User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-95.112.161.208-20150915211422/@comment-24834479-20150916013713

Heaven is comprised of countless different realities, which is where the string theory comes in. They aren't necessarily different universes, but they are within their own realities. Ash was able to his knowledge of string theory and magic to cross into others' Heavens.