Alan Guth (born 1947) started as a theoretical physicist but became increasingly interested in cosmology during post-doctoral work at Princeton, Columbia, Cornell and the Stanford Linear Accelerator Center (SLAC). The Big Bang version of the early universe at the time still left significant issues unexplained. Guth's suggestion that the nascent universe passed through a phase of exponential expansion (inflation), at a rate corresponding to a billion times the speed of light, addressed many of those issues. See Dennis Overbye, Lonely Hearts of the cosmos: the scientific quest for the secret of the universe, Chapters 12, 13, 14 for a readable discussion of some very high-end issues in theoretical cosmology.