The inner light by Jay

Here's what I think happened, with the alt universe, on island, etc.

When they detonated the bomb in the Incident, they released the light at the heart of the island (hence the fade to white at the end of season 5).

Due to the unique electromagnetic properties of the island+hydrogen bomb, they were all physically pushed forward to the 2007 timeline, but the light released by the bomb couldn't go with them - it had to stay behind - trapped within each of them, creating a new, alternate timeline at the point where they would have crashed onto the island - but they didn't crash because the bomb in the 1970's ensured that the actual island wasn't even there anymore.

“a little bit of this very same light is inside every man, but they always want more" - the reason they always want more of it, is because this light is the potential for happiness.

We all have our little bit of it, but we're not meant to have any more than that, and so in this alternate timeline, instead of simply remaining trapped within each individual as a result of the explosion and timeline split, it needed to converge again, and go back to the way it was supposed to be all along, together once again.

The universe has a way of course correcting itself - Desmond was the agent of course correction, because Jacob, the protector of the light, encouraged Widmore to get him to correct the course in the alternate timeline and bring the light back where it belongs - together once again. In this sense, he wasn't simply the protector of the light on the island, but the protector of the inner light that each of these people had within them - and he brought them together, out of their flawed lives, not just to protect the light on the island but to actually give each of them a chance at that happiness as well.

What brought the light back together in the alt timeline was ultimately the people and their emotional experiences - because while the light is the constant, but ultimately, what we do with that light is up to us - the people are the variables.

On-island Jack was able to see into this alternate world when he entered the light underground, and his 'funeral' was really his reunification with the part of the light that was still trapped in the other world - his part of the light and others as well - which he got to see at the moment of his death, just like Juliet did at the beginning of season 6 - after all of the on-island events which had all really happened.