“they [the elves] hold that all Creation of any sort must be in Eä [the actual, existing universe], proceeding from Eru in the same way, and therefore being of the same Order. They do not believe in contemporaneous non-contiguous worlds except as an amusing fantasy of the mind. They are (say they) either altogether unknowable, even as to whether they are or are not, or else if there are any intersections (however rare) they are only provinces of one Eä” (Morgoth’s Ring 252)
Does this mean that there are no possible, alternate worlds at all, or just that there are no actual worlds that are not already “contiguous” with, and hence part of, this world? Compare this with St. Thomas:
The very order of things created by God shows the unity of the world. For this world is called one by the unity of order, whereby some things are ordered to others. But whatever things come from God, have relation of order to each other, and to God Himself, as shown above (Q[11], A[3]; Q[21], A[1]). Hence it must be that all things should belong to one world. Therefore those only can assert that many worlds exist who do not acknowledge any ordaining wisdom, but rather believe in chance, as Democritus, who said that this world, besides an infinite number of other worlds, was made from a casual concourse of atoms. (ST 1.47.3)
The Flame Imperishable 