When quantum physicists talk about alternate universes and the multiverse, it's a different thing to what astrophysicists and cosmologists mean when they talk about the multiverse and alternate universes.
As explained to me by an astrophysicist at the local Science Festival Saturday (so if this is wrong it's down to me), when quantum physicists talk about the multiverse, they mean all the quantum universes where any time a particle does one thing or another, those universes split off, e.g. one where Shroedinger's cat is dead, one where it's alive; or one (or several) where Napoleon won Waterloo, and a bunch where he lost. Alternate histories in sci-fi terms. These all "occupy" the same space our universe does, and getting from one to another would mean doing something spooky to our quantum states or something.
Meanwhile, a cosmologist may be talking about different branes, where different three-dimensional spaces may be floating around n-dimensional super-spaces, coinciding with ours only in that the first three dimensional co-ordinates are the same but the higher dimensional co-ordinates are different. Since their origins are different, they have no relationship to ours, and may enjoy different physical laws. Travelling to them would mean finding a way to move through those higher dimensions.
Or, they may be different universes only in that they are beyond the event horizon of our universe, and they were pinched off from ours during the inflationary period when the universe expanded faster than light. Being permanently out of contact with us (until FTL is invented) they may have different physical laws pertaining locally.