Why are there only 2 (known) parallel universes in Dragon Ball Z?


In the Androids/Cell Saga, when Trunks traveled back in time, he changed
history and created a parallel universe.

But Cell also traveled back in time, and apparently arrived before Trunks was
and responsible for changes in history, too.

One would think that Trunks would arrive in a parallel universe without a
future Cell (in essence, one that was exactly the same as his, up until the
point where he arrived.)

Why did Trunks arrive in the same parallel universe as Cell?




# Alternate Timelines are not strict branches

Parallel universes can often mess with your mind if you're not careful. You
need to pay careful attention to both where you are, what are the
possibilities, and what has occurred to affect change. DBZ uses the alternate
timeline version of time travel, so this means that many universes can exist,
shown or not. A diagram will be easiest.

# Timeline

![Timeline for Cell/Trunks in DBZ](https://i.stack.imgur.com/x49OF.png)

There are actually 4 different timelines involved in the Cell/Trunks/Android
Saga (excluding ones created/changed by time travel).

## Cell to DBZ

The _first timeline_ is one in which the Androids have ravaged the earth, and
cause chaos. All of the main heroes have died, and Trunks (I'm calling him
Trunks "killed") is left to fend for himself. He travels back in time to _an
alternate DBZ timeline_ (one we don't see or know much about, but there is
only 1 version of Cell in this timeline) to give Goku the medicine.
Presumably, he defeats the androids and/or give Goku the medicine, then
travels back to his own timeline. At this point, he (Trunks "killed") is
killed by [Cell](http://dragonball.wikia.com/wiki/Cell) (who is the main Cell
in the series), and Cell (main) steals his time machine to travel back in
time.

## Trunks to DBZ

Cell (main), travels to the _known DBZ timeline from the show_. He shows up
and that process starts. Trunks (the one called Future Trunks often, the one
in the show and in History of Trunks) meanwhile, in _his own, future timeline_
(likely related, see below), also travels back in time, also to the current
DBZ known timeline, to warn about the Androids. When he arrives, there are 2
Cells in this timeline, the main one, and one that already existed. The one
that is native to this timeline (called [Present
Cell](http://dragonball.wikia.com/wiki/Present_Cell) often), is destroyed with
the destruction of the lab by Future Trunks, and Krillin.

## Loose Ends

Blah, blah, blah. Cell saga continues until Cell (main) is destroyed. At this
point, Future Trunks, travels back to his own timeline, and there, already
knows about, and destroys the [Cell in that timeline (Future
Cell)](http://dragonball.wikia.com/wiki/Future_Cell). In total, there were:

  * 5 Cells (the forth is never mentioned, but likely grew into a full Cell later in the alternate DBZ timeline; fifth is assuming Future Trunks' timeline is derivative, see below)
  * 4 timelines
  * 2 Trunks from the Future
  * 2 Trunks from the present
  * 2 time machines

# The question: Why?

So why did Trunks arrive in the same Universe as Cell? Well, that's easy. He
did, and he didn't. He arrived in the timeline we were shown (the main DBZ
continuity), but the previous Trunks arrived in one in which the Future Cell
had not arrived yet. The theory of alternate timelines often involves time
travel on a single thread. That is, Future Trunks' universe likely existed
from a timeline branch in which Cell (main) had already arrived. When Future
Trunks traveled back in time, he was traveling to his past, which included
Cell (main) being there, but none of the events that had been set into motion
to lead to his timeline. His presence then altered the timeline to include the
events we saw, and to not include the ones that lead to his time.

Basically, if Trunks didn't go back in time, things _could_ have happened in a
way which lead to Trunks going back in time, which caused the series of events
in which we saw. Simple, right?

## What happened to Cell (main) in Future Trunks' timeline?

We don't know. Likely something. But we don't have anything to state what
might have happened.




# Alternate Timelines are not strict branches

Parallel universes can often mess with your mind if you're not careful. You
need to pay careful attention to both where you are, what are the
possibilities, and what has occurred to affect change. DBZ uses the alternate
timeline version of time travel, so this means that many universes can exist,
shown or not. A diagram will be easiest.

# Timeline

![Timeline for Cell/Trunks in DBZ](https://i.stack.imgur.com/x49OF.png)

There are actually 4 different timelines involved in the Cell/Trunks/Android
Saga (excluding ones created/changed by time travel).

## Cell to DBZ

The _first timeline_ is one in which the Androids have ravaged the earth, and
cause chaos. All of the main heroes have died, and Trunks (I'm calling him
Trunks "killed") is left to fend for himself. He travels back in time to _an
alternate DBZ timeline_ (one we don't see or know much about, but there is
only 1 version of Cell in this timeline) to give Goku the medicine.
Presumably, he defeats the androids and/or give Goku the medicine, then
travels back to his own timeline. At this point, he (Trunks "killed") is
killed by [Cell](http://dragonball.wikia.com/wiki/Cell) (who is the main Cell
in the series), and Cell (main) steals his time machine to travel back in
time.

## Trunks to DBZ

Cell (main), travels to the _known DBZ timeline from the show_. He shows up
and that process starts. Trunks (the one called Future Trunks often, the one
in the show and in History of Trunks) meanwhile, in _his own, future timeline_
(likely related, see below), also travels back in time, also to the current
DBZ known timeline, to warn about the Androids. When he arrives, there are 2
Cells in this timeline, the main one, and one that already existed. The one
that is native to this timeline (called [Present
Cell](http://dragonball.wikia.com/wiki/Present_Cell) often), is destroyed with
the destruction of the lab by Future Trunks, and Krillin.

## Loose Ends

Blah, blah, blah. Cell saga continues until Cell (main) is destroyed. At this
point, Future Trunks, travels back to his own timeline, and there, already
knows about, and destroys the [Cell in that timeline (Future
Cell)](http://dragonball.wikia.com/wiki/Future_Cell). In total, there were:

  * 5 Cells (the forth is never mentioned, but likely grew into a full Cell later in the alternate DBZ timeline; fifth is assuming Future Trunks' timeline is derivative, see below)
  * 4 timelines
  * 2 Trunks from the Future
  * 2 Trunks from the present
  * 2 time machines

# The question: Why?

So why did Trunks arrive in the same Universe as Cell? Well, that's easy. He
did, and he didn't. He arrived in the timeline we were shown (the main DBZ
continuity), but the previous Trunks arrived in one in which the Future Cell
had not arrived yet. The theory of alternate timelines often involves time
travel on a single thread. That is, Future Trunks' universe likely existed
from a timeline branch in which Cell (main) had already arrived. When Future
Trunks traveled back in time, he was traveling to his past, which included
Cell (main) being there, but none of the events that had been set into motion
to lead to his timeline. His presence then altered the timeline to include the
events we saw, and to not include the ones that lead to his time.

Basically, if Trunks didn't go back in time, things _could_ have happened in a
way which lead to Trunks going back in time, which caused the series of events
in which we saw. Simple, right?

## What happened to Cell (main) in Future Trunks' timeline?

We don't know. Likely something. But we don't have anything to state what
might have happened.

