 The observational cosmology program (Kristian &amp; Sachs 1966; Ellis et al. 1985) clarifies the extent to which a set of ideal observations can determine the spacetime geometry directly with minimal cosmological assumptions. (By contrast, the standard approach starts by assuming a background cosmological model and then finding an optimal parameter fit.) Roughly put, the ideal data set consists of a set of astrophysical objects that can be used as standard candles and standard rulers. If the intrinsic properties and evolution of a variety of sources are given, observations can directly determine the area (or luminosity) distance of the sources, and the distortion of distant images determines lensing effects. These observations thus directly constrain the spacetime geometry of the past light cone \(C^-(p)\). Number counts of discrete sources (such as galaxies or clusters) can be used to infer the total amount of baryonic matter, again granting various assumptions. Ellis et al. (1985) proved the remarkable result that an appropriate idealized data set of this kind is sufficient, if we grant that EFE hold, to fully fix the spacetime geometry and distribution of matter on the past light cone \(C^-(p)\), and from that, in the causal past \(J^-(p)\) of the observation point \(p\).[18] Observers do not have access to anything like the ideal data set, obviously, and in practice cosmologists face challenges in understanding the nature of sources and their evolution with sufficient clarity that they can be used to determine spacetime geometry, so this is the ideal situation.