Um, it's highly possible that going back in time wouldn't change
your future (where you come from), but open another universe/dimension with those changes.
Say, for example, you prevent (just imagine) the meteorite to crash on Earth, thus the dinosaurs won't die. When you go back, you will not see dinosaurs everywhere, but there will be another universe/dimension that might be totally different from yours.
The Butterfly effect, for example... Imagine that all the changes he made opened different universes.
Not only that, but all the time, different universes are "started" because of different choices we make.