OK then here I go. The theory of multiverses can be refuted by simply noting that in order to create any theory about multi-dimensional reality one must assume that parallel universes exist. This applies to String, M-Theory, Universe Bubbles and the 4 levels of the Multiverse Theory. Making this assumption is a huge leap in logic. It defies the much simpler reasoning that can be accomplished by just dealing with the one universe that we know exists.
Furthermore, just to play devil's advocate and agree that parallel universes exist and are created every moment, then why should anyone reason that their futures will differ from the future of this current universe? Why would Hitler be alive? Why wouldn't the exact same set of effects take place given the exact same set of causes? I propose that if there were innumerable parallel universes then they would all be exactly the same and therefore useless in time travel because they should be advancing in terms of time identically with the universe from which they spawned.
Now, to continue making different points the Bubble Theory is interesting because it suggests the creation of universes from the quantum stuff of a parent universe. On a very small scale there are energy fluctuations. These fluctuations may create tiny bubbles and wormholes. If the energy fluctuation is not very large, a tiny bubble universe may form, experience some expansion like an inflating balloon, and then contract and disappear from existence. However, if the energy fluctuation is greater than a particular critical value, a tiny bubble universe forms from the parent universe, experiences long-term expansion, and allows matter and large-scale galactic structures to form. Now, we would stil have to make a huge assumption and say that the energy difference will effect time and/or physics.
For now, I'm going to stick with a one universe ideology. In this universe we cannot travel back in time. We can travel forward in a sense as harvey45 pointed out (also works with long-term frozen hibernation). I believe, for now, that the multiverse and other theories assume too much and then theorize based off of those unproven assumptions.