In Western philosophy, the theory of infinite worlds dates back 2500 years to the “atomists” of ancient Greece. For philosophers like Leucippus, Democritus and Epicurus, the universe was composed of microscopic atoms, moving eternally in a void, colliding haphazardly with one another until they formed a vortex.
In this vortex, the heavy elements clustered together to form earth, light elements scattered far from the centre as fire, and mid-weight particles formed water and air in between them. Crucially, this void was spatially infinite. So particles elsewhere were forming vortices to create other worlds, even an infinite number of them. When these worlds died, their atoms would go on to form new worlds elsewhere. Some of these worlds were small, some large, some lifeless, and some just like ours. But all of them were the result of random collisions in infinite space, no God required. For the atomists, “gods” were enlightened sages engaged in blissed-out contemplation. They existed, but did not create, sustain or intervene in our world.
If the atomists gave us a spatial model of the multiverse, their Stoic rivals gave us a temporal one. For the Stoics, matter is continuous. There are no atoms, no void and nothing beyond our cosmos that might form other worlds. In short, ours is the only world in the universe. The cosmos will eventually end when the sun dries up Earth and consumes it in flames, only for it to be born again. The periodic destruction and regeneration of the universe is the stand-out feature of early Stoic cosmology, a process called ekpyrosis (“out of fire”).
If the atomists pushed their gods out of the cosmos, the early Stoics pulled them all the way in. The Stoic god was the animating force of the cosmos itself: creating, sustaining, unravelling, and regenerating all things. If the atomists were effectively non-theists, then the Stoics were effectively pantheists: the world itself was what they meant by “god”.
The Atomists got it right. They are the fathers of modern cosmology, even if no one talks today less about individual atoms, but their consituent parts.
I think we have much to learn from the Stoics, though perhaps not from their views on cosmology. The Stoics were very popular in their time because they gave people a philosophy they needed to carry on. In a world of struggle, Stoicism could be very comforting.
Stoics also had a more realistic view of the spiritual realm than other theists. If there is such a thing as the supernatural, why would it only intervene occasionally, as Christians seem to think? Rather, it makes more sense to believe that the supernatural is intimately involved in everyday life, and that everything happens because of something supernatural. This holds the supernatural to account for everything. Conversely, theists only think God is responsible for good things, and abdicate him from responsibility for anything bad- a far more absurd view than Stoic pantheism.