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Observational and astrophysical cosmology: 1940–1980

Following the pioneering studies of galaxies and the universe during the pre-Second World War years, the period 1940-1980 saw the consolidation of the observational and theoretical basis of geometrical and astrophysical cosmology. By the early 1950s, the cosmological time-scale problem had been resolved by Baade’s recalibration of the Cepheid distance scale, but new disputes arose about the best estimate of Hubble’s constant, the value of the deceleration parameter and the presence or otherwise of the cosmological constant in the cosmological field equations. The evolution of the contents of the universe was established by radio astronomical observations of active galaxies and, most spectacularly, by the discovery of the cosmic microwave background radiation. The latter enabled the problems of the origin of galaxies and large scale structures in the universe to be placed on a secure physical basis, but many issues remained unresolved, including the dark matter problem.