Measuring the metric: A parametrized post-Friedmannian approach to the cosmic dark energy problem

We argue for a parametrized post-Friedmannian approach to linear cosmology, where the history of expansion and perturbation growth is measured without assuming that the Einstein field equations hold. As an illustration, a model-independent analysis of 92 type Ia supernovae demonstrates that the curve giving the expansion history has the wrong shape to be explained without some form of dark energy or modified gravity. We discuss how upcoming lensing, galaxy clustering, cosmic microwave background and Lyalpha forest observations can be combined to pursue this program, and forecast the accuracy that the proposed SNAP satellite can attain.