ is potentially the most direct measure of the mass power spectrum, as it is unaffected by poorly justified assumptions made concerning the biasing of the distribution. Our detection is based on an initial imaging study of 14 separated 8×16 arcmin2 fields observed in good, homogeneous conditions with the prime focus EEV‐CCD camera of the 4.2‐m William Herschel Telescope. We detect an rms shear of 1.6 per cent in 8×8 arcmin2 cells, with a significance of 3.4σ. We carefully justify this detection by quantifying various systematic effects and carrying out extensive simulations of the recovery of the shear signal from artificial images defined according to measured instrument characteristics. We also verify our detection by computing the cross‐correlation between the shear in adjacent cells. Including (Gaussian) cosmic variance, we measure the shear variance to be (0.016)2±(0.012)2±(0.006)2, where these 1σ errors correspond to statistical and systematic uncertainties, respectively. Our measurements are consistent with the predictions of cluster‐normalized cold dark matter (CDM) models (within 1σ) but a Cosmic Background Explorer normalized standard cold dark matter model is ruled out at the 3.0σ level. For the currently favoured ΛCDM model (with Ωm=0.3), our measurement provides a normalization of the mass power spectrum of σ8=1.5±0.5, fully consistent with that derived from cluster abundances. Our result demonstrates that ground‐based telescopes can, with adequate care, be used to constrain the mass power spectrum on various scales. The present results are limited mainly by cosmic variance, which can be overcome in the near future with more observations.