Facilities for Dark Energy Investigations

The discovery of cosmic acceleration has inspired ambitious experimental and observational efforts to under-stand its origin. Many of these take the form of large astronomical surveys, sometimes using new, special-purpose instrumentation, and in some cases entirely new facilities. These efforts are frequently referred to as “dark energy experiments, ” which is the terminology we will adopt in this Report; we will not try to draw any fine distinctions between “experiments ” and “observations. ” The basic thrust of these experiments is to measure the history of expansion and growth of structure in the universe with steadily increasing precision over the broadest achievable range of redshifts. The hope is that these measurements may eventually reveal discrepancies with the predictions of a simple cosmological constant model, perhaps pointing towards a particular class of dynamical field or a breakdown of General Relativity (GR) on cosmological scales. The approach is in many ways analogous to high-energy physics experiments that pursue increasing measurement precision or new ranges of energy, hoping to “break ” the standard model in a way that points towards deeper physics that underpins it. Most existing and planned dark energy experiments employ one or more of the following approaches: 1. Wide-field imaging, to measure weak gravitational lensing, the galaxy content and weak lensing signal of galaxy clusters, and angular clustering of galaxies in bins of photometrically estimated redshift