Astronomy Tea Talk

  Surveys of galaxy clusters provide a sensitive probe of cosmology by measuring the evolution of the halo mass function. With a number of surveys at optical, millimeter, and X-ray wavelengths on-going or starting in the near futures, cluster count experiments will be one the most important cosmological probes over the next decade. However, none of the typical survey observables (X-ray luminosity, optical richness, SZ flux) directly measures the cluster mass. Already for  current cluster surveys, the dominant uncertainty in the determination of sigma_8 and other parameters (e.g. the species-summed neutrino mass) comes from the uncertainty in the scaling relation between cluster mass and observables. Cluster weak lensing is the most promising observational method to calibrate the mass scaling to the required precision, but requires the control of systematic errors to a a few percent each. I will review our "Weighing the Giants" project to  measure accurate weak lensing masses for the largest sample of clusters to date, and discuss its first cosmological applications for the baryonic mass fraction test, the Planck cluster cosmology analysis, and for new results from ROSAT-selected cluster counts.