When we speak about parallel universes we literally bring about the concept of alternate history. It's quite interesting to note that alternate histories is not some relatively new concept thought about in the last hundred years or so.
The earliest example of an alternate history is Book IX, sections 17–19, of Livy's Ab Urbe condita. Livy contemplated an alternative 4th century BC in which Alexander the Great expanded his empire westward instead of eastward; Livy asked, "What would have been the results for Rome if she had been engaged in war with Alexander?"
Ab urbe condita — is a monumental history of ancient Rome written in Latin sometime between 27 and 25 BC by the historian Titus Livius.
The period around the second World War also saw the publication of the time travel novel Lest Darkness Fall by L. Sprague de Camp where an American academic travels to the Italy of the Ostrogoths at the time of the Byzantine invasion led by Belisarius. De Camp's work is concerned with the historical changes wrought by his time traveler, Martin Padway, thereby making the work an alternate history. Padway is depicted as making permanent changes and implicitly forming a new time branch.
The physicist David Deutsch, a strong advocate of the many-worlds interpretation of quantum mechanics, has said that "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives. When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen."
You can look at both, The Montauk Project and the Philadelphia Experiment where the Unified Field Theory was implemented and time and space were, for a time meshed and Hyperspace was achieved. the original goal was for the potential of radar invisibility but many other doors were eventually opened that were chanced upon and explored in secret.
Hitler was very intrigued with the concept of time travel among other things, and a facility which was in another European country prior to WW2 that was actually experimenting with the theory, was dismantled and taken to Germany once Hitler had found out about it.
You have to remember that many esoterica avenues were being explored by the 3rd Reich that were light years ahead of anything the US could have conceived of at the time. Time travel, parallel dimensions, immortality etc...etc...
A fairly good rendition of this was the movie Hellboy as well as the Indiana Jones movie Raiders of the Lost Ark and The Keep which was supposed to be in Romania, the same place where Hitler removed the time travel experiment from in WW2.
