The Bootstrap Paradox is thought to be the most respected theory in the connected timeline. Why? It follows all the rules of the constant past concept without breaking those laws. Basically, in this paradox, you experienced something in the past that was always going to happen. It is part of a casual loop.
In this format, one event causes the second event. Yet it was the second event that was actually the cause of the first. A great example of this can be seen in DC Comics’ The Flash character. Reverse Flash goes back in time to kill young Barry Allen but fails, then the Reverse Flash decides to instead kill Barry’s mother.
Barry got himself out of the home but could not save his mother. By going back in time to save his mother, he’d never become The Flash. Therefore, he has to let his mother die to become the hero the world needs. If she never does, he does not become a forensic specialist for the CCPD the night of a Particle Accelerator Explosion, which gave him his powers. Yet if he’s never The Flash, he cannot run back in time to stop himself from being killed either. Which creates a loop of needed actions.
It’s interesting that we bring up The Flash in this scenario of time travel. He actually does go back in time to stop Reverse Flash from killing his mother in the Flashpoint Paradox story-arc. This is a key part of DC Comics that causes a few rifts in the multi-universe itself and ends up causing a Crisis Event.
In any case, when Flash does this he technically creates a new timeline and universe where his mother exists yet he is not The Flash. A lot of things are the same but many things are not. Bruce Wayne is killed instead of his parents, causing his father Thomas to become Batman while his mother actually becomes this universe’s Joker.
Superman does not land in a field but rather, in the center of the Metropolis. Wonder Woman and Aquaman have an affair leading to the death of Mera, the Queen of Atlantis. Thus causing a war that will wipe out the planet. This must be changed, so Flash finds a way to recreate the conditions that gave him powers, nearly killing himself to do so. All to go back in time. Yet this does not stop changes from happening because it still causes a rift where a third universe or timeline comes about.
If we did go back in time, it is most likely that we would end up being connected to the laws of the second timeline theory we proposed. This is the timeline where the past is constant and cannot technically be changed. When it does change, it would then create another universe branched off on its own.
Basically, if we kill Baby Hitler then we will not see the result of this in the timeline we return to. Hitler always existed in our timeline and always will. However, the interesting notion is to consider how we could end up remaining in this Hitlerless universe. One would conclude the only way to do this is to remain in the timeline we’ve gone back to.
Therefore, the changes you make would branch off a different timeline and you’d remain in it. However, you might not have to remain behind. It is interesting to note that if we did use the mirrored parallel universe to go back in time, to begin with, we’d likely be able to visit this new universe we created eventually. If we can access one universe, what stops us from accessing another? We just shouldn’t assume our time travel changes will affect our present timeline.