Special relativity the twin paradox.

[can't_think_of_a_name]

When I was learning about the twin paradox I learned that in Bob's frame the space time diagram is represented by the diagram on the far right.
In Alice's frame the diagrams are represented by the far left and middle diagrams.
Doesn't this assume acceleration because there are 2 frames. How do I solve if there are no acceleration?  

https://imgur.com/a/mEaDPcP

Edited October 18, 2020 by can't_think_of_a_name

[Markus Hanke]

3 hours ago, can't_think_of_a_name said:

Doesn't this assume acceleration because there are 2 frames. How do I solve if there are no acceleration?

In the twin paradox, there is always acceleration in one of the frames, even if just for a very short time on a very short section of the world line. If there is never any acceleration in either one of the two frames, then it is not a “twin paradox” type of scenario. It is the very presence of acceleration - no matter how short in duration - that destroys the symmetry between the two frames.