This contains spoilers. Lots of them. so please, pretty please, don’t read this unless you are up to date on Lost.
I have a theory about the “Parallel Universe”
It isn’t a parallel universe. It is now. the Now Now.
Ok, so it goes like this, the one group bounces around time, ends up in the ’70’s working for Dharma, yes? They are going about their lives like everything is dandy because the island at some point will do something. But three years goes by and it doesn’t.
So they decide to do something. Specifically Jack, but the others decide it is a good idea. What is this something? Why they are going to detonate a nuke. Pow! right at the center of the magic energy of the island. (Yes you can picture Gandolf there)
So they get the nuke bits in place, drop it down the hole, Juliet hits is and BOOM white screen, lots of LOST fans cursing and throwing things and some even crying in each others arms that the season was over.
Season starts. Now remember something very important: the nuke went off in the 1970’s. Not the present. The island was still in the past. This is important for two reasons: 1. This means that EVERYTHING timeline wise has now changed from that moment on and 2. everything we know about the ‘present’ has changed.
The ‘alt’ timeline IS the timeline. The nuke went off, the energy went, the plane never crashes, Jacob never shows up and does whatever he does, life happened as if Oceania and all of us had never been on the show. Err, affected by the island. So the ‘alt’ timeline is reality. This is now, the present (or a few years ago, but close enough). There are no others, there is no island, they are all free of their destiny (and those numbers).
In the ‘alt’ timeline the island is underwater. What does this mean? Well it means the island isn’t in the present. It can’t be. So where is it? Hell if I know. I do find it interesting that the people are still draw to each other, maybe this will work itself in. But the way i see it now? LA timeline has the real people and the ones left on the island are in purgatory.
Otherwise you have one of those paradox things that Daniel would need to awkwardly explain for at least two episodes.