I always have found this question interesting...the whole...time travel paradox.
Lets say you go back in time. You'll now be arriving in a place where everything has a set path toward its future. Definite fate perhaps. Either way everything you see is part of a path that has been traveled, if you interfere with that path the fate of those actions will change, therefore changing the future and so on down the line. I don't think I quite side with the idea of two universes from this point on because there would actually be at least 3 brought on by time travel:
The path that was going to be carried out from the past to the present to the future...without traveling through time.
The path that would be created by traveling through time, and coming back(to the future
) and moving onward through time.
The path that would be created by changing the past and altering that future.
So there are 3 at least. If you want to consider that there can be no matter created nor destroyed throughout the universe. (assumption of course, who knows some day) then we have to side with an idea that there were already these 3 universes around or else we will have created a problem with the physics.
Furthermore, if there are these 3(or even millions) of undisturbed universes/dimensions we then would have altered the course of these dimensions creating at least two futures for these "undisturbed" dimensions....and so on....so it would seem more likely that traveling in time and changing something can only result in one path toward the future...right?
well, the whole idea of traveling back and changing your own future would get knocked into the can because if we are to assume that the separate dimensional paths theory is false due to conservation of energy/matter throughout the universe we must say that traveling back in time is impossible because we would be adding to a closed system (past)
however, if we could alter the past remotely somehow to create a place where we could subtract one object (say, a tree) and insert another object (person)....we could stay in tune with the physics if the premise of conservation is correct....
then we'd just have a problem with altering the future....so yeah...anyway you slice it....you'll be left with a pickle....
footnotes: time travel should'nt be possible due to conservation of matter (IMO) because the past has no space for addition (matter/energy), and creating multiply dimensions can't happen cus of this either..