The chapter presents the key properties and the evolution of satellite measurements before diving into the history of various explanations of the CMB. The shape of the CMB spectrum reflecting the black body radiation and its unusual isotropy are the essential properties telling us about the origin, but its temperature is also dependent on other contingent factors. The chapter briefly discusses details of the physical process behind such properties and the relevance of their ever-more precise satellite measurements and presents these in diagrams.

Which of the four-parameter family of Friedman–Robertson–Walker (FRW) cosmological models best fits our universe and why? This chapter addresses these two central questions for observation and theory in cosmology. Of the four parameters that define an FRW model, only two are determined by observations so far: the Hubble constant; and the ratio of energy density in radiation to the critical density. To determine the others, the spacetime geometry of the universe must be measured on large scales through a study of how matter moves through it. We describe two illustrative ways of doing that – one based on observations of distant supernovae, and the other on observations of the cosmic background radiation. Remarkably, the best cosmological parameter values are consistent with the universe being spatially flat – right on the borderline between positive and negative spatial curvature.