He applied his early training in nuclear physics to astrophysics and cosmology. He developed his version of the Big Bang theory in the late 1940s, with the American physicists Ralph Asher Alpher (1921–2007) and Robert Herman (1914–97). It proposed that the Universe underwent a rapid expansion from a hot concentration of atomic particles and radiation, which he called ‘ylem’, building the nuclei of the elements in the process. The theory predicted a Universe containing three-quarters hydrogen and one-quarter helium, roughly in accordance with present-day measurements, and that it would cool as it expanded. Alpher and Herman suggested that the present, cooled-down Universe should be pervaded by a cosmic microwave background with a temperature of about 5 K; their suggestion was forgotten until the radiation was detected in 1964.