An Introduction to Radio Astronomy

For astronomers prior to 1925, the stars of the Milky Way seemed to form a Universe of immense size, but cosmology was transformed from philosophy to science in that year, when Edwin Hubble announced that he had identified Cepheid variables in the Andromeda nebula, M31. Through the known period-luminosity relation of Cepheids, he proved that M31, contrary to majority opinion, was a giant system of stars, comparable to the Milky Way. The stars of the Milky Way represented only a tiny fraction of a Universe populated by other galaxies fully as rich as our own. The climax of the Hubble revolution was reached in 1929 when, through measuring the Doppler shifts of the external galaxies, he demonstrated that the entire system of galaxies was expanding. The more distant the galaxy, the higher its radial velocity. This fitted nicely with cosmological models derived from solutions of Einstein's general theory of relativity (GR), whose cosmological interpretation of GR had puzzled theorists for over a decade, but it was now applied to Hubble's discovery and provided the intellectual framework within which a consistent cosmology could be developed. The ‘standard cosmological model’ of a Universe of galaxies, expanding from an initial state of high density, became the principal focus of theoretical cosmology for 20 years.

The second revolution in cosmology took place 35 years after the discovery of the expanding Universe, not from conventional optical observations, but from radio astronomy.