This chapter discusses the laws governing the evolution of the scale factor as well as Hubble’s law, which is historically the first observational signature of cosmic expansion. Hubble’s law relates two measurable quantities, the redshift and the luminosity distance of a galaxy. The chapter also introduces the Weyl postulate (1923), which stipulates that the ‘cosmological fluid’ consisting of galaxies, quasars, and so on, visible or invisible, follows such geodesics. It then presents the Friedmann–Lemaître equations. Finally, the chapter discusses the first models of the universe, from 1917–60: the static Einstein model and the de Sitter and steady state models.
Keywords: scale factor, Hubble’s law, Weyl postulate, cosmic expansion, Friedmann–Lemaître equations, Friedmann–Lemaître spacetimes, models of the universe, static Einstein model, de Sitter and steady state models
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