Dogen Revealed what the sideways universe IS by Jack's Destiny

This theory touches upon some ideas that people have already mentioned, but this is an entirely new theory directly relevant after watching "Sundown". We know Darleton has been teasing fans for a few weeks by stating that Dogen holds a huge clue concerning the flash-sideways' and I believe that Dogen subtly explained what is going on.

Dogen said a man named Jacob approached him after his son was killed and said he could make it so Dogen's son lived, but Dogen would have to go to the island and....(this is the really important part) DOGEN COULD NEVER SEE HIS SON AGAIN.

I believe that these flash-sideways' are a seperate timeline, created by Jacob, as a chance for redemption with many of these flawed characters brought to the island. However, just like the belief in parallel universes today, the dogen that we know is not simply transplanted into the sideways universe after death, rather it is a different dogen, a different being. This goes for all of our characters. The charlie we know and love is dead, yet his parallel universe 'brother'(for lack of a better word) is not.