I've been thinking about the parallel universe thing... We've seen so far that there are some obvious changes in the other timeline, and rightly so. People have been wondering why there are so many changes... Maybe its Jacob's lack of influence? Maybe Its MIB's lack of influence ? Maybe its both ? Whatever the case, I really don't think "Destiny" decided to go nice on them in this particular timeline, or anything like that... For instance, Locke being married to Helen MUST be the direct result of Jacob or Monster or anything else being different in this timeline as opposed to the original... Make sense ???
Basically, to make it simpler, I think that if Locke is still with Helen in the newline, then its because in the original, Jacob made it that Locke broke up with Helen... Whereas he didn't in the new one (Maybe because he's dead, or simply that there's no point in trying to shape him into a man he doesn't need to be if the island's gone)... By the way, i'm only using Locke as an example. This theory goes for all the characters...
So, Parallel universes are basically universes that exist with ours, whilst having nothing to do with us. Though these PU's can be exactly the same, they can have differences, such as the one we're witnessing in Season 6.
However, personally, I believe that all PU's are fixed, and exactly identical to every little detail, UNLESS, something in one Universe directly alters the other... Using great amounts of Electromagnetic energy, classic LOST style :) Think of a type of gateway (Sliders style), and say youu threw a cricket ball into this PU. The PU would have the cricket ball now, and the PU would be changed in this way, and therefore not identical. Granted, the cricket ball would only have the result of their being an extra cricket ball in that universe, and other than that, would be quite uneffective. However, say you were to put a person through this gate, the PU would be altered in the way that now there is a person in that PU (One that shouldn't technically be there). This person could alter loads of things that the universe he's originally from never had. Is this making sense ???
So say if a nuclear bomb were to end up travelling from the OU to the PU, the PU would be forever altered by this one object, hence making it different from the OU. The only thing is, that once the bomb reached the PU, it exploded, which set off the chain of events that make THIS particular PU dramatically different!
Please Please PLEASE let me know if this post is confusing, and i'll try and clear things up a bit... Thanks :)