Although theoretical physics and temporal mechanics aren't my strong points, I always thought time travel wouldn't be about punching a hole, as it were, through to the past, but more of a series of actions involving quantum 'foam'. In this case, the theory is, the universe at the extreme quantum level is nothing so more than a collection of particles popping in and out of existence, and doing whatever it is quantum particles do. Now, if all points in time are now viewed not as a linear progression, but rather as what the quantum strata 'looked like' at that instant, then the question is raised: if you can reset the particles to what they looked like at that instant, have you gone back in time (ie, to the period when the image was taken)?
As to the whole alternate universe dealie, well, again, I think it's a function of quantum mechanics. If every particle has 32 (was it 32? something like that) possible quantum states, then for each combination of states that the particles are in, a different universe could exist. For what it's worth, that's the way I view the progression of time.
