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.

Studies of galaxy clusters have proved crucial in helping to establish the standard model of cosmology, with a universe dominated by dark matter and dark energy. A theoretical basis that describes clusters as massive, multi-component, quasi-equilibrium systems is growing in its capability to interpret multiwavelength observations of expanding scope and sensitivity. We review current cosmological results, including contributions to fundamental physics, obtained from observations of galaxy clusters. These results are consistent with and complementary to those from other methods. We highlight several areas of opportunity for the next few years, and emphasize the need for accurate modeling of survey selection and sources of systematic error. Capitalizing on these opportunities will require a multi-wavelength approach and the application of rigorous statistical frameworks, utilizing the combined strengths of observers, simulators and theorists.

We present a 90 per cent flux‐complete sample of the 201 X‐ray‐brightest clusters of galaxies in the northern hemisphere (δ ≥ 0°), at high Galactic latitudes (|b| ≥ 20°), with measured redshifts z ≤ 0.3 and fluxes higher than 4.4 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All‐Sky Survey data and is the largest X‐ray‐selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X‐ray‐brightest Zwicky clusters and other clusters selected from their X‐ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non‐cluster X‐ray sources. X‐ray fluxes are computed using an algorithm tailored for the detection and characterization of X‐ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1σ error).  We find the cumulative log N–log S distribution of clusters to follow a power law κSα with α = 1.31+0.06−0.03 (errors are the 10th and 90th percentiles) down to fluxes of 2 × 10−12 erg cm−2 s−1, i.e. considerably below the BCS flux limit. Although our best‐fitting slope disagrees formally with the canonical value of −1.5 for a Euclidean distribution, the BCS log N–log S distribution is consistent with a non‐evolving cluster population if cosmological effects are taken into account.  Our sample will allow us to examine large‐scale structure in the northern hemisphere, determine the spatial cluster–cluster correlation function, investigate correlations between the X‐ray and optical properties of the clusters, establish the X‐ray luminosity function for galaxy clusters, and discuss the implications of the results for cluster evolution.