Dark Energy Survey Year 1 Results: Cosmological Constraints from Cluster
  Abundances and Weak Lensing
  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.
