The Emergence of Modern Cosmology

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. Whereas philosophers and astronomers have always been interested in the universe at large, cosmology as a physical science is of relatively recent origin. Two roots of so-called modern cosmology can be identified, one observational and the other theoretical. On the observational side, spectroscopic study of the distant nebulae in the late 19th century was one of the roots. The other root, purely theoretical in nature, was Einstein’s cosmological model of 1917 based on general relativity. For a long time, the two traditions developed independently, but with the recognition of the expanding universe at about 1930 they merged into one. Physical cosmology in more or less the modern sense of the term relied on quantum mechanics and advances in nuclear physics, which provided the basis for the first theories of the very early universe established in the late 1940s. During the following decade the new theory of the hot big bang met stiff competition from the rival steady state theory. However in 1965, and largely as a result of the discovery of the cosmic microwave background, the latter theory was abandoned or at least marginalized. Five years later, the standard hot big bang theory of the universe had obtained an almost paradigmatic status in the small but growing community of cosmologists. Modern cosmology had come into being. This was only a beginning and over the next decades the paradigm (if so it was) continued to be refined and on occasions even questioned.