The classic alternate universe book is Edwin Abbott's 1884 story Flatland, in which there is a universe that has only two dimensions. Other early alternate universe stories include Homer Eon Flint's and Austin Hall's The Blind Spot (1921), Edmond Hamilton's "Locked Worlds" (1929), Murray Leinster's "The Fifth Dimension Catapul" (1931), William Hope Hodgson's The House on the Borderland (1908), and Clifford Simak's Ring Around the Sun (1953).
In Jane Lindskold's 1996 novel Chronomasters, the protagonist, Rene Kordal, is one of a handful of specialists who can create "pocket universes" -- small-scale alternate universes located within our own universe.
Larry Niven has speculated that parallel universes exist apart from our own in another dimension, not another location in space. He whimsically imagines that perhaps fog is caused not by condensed water vapor, but when two parallel universes intersect in the same time and space. If you wander out on a foggy night, you may end up in a world that is not quite your own, never able to find your way back.
Cosmologists Max Tegmark, Andrei Linde, and other scientists argue that alternate universes could be real. One argument in favor of alternative universes says that there's no reason to think that the Big Bang was a unique event. If other big bangs - like the one that formed our own universe -- take place occasionally, at vast distances throughout an infinite space, then each might have created its own universe.
The universe we can actually observe is about 14 billion light-years long. What if this "reality" of ours is only a small patch in an infinite space filled with other universes, each created by its own big bang?
The laws of physics in the alternate universes may be different than our own. Andrei Linde theorizes that wormholes may link these universes together.
If these alternative universes spring up often enough, the pattern of matter in our universe may be randomly replicated in one of these alternate universes. Such a universe would be a "parallel" universe in that it is a replica of our own.