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Although modern cosmology is essentially a twentieth-century science, its birth can reasonably be traced back to discussions about the universe in the previous century. With the emergence of astrophysics in the 1860s astronomy was substantially changed and the material content of the universe became an issue of science. At about the same time thermodynamics was applied to the universe at large, with the result that the beginning and end of the universe entered astronomical thought. Moreover, it became slowly realized that space can be described as curved rather than flat. In that case it would be possible to speak about a finite and yet unbounded universe and in this way to solve some of the problems associated with the traditional view of an infinite number of stars. These and other problems were only fully understood in the twentieth century, but they were discussed before Einstein revolutionized cosmology.

Although relativistic astrophysics began in the 1930s with study of supernovae and neutron stars, it was only three decades later that the discovery of extragalactic radio sources, quasars and pulsars marked the emergence of special and general relativity as essential tools of the high energy astrophysicist. X-ray and γ -ray astronomy provided many new insights, culminating in the discovery of γ -ray bursts at cosmological distances in 1997. Supermassive black holes in active galactic nuclei provided major new challenges for theorists and observers alike, revealing many remarkable relativistic phenomena, such as superluminal motions observed in some of the most active galaxies. Einstein’s prediction of gravitational waves of 1916 was substantiated exactly 100 years later with their discovery in coalescing binary black hole systems by the LIGO project. These remarkable discoveries, mostly in the non-optical wavebands, brought a wide range of physicists into the astronomical and cosmological communities.

The origin and evolution of the universe constitutes one of the most fascinating and challenging questions in the scientific investigation of nature. The general theory of relativity has made it possible to properly address this question. Einstein transformed cosmology when he formulated, in 1917, a relativistic model that could describe the universe in its entirety. The incorporation of the observational evidence of extragalactic recession into relativistic world models culminated in 1930 with the recognition of the expanding universe, which was a breakthrough in the scientific understanding of the universe as a whole. This chapter traces the history of the early phase of modern cosmology, from the formulation of the first cosmological models based on general relativity to the acceptance of the expanding universe and the early systematization of relativistic cosmology as a new scientific discipline.

This chapter deals with the development of modern cosmology as a consequence of relativity theory testing and the space race during the Cold War. The chapter describes the dynamics of competition and collaboration among the two blocs with emphasis on the sectors of Soviet radio astronomy and space science. Developments of both fields are analysed in relationship with the military context and the technological development taking place in the USA. The chapter also takes into account the relationship between the two blocs and other Countries, such as France and Australia in order to show how the transfer of knowledge and know-how played a role in the extraordinary impulse that cosmology received during the Cold War.