The Dark Matter Telescope

Weak gravitational lensing enables direct reconstruction of dark matter maps over cosmologically significant volumes. This research is currently telescopelimited. The Dark Matter Telescope (DMT) is a proposed 8.4 m telescope with a 3◦ field of view, with an etendue of 260 m deg, ten times greater than any other current or planned telescope. With its large etendue and dedicated observational mode, the DMT fills a nearly unexplored region of parameter space and enables projects that would take decades on current facilities. The DMT will be able to reach 10σ limiting magnitudes of 27-28 magnitude in the wavelength range .3−1μm over a 7 deg field in 3 nights of dark time. Here we review its unique weak lensing cosmology capabilities and the design that enables those capabilities. 1 Breaking Degeneracies, Testing Foundations Direct information on cosmology – and thus on the early history of the universe – can in principle be obtained by measuring the spectrum of mass as it evolves with cosmic time. The current set of cosmological models (cf. Zaldarriaga etal 1997, Hu 1998, Turner & Tyson 1999), has over ten free parameters. Each type of observational test encounters degeneracies among the parameters that cannot be resolved without additional information. Measuring the mass distribution at redshifts 0.2–1 breaks this degeneracy because it is sensitive to different parameter combinations than is the cosmic microwave background (CMB). More importantly, such observations and their concordance with the CMB results will provide an independent test of the validity of the entire theoretical framework. From work spanning more than five decades, it has become apparent that light and mass are not identically distributed. It is now possible to discover mass concentrations that are not evident in the light distribution. Ultimately, such studies will define the distribution of mass on a variety of scales. What do the inferred dark mass concentrations imply for cosmological simulations that are normalized to the number density of luminous mass concentrations? Such questions will not be addressed by spectroscopic surveys, even those as large as the Sloan survey. The only way to directly “weigh” the mass distribution is through weak gravitational lensing. In its simplest form, gravitational lens distortions of the distant galaxies enables a reconstruction of the projected mass density map for the intervening lens. But one can do even better: photometric redshifts enable us to slice the projected sky in redshift bins. By obtaining weak lensing maps for sources at a variety of redshifts, we could obtain a three-dimensional mass map of