About a decade later, in the midst of what was dubbed the " Great Debate ", Hubble and Slipher discovered the expansion of universe in the 1920s measuring the redshifts of Doppler spectra from galactic nebulae. Using Einstein's general relativity, Lemaître and Gamow formulated what would become known as the big bang theory. A rival, called the steady state theory, was devised by Hoyle, Gold, Narlikar and Bondi.