Right now when physicists talk of other alternate universes they mostly talk of varying the laws of physics, changing a few parameters, but I've not heard of the possible existence of an unlimited world. When it comes to a beginning, a first moment, some models have suggested the possibility of an unlimited past, that is an eternal line of events preceding the present with no first moment. This is one aspect that's been considered as possibly unlimited. But what about the other aspects? What about speed? time and spatial divisions, and rate of change in matter? Could there not be a world where the maximum speed is unlimited? That is there is no maximum speed. What about a world where there are stable structures of smaller and smaller dimensions and also no limit to the magnification that could be obtained? Also a world were temporal divisions could be carried unlimited meaningfully , there being no smallest interval? It would be similar to the case with the real numbers were between any two numbers there are an infinity of numbers. If such a world could exist, it could be considered a root world, in a sense. As it could possibly provide infinite computational capacity and storage to its inhabitants. The inhabitants of said world could simulate all the possible finite universes with limited variables and store them in their unlimited universe. The only thing that might make such a world impossible is if there's some reason why making so many parameters unlimited is fundamentally impossible. But being as some have considered making the past unlimited in extent, without this being somehow indefensible, it seems safe to suggest that doing the same with the other parameters should not bring problems different from those already brought by making the past unlimited.