Understanding the Dark Components of the Universe

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Current observations of the universe over a wide range of scales and redshifts seem consistent with a single, standard model of cosmology. The ingredients of this model are surprising, however; it includes three radically new components, unknown from laboratory physics - an inflationary field, dark energy, and dark matter. Establishing the reality and nature of these new components remains one of the most important tasks in cosmology. The aim of this proposal is to better understand the two components - dark energy and dark matter - that dominate the late-time universe, and to determine how dark and luminous structures are related. The methods will include numerical simulations of structure formation, statistical analyses of the local galaxy distribution, and cosmological tests based on gravitational lensing. I outline the two main components of the program below. Relating Dark and Luminous Matter on Small Scales: I will focus on small-scale (galactic or subgalactic) dark matter structure, since its existence is poorly constrained, and since predictions for different dark matter candidates diverge on small scales. My group is currently engaged in projects using the clustering of faint sources in two nearby galaxy samples to detect satellite populations around isolated Milky Way-like galaxies. The initial results show a clustering signal at signal-to-noise ~10 in both cases. I propose to extend this work to deeper surveys, to characterize the population of satellites around a larger set of primaries. Using numerical simulations, we will then compare the abundance and distribution of observed satellites to models of dark matter substructure in galaxy halos, to determine whether the theory is consistent with observations, and if so, how satellites trace dark matter on small scales. Combining theory and observations, we will measure the net efficiency of galaxy formation as a function of dark matter halo mass (the stellar-to-halo mass ratio) in a regime where it has not yet been explored. We have recently completed an initial comparison between predicted structure and visible galaxies in the core of the Virgo cluster, as part of the Next Generation Virgo Survey. I plan to extend this study to the entire cluster volume. These studies should provide a more precise framework for models of galaxy formation and evolution in different environments. Constraining Dark Energy: Current observations of dark energy are consistent with a cosmological constant, a simple possibility but a difficult one to explain. Any detected variation with redshift in the energy density of this component would provide a vital clue to its nature, but extremely precise (percent-level) measurements are required. I recently developed a new version of a classic cosmological test, measuring the mean lensing signal behind a set of gravitational lenses as a function of redshift. The strength of the lensing signal provides an indirect measure of distance, and the resulting distance-redshift relation constrains the expansion history and thus dark energy. As a proof of concept, I have applied this test to galaxy groups in the COSMOS survey, constraining the dark energy density to within 12%. I propose to extend this test to the larger CS82 and CFHTLS surveys, where it should be possible to reduce the uncertainties to 3-4% or less. I am also developing tests of the cosmic growth rate based on non-linear structure formation. The internal relaxation of galaxy clusters as they evolve provides a means of estimating their growth history. This, in turn, can constrain the overall growth rate of cosmological structure, and thus dark energy. Such tests are becoming increasingly important, given the wealth of data on clusters expected from current and forthcoming surveys.