Abstract
We perform a joint analysis of the counts and weak lensing signal of redMaPPer clusters selected from the Dark Energy Survey (DES) Year 1 dataset. Our analysis uses the same shear and source photometric redshifts estimates as were used in the DES combined probes analysis. Our analysis results in surprisingly low values for $S_8 =\sigma_8(\Omega_{\rm m}/0.3)^{0.5}= 0.65\pm 0.04$, driven by a low matter density parameter, $\Omega_{\rm m}=0.179^{+0.031}_{-0.038}$, with $\sigma_8-\Omega_{\rm m}$ posteriors in $2.4\sigma$ tension with the DES Y1 3x2pt results, and in $5.6\sigma$ with the Planck CMB analysis. These results include the impact of post-unblinding changes to the analysis, which did not improve the level of consistency with other data sets compared to the results obtained at the unblinding. The fact that multiple cosmological probes (supernovae, baryon acoustic oscillations, cosmic shear, galaxy clustering and CMB anisotropies), and other galaxy cluster analyses all favor significantly higher matter densities suggests the presence of systematic errors in the data or an incomplete modeling of the relevant physics. Cross checks with X-ray and microwave data, as well as independent constraints on the observable--mass relation from SZ selected clusters, suggest that the discrepancy resides in our modeling of the weak lensing signal rather than the cluster abundance. Repeating our analysis using a higher richness threshold ($\lambda \ge 30$) significantly reduces the tension with other probes, and points to one or more richness-dependent effects not captured by our model.
Highlights
The flat ΛCDM model, despite its apparent simplicity—six parameters suffice to define it—has proven able to describe a wide variety of observations, from the low to the high redshift Universe
We perform a joint analysis of the counts and weak lensing signal of redMaPPer clusters selected from the Dark Energy Survey (DES) Year 1 dataset
The fact that multiple cosmological probes, and other galaxy cluster analyses all favor significantly higher matter densities suggests the presence of systematic errors in the data or an incomplete modeling of the relevant physics
Summary
The flat ΛCDM model, despite its apparent simplicity—six parameters suffice to define it—has proven able to describe a wide variety of observations, from the low to the high redshift Universe. Ongoing (e.g., the Dark Energy Survey (DES), Hyper Suprime-Cam, Kilo-Degree Survey eRosita, South Pole Telescope (SPT), Atacama Cosmology Telescope (ACT)6) and future surveys (e.g., Euclid, Large Synoptic Survey Telescope, WFIRST9) aim to further test the ΛCDM paradigm, as well as the mechanism that drives the cosmic. Lacking a fundamental theory to test, one way to shed light on the latter is by looking at the evolution of cosmic structures over the past few Gyr, when the dark energy becomes dominant, and searching for discrepancies between the observables in the low-redshift Universe and the predictions for said observables derived from the high-redshift Universe as measured through observations of the cosmic microwave background (CMB) anisotropies [e.g., [1,2] ]. DES was designed with the primary goal of testing the ΛCDM model and studying the nature of dark energy through four key probes: cosmic shear, galaxy clustering, clusters of galaxies, and Type Ia supernovae
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