Why exactly is time travel forbidden?
Hint: Parallel worlds are only created by rewriting history because time can only flow in a single direction; if the past is altered by time travel, then the entire temporal axis separates the altered past away from the original future.
Question: Why did Beerus destroy Zamasu?
Answer: Because Future Trunks used his Time Machine to time travel back from his alternate future to the alternate past of Goku's world and then informed the Z-Fighters of the past of Goku Black's existence.
The "any small change" bit is directly referring to the action of rewriting history. The example of a "small change" Future Trunks uses is
him traveling to the past and giving Goku the heart medicine, instead of Goku dying from the heart virus. You can only create
parallel worlds from a "small change" if you travel back in time. You
cannot make a "small change" in your present or your future because your present and your future are currently being written, and any "change" you in the present could think of is simply an action pre-determined by the flow of causality, which expands forward endlessly; hence why travelling to the future is
not prohibited by cosmic law.
You are thinking of the
many-worlds theory, which establishes an infinite number of parallel universes existing simultaneously. To the human observer, the difference between the movement of an atom creates an infinite number of possible futures; in reality, said possible futures would have already existed across the infinite multiverse, such is the nature of infinity.
Dragon Ball is
not the
many-worlds theory.
Time travel and
parallel worlds are
very explicit.
