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Galaxy clusters provide a powerful toolset with diverse applications in cosmology and fundamental physics. The observed number density and clustering of galaxy clusters as a function of mass and redshift are sensitive to both the expansion history and growth of structure in the Universe, enabling powerful constraints on dark energy and providing critical distinguishing power between dark energy and modiﬁed gravity models for cosmic acceleration. Measurements of the baryonic mass fraction in clusters, and of the tomographic lensing signatures through clusters, provide additional ways to measure cosmological parameters. Galaxy clusters provide sensitive probes of the physics of inﬂation and, in combination with CMB data, currently provide our best constraints on the species-summed neutrino masses.
As with all cosmological probes, the key to extracting robust cosmological constraints from galaxy clusters is the control of systematic uncertainties, particularly those associated with ﬁnding clusters and relating the observed properties of clusters to the underlying matter distribution. This requires a coordinated, multiwavelength approach, with LSST at its core, and the application of rigorous statistical frameworks informed by cosmological simulations. (For a recent review of galaxy cluster cosmology see Allen et al. (2011).)
Image: Galaxy Cluster MACS J1206 (NASA, ESA, M. Postman (STScI) and the CLASH Team).