Spoiler alert: This post has massive spoilers for Avenger: Endgame.
Watching Avengers: Endgame was one hell of an exhilarating experience. However one of the most mind-boggling aspects of it was time travel.
There are no rules to the phenomenon of time travel as such. And whatever we do know about it comes from the fictional universe itself.
So it’s safe to assume that one has the artistic freedom to approach time travel in a way that suits the plot in the most ‘logical’ way.
Avengers: Endgame uses a linear form of time travel for the most part.
And towards the end, it does get confusing, breaking its own rules from time to time. So we tried to wrap our heads around what it entailed in the film and tried simplifying it as much as possible.
In the first act of the film, Bruce Banner nullifies previous cinematic representations of time travel, like Back to the Future. He clearly states,
Time doesn't work that way... If you travel to the past, that past becomes your future, and your former present becomes the past, which can't now be changed by your new future.
Essentially the concept of time travel here is multi-dimensional.
Consider each pocket in time as an independent location and you travelling to it as your current self. Time for you is still advancing at a natural pace, yet you are just placed at a different location in time while this is happening. An earlier version of you residing in the past timeline still exists. You, who’s a different and older you is a mere visitor in this past.
This is succinctly explained by Nebula's killing of her own past self which ensures she is unscathed.
In this school of time travel whatever has happened is set in stone. Just like what you do in the future can’t change your past, what you do in the past cannot change the present.
However, a drastic change could create an alternate timeline where said changes will create an alternate reality affected by it.
As explained by The Ancient One to Banner, something as drastic as changing the location of an infinity stone will tend to create an alternate timeline.
But since the Avengers aimed to simply ‘borrow’ the stones from the past the timeline was meant to stay intact.
Bruce also mentions,
Once we're done with the stones, we can return each one to its own timeline at the moment it was taken — so chronologically, in that reality, it never left.
Basically, the Avengers saved the universe. And whatever ‘alternate timelines’ were to be created with the displacement of the stones were immediately erased as soon as they were restored.
Now, while the stones and Mjolnir were put back at the exact place and time from where they were taken, the Space stone is still absconding along with 2012 Loki. This is never explained in the film so it is essentially open-ended.
Whether they go retrieve it in future projects or this creates an alternate timeline for Loki is yet to be known. All we can speculate is that this version of Loki’s reality may be the setting for the upcoming show about him on Disney Plus.
Another loose end is the timeline where Captain America, aka Steve Rogers, lives a long fulfilling life with Peggy Carter and ages naturally. So in this narrative, Cap may have gone on that date with her after all or met her after the events of the series Agent Carter.
One explanation for this is that while this timeline’s Cap was cryogenically frozen our old-man Cap went back and let life take its natural course. He may have explained things to Peggy, hidden or changed his identity, and had a family life with her.
One can argue that in the scene in Civil War, when Peggy is on her death bed, there are only pictures of her kids and not her husband. Even the mention of her husband is alluded to without a name or anything specific. Could this have been the alternate Steve Rogers all along?
Even in the scene where he spots her in her office, Cap finds a pre-transformation picture of himself which possibly means she’s either still mourning his loss or simply has an old picture of her husband on her desk.
While the film is neatly packed, some things could have been left open-ended on purpose. Possibly to explore in future MCU projects.
Only time will tell whether Marvel will explain a multiverse timeline in the future where all realities have a parallel existence. Until then we’re just going to keep re-watching Endgame and soak in the complex fabric of time in the film.