Cosmology is the study of the origin, evolution, composition, and structure of the Universe. As a scientific discipline cosmology began only in the twentieth century. Among the fundamental theoretical and observational developments that established the Big-Bang model were the general theory of relativity proposed by Albert Einstein in 1915, the development of the theory of an expanding relativistic cosmology by Alexander Friedmann in 1922 and Georges LemaÎtre in 1927, the observation of the expansion of the Universe by Edwin Hubble in 1929, the development of the theory of Big-Bang nucleosynthesis by Ralph Alpher, George Gamow, and Robert Herman in the early 1950s, and the discovery of the cosmic background radiation by Arno Penzias and Robert Wilson in 1964.
Traditionally, cosmology has been a data-starved science, but cosmology today is experiencing a fertile interplay between observation and theory. Precision measurements of the expansion rate of the Universe, the large-scale homogeneity and isotropy of the distribution of galaxies, the existence and high degree of isotropy of the 3-K cosmic microwave background radiation, and the abundances of the light elements support the basic picture of an expanding hot-Big-Bang Universe.