The biggest proof that the alternate flash actually are flash forwards after all the events we're watching on the island now, somehow. They may still be an alternate universe, but they come after the main action of the series, and I am absolutely positive of that now.
Why?
Remember that cut Jack found on his neck while aboard flight 815 at the very start of the season, and he seemed to be quite confused about where it came from?
Well, check out this screencap from last week's episode, and look closely at Jack's neck.
J
