Are there other universes?
Possibly. OPH starts from a single self-reference equation: the universe is whatever comes back unchanged when its own records are read and the reading is run again. Any solution of that equation is a valid universe, so other universes exist if the equation has other solutions. We could never reach them. Space, time and every channel a signal could travel along are built inside one solution, from its own observers' records, so nothing connects one solution to another and there is nowhere for a bridge to stand. Inside our own universe there are no alternatives on the shelf. One cell of an observer's boundary, read as an area and as the smallest electromagnetic act, must give the same number, and that fixes the fine-structure constant at a single value. A universe like ours with different constants names nothing. Another universe would be a different solution altogether, with its own observers and its own physics.
Also in Chapter 33: Why Was Every Open Problem the Same Problem?, Chapter 2: Why Is There Anything Rather Than Nothing?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 36: Why These Laws and Not Others?watch a universe make its own particles