We present cosmological results from a combined analysis of galaxy clustering and weak gravitational lensing, using $1321{\mathrm{deg}}^2$ 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 $\mathrm{\Lambda }\mathrm{CDM}$ and $w\mathrm{CDM}$ cosmologies, marginalizing over 20 nuisance parameters, varying 6 (for $\mathrm{\Lambda }\mathrm{CDM}$) or 7 (for $w\mathrm{CDM}$) cosmological parameters including the neutrino mass density and including the $457\times 457$ element analytic covariance matrix. We find consistent cosmological results from these three two-point functions and from their combination obtain $S_8\equiv {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}=0.773_{-0.020}^{+0.026}$ and ${\mathrm{\Omega }}_m=0.267_{-0.017}^{+0.030}$ for $\mathrm{\Lambda }\mathrm{CDM}$; for $w\mathrm{CDM}$, we find $S_8=0.782_{-0.024}^{+0.036}$, ${\mathrm{\Omega }}_m=0.284_{-0.030}^{+0.033}$, and $w=-0.82_{-0.20}^{+0.21}$ at 68% C.L. The precision of these DES Y1 constraints rivals that from the Planck cosmic microwave background measurements, allowing a comparison of structure in the very early and late Universe on equal terms. Although the DES Y1 best-fit values for $S_8$ and ${\mathrm{\Omega }}_m$ are lower than the central values from Planck for both $\mathrm{\Lambda }\mathrm{CDM}$ and $w\mathrm{CDM}$, the Bayes factor indicates that the DES Y1 and Planck data sets are consistent with each other in the context of $\mathrm{\Lambda }\mathrm{CDM}$. Combining DES Y1 with Planck, baryonic acoustic oscillation measurements from SDSS, 6dF, and BOSS and type Ia supernovae from the Joint Lightcurve Analysis data set, we derive very tight constraints on cosmological parameters: $S_8=0.802\pm 0.012$ and ${\mathrm{\Omega }}_m=0.298\pm 0.007$ in $\mathrm{\Lambda }\mathrm{CDM}$ and $w=-1.00_{-0.04}^{+0.05}$ in $w\mathrm{CDM}$. Upcoming Dark Energy Survey analyses will provide more stringent tests of the $\mathrm{\Lambda }\mathrm{CDM}$ model and extensions such as a time-varying equation of state of dark energy or modified gravity.

We describe the derivation and validation of redshift distribution estimates and their uncertainties for the populations of galaxies used as weak-lensing sources in the Dark Energy Survey (DES) Year 1 cosmological analyses. The Bayesian Photometric Redshift (bpz) code is used to assign galaxies to four redshift bins between z ≈ 0.2 and ≈1.3, and to produce initial estimates of the lensing-weighted redshift distributions $$n^i_{\rm PZ}(z)\propto \mathrm{d}n^i/\mathrm{d}z$$ for members of bin i. Accurate determination of cosmological parameters depends critically on knowledge of nⁱ, but is insensitive to bin assignments or redshift errors for individual galaxies. The cosmological analyses allow for shifts $$n^i(z)=n^i_{\rm PZ}(z-\Delta z^i)$$ to correct the mean redshift of nⁱ(z) for biases in $$n^i_{\rm PZ}$$. The Δzⁱ are constrained by comparison of independently estimated 30-band photometric redshifts of galaxies in the Cosmic Evolution Survey (COSMOS) field to bpz estimates made from the DES griz fluxes, for a sample matched in fluxes, pre-seeing size, and lensing weight to the DES weak-lensing sources. In companion papers, the Δzⁱ of the three lowest redshift bins are further constrained by the angular clustering of the source galaxies around red galaxies with secure photometric redshifts at 0.15 < z < 0.9. This paper details the bpz and COSMOS procedures, and demonstrates that the cosmological inference is insensitive to details of the nⁱ(z) beyond the choice of Δzⁱ. The clustering and COSMOS validation methods produce consistent estimates of Δzⁱ in the bins where both can be applied, with combined uncertainties of $$\sigma_{\Delta z^i}=0.015, 0.013, 0.011,$$ and 0.022 in the four bins. Repeating the photo-z procedure instead using the Directional Neighbourhood Fitting algorithm, or using the nⁱ(z) estimated from the matched sample in COSMOS, yields no discernible difference in cosmological inferences.

We present results from a study of the photometric redshift performance of the Dark Energy Survey (DES), using the early data from a Science Verification period of observations in late 2012 and early 2013 that provided science-quality images for almost 200 sq. deg. at the nominal depth of the survey. We assess the photometric redshift (photo-z) performance using about 15 000 galaxies with spectroscopic redshifts available from other surveys. These galaxies are used, in different configurations, as a calibration sample, and photo-z's are obtained and studied using most of the existing photo-z codes. A weighting method in a multidimensional colour–magnitude space is applied to the spectroscopic sample in order to evaluate the photo-z performance with sets that mimic the full DES photometric sample, which is on average significantly deeper than the calibration sample due to the limited depth of spectroscopic surveys. Empirical photo-z methods using, for instance, artificial neural networks or random forests, yield the best performance in the tests, achieving core photo-z resolutions σ₆₈ ∼ 0.08. Moreover, the results from most of the codes, including template-fitting methods, comfortably meet the DES requirements on photo-z performance, therefore, providing an excellent precedent for future DES data sets.

We measure the clustering of DES year 1 galaxies that are intended to be combined with weak lensing samples in order to produce precise cosmological constraints from the joint analysis of large-scale structure and lensing correlations. Two-point correlation functions are measured for a sample of $6.6\times {10}^5$ luminous red galaxies selected using the redMaGiC algorithm over an area of 1321 square degrees, in the redshift range $0.15<z<0.9$, split into five tomographic redshift bins. The sample has a mean redshift uncertainty of ${\sigma }_z/(1+z)=0.017$. We quantify and correct spurious correlations induced by spatially variable survey properties, testing their impact on the clustering measurements and covariance. We demonstrate the sample’s robustness by testing for stellar contamination, for potential biases that could arise from the systematic correction, and for the consistency between the two-point auto- and cross-correlation functions. We show that the corrections we apply have a significant impact on the resultant measurement of cosmological parameters, but that the results are robust against arbitrary choices in the correction method. We find the linear galaxy bias in each redshift bin in a fiducial cosmology to be $b({\sigma }_8/0.81){|}_{z=0.24}=1.40\pm 0.07$, $b({\sigma }_8/0.81){|}_{z=0.38}=1.60\pm 0.05$, $b({\sigma }_8/0.81){|}_{z=0.53}=1.60\pm 0.04$ for galaxies with luminosities $L/L_{*}>0.5$, $b({\sigma }_8/0.81){|}_{z=0.68}=1.93\pm 0.04$ for $L/L_{*}>1$ and $b({\sigma }_8/0.81){|}_{z=0.83}=1.98\pm 0.07$ for $L/L_{*}>1.5$, broadly consistent with expectations for the redshift and luminosity dependence of the bias of red galaxies. We show these measurements to be consistent with the linear bias obtained from tangential shear measurements.

We calculate the volumetric rate of superluminous supernovae (SLSNe) based on five events discovered with the Robotic Optical Transient Search Experiment-IIIb (ROTSE-IIIb) telescope. We gather light curves of 19 events from the literature and our own unpublished data and employ crude k-corrections to constrain the pseudo-absolute magnitude distributions in the rest-frame ROTSE-IIIb (unfiltered) bandpass for both the hydrogen-poor (SLSN-I) and hydrogen-rich (SLSN-II) populations. We find that the peak magnitudes of the available SLSN-I are narrowly distributed (M = −21.7 ± 0.4) in our unfiltered band pass and may suggest an even tighter intrinsic distribution when the effects of dust are considered, although the sample may be skewed by selection and publication biases. The presence of O ii features near maximum light may uniquely signal a high-luminosity event, and we suggest further observational and theoretical work is warranted to assess the possible utility of such SN 2005ap-like SLSN-I as distance indicators. Using the pseudo-absolute magnitude distributions derived from the light-curve sample, we measure the SLSN-I rate to be about (32$$ {^{+ 77}_{-26}} $$) events Gpc⁻³ yr⁻¹ h³₇₁ at a weighted redshift of $$\overline{z} = 0.17$$, and the SLSN-II rate to be about (151$$ {^{+ 151}_{-82}} $$) events Gpc⁻³ yr⁻¹ h³₇₁ at $$\overline{z} = 0.15$$. Given that the exact nature and limits of these populations are still unknown, we discuss how it may be difficult to distinguish these rare SLSNe from other transient phenomena such as active galactic nucleus activity and tidal disruption events even when multiband photometry, spectroscopy or even high-resolution imaging are available. Including one spectroscopically peculiar event, we determine a total rate for SLSN-like events of (199$$ {^{+ 137}_{-86}} $$) events Gpc⁻³ yr⁻¹ h³₇₁ at $$\overline{z} = 0.16$$.