Okay, parallel universe are formed when there is a decision to be made in which one decision goes one way while another decision goes another way. Basically where the path diverges. Now add in time travel in which one goes back in his own time line. When that person goes back and purposely "derails" the timeline by making a different decision he forms a new parallel universe also known as a tangent timeline.
Tangents refers to both alternate timelines and parallel universe theory and expands the dimensions on fix axis points: V, W, X, Y, and Z. V being the backwards and forward movement of time with W representing sideways movement through time (parallel universes). When there is a divergent in the primary timeline that divergent moves a point away fromthe baseline so that it remains true and unchanged while the tangent and the one who caused the tangent follows a new path.
There is no single link that forms this theory but something I came up with by reading a bunch of material on particle physics, time travel, and parallel universes. When I have more time I'll track down some relevant links for you.