In Fictional Realism, all possible universes exist, and therefore all fictional, constructed and theoretical universes exist.
Astract Object TheoryEdit
Brane worlds are a theory in which our universe is a 3 dimensional membrane in higher dimensional space. It postulates that multiple branes may exist, and if this hyperdimensional universe in infinite, all possible universes exist within it, including constructed universes.
Hugh Everett's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics. In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different probability. According to the MWI, each of these possible observations corresponds to a different universe. Suppose a die is thrown that contains six sides and that the numeric result of the throw corresponds to a quantum mechanics observable. All six possible ways the die can fall correspond to six different universes. (More correctly, in MWI there is only a single universe, but after the "split" into "many worlds" these cannot in general interact.)
Some theories suggest that space is infinite, and that all possibilities, including constructed universes, exist within the same 3 dimensional space.
In the chaotic inflation theory, a variant of the cosmic inflation theory, the multiverse as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread. Such bubbles are embryonic level I multiverses. Different bubbles may experience different spontaneous symmetry breaking resulting in different properties such as different physical constants. This level also includes John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.