For over three millennia, cosmology had closer connections to myth, religion, and philosophy than to science. Cosmology as a branch of science has essentially been an invention of the twentieth century. Because modern cosmology is such a diverse field and has ties with so many adjacent scientific disciplines and communities (mathematics, physics, chemistry, and astronomy), it is not possible to write its history in a single chapter. Although there is no complete history of modern cosmology, there exist several partial histories that describe and analyze the main developments. The following account draws on these histories and presents some major contributions to the knowledge of the universe that emerged during the twentieth century. The chapter focuses on the scientific aspects of cosmology, rather than on those related to philosophy and theology.

THE NINETEENTH-CENTURY HERITAGE

Cosmology, the study of the structure and evolution of the world at large, scarcely existed as a recognized branch of science in the nineteenth century; and cosmogony, the study of the origin of the world, did even less. Yet there was, throughout the century, an interest, often of a speculative and philosophical kind, in these grand questions. According to the nebular hypothesis of Pierre-Simon de Laplace and William Herschel, some of the observed nebulae were protostellar clouds that would eventually condense and form stars and planets in a manner similar to the way in which the solar system was believed to have been formed. This widely accepted view implied that the world was not a fixed entity, but in a state of evolution.