The Friedmann–Lemaître–Robertson–Walker metric that is necessary for the Big Bang and Steady State models emerged in the decade after the development of Einstein's general relativity and was accepted as a model for the universe after Edwin Hubble's discovery of his eponymous law. It was not clear early on how to find a "universe solution" to Einstein's equations that allowed for a universe that was infinite, unending, and immutable (scientists of the time assumed for philosophical reasons the universe should have such a character).
Even after the development of expanding universe theories, people would engage in this exercise from time to time when looking for a replacement for general relativity. Any alternative theory of gravity would imply immediately an alternative cosmological theory since current modeling is dependent on general relativity as a framework assumption. What is included are a number of models based on alternative gravitational scenarios as well as early attempts to derive cosmological solutions from relativity.