It may aid in writing for events and nuances as dizzying as comic book material, but when the Marvel Cinematic Universe brought the multiverse and branching timelines into the mix, surely they were aware of the complications that would come with presenting their material.
We got a glimpse of how the MCU’s branching timelines work in Avengers: Endgame, neatly summed up by Bruce Banner and his line “changing the past does not change the present,” diverting from most all time travel physics from other fictional worlds. In the MCU, whenever the past is changed, it instead branches off into a new timeline with a new set of events brought on by the change.
Loki fleshed out the MCU’s timeline physics immensely, introducing us to the Time Variance Authority, a collective of time-jumping secret police whose goal is to guard the plot of Sacred Timeline, doing so by eliminating (“pruning”) any branching timeline whose varying events create an end result that differs too much from the Sacred Timeline.
That’s the gist of it, but reading about it and learning about it while watching Loki are two different beasts entirely, and it was inevitable that some viewers would get lost along the way. One redditor was a part of these casualties, and sought answers in a post on r/marvelstudios.
For context, Loki deuteragonist Sylvie sought to destroy the Sacred Timeline and, eventually, the Time Variance Authority after they took away her future by trying to eliminate her when she was just a small child. The bombs she used, shown in the photo above, are usually used to destroy branching timelines that deviate too much from the Sacred Timeline. But, by bombing the Sacred Timeline, which is something that wasn’t supposed to happen, and is perhaps the largest deviation from the Sacred Timeline that could possibly happen, it caused a myriad of new branching timelines to occur due to the sheer size of that event.
Responders tried their best to explain everything to the user…
…but it was largely an uphill battle.
In any case, it may be for the best if we just suspend our disbelief when it comes to the physics surrounding the multiverse and time travel and the like; best-case scenario, it will all make sense with time. Worst case scenario, the plot of the MCU moves forward.