ABSTRACT
This chapter broads the coverage of observations that inform our understanding of the cosmic spacetime, encompassing several measurable signatures, always with a focus on refining the equation-of-state of the cosmic fluid. It gauges how strongly the wide assortment of observations now at our disposal supports the condition over the unconstrained blend of matter, radiation and cosmological constant that drive the expansion in ΛCDM. The statistical method typically involves use of a parametric model fit. A very different approach for evaluating the viability of a chosen cosmology avoids such formulations altogether, and instead relies on the use of ‘two-point’ diagnostics, which differ from the parametric fitting in several distinct ways. An entirely different measure of the cosmic expansion is based on the integrated luminosity distance as a function of redshift. The Hubble diagram provides us with the cumulative expansion as function of ‘look-back’ time, and is drawn from observation of ‘standard candles’—sources whose luminosity is known in their respective rest frames.
