In Avengers: Endgame, audiences are presented with a completely new interpretation of time travel, and it's an interpretation that is completely and utterly blatant in dismissing and disregarding the "rules of time travel" that have been established in countless other films involving the phenomenon.
Time travel is not exactly a real thing, so when utilizing it in a film, it's up to the creative team behind the scenes to come up with rules that work in the story they're trying to tell. The only issue with these rules in Avengers: Endgame is that not even the screenwriters, Christopher Markus and Stephen McFeely, or the directors, Anthony and Joe Russo, can agree on how exactly the rules of time travel work in the film, in particular relating to the closing moments with Steve Rogers.
In the closing moments of Endgame, Rogers is the one who elects to go back in time again to return the Infinity Stones and Mjolnir to their respective places in time, but instead of coming back through the time machine, the next time we see Rogers is as an old man sitting on a bench overlooking a lake not far from the time machine.
Markus rejects the notion that Rogers lived in an alternate reality, citing the Ancient One's claims that only the removal of an Infinity Stone would create a divergent reality. Joe Russo, however, claims that when Steve returned to 1945 to live with Peggy Carter, he was living in an alternate reality.
The different interpretation of time travel in Endgame has created confusion among fans who are wondering how exactly Steve ended up back on that bench. As with anything involving time travel, there are bound to be some gray areas, but that doesn't mean the rules need to be so confusing. And, within the context of Endgame, they're really not. In fact, both Markus and Russo are technically correct in their interpretations of the ending. However, the clarification of the rules could be simpler.
The easiest way to interpret the rules of time travel in Avengers: Endgame is to consider time as a continuous line that is always moving in the same direction, forward. If we consider time as continuously flowing in one direction, and one direction only, it becomes easy to understand how both Markus and Russo are correct in their assertions based on a key moments of exposition.
When Rogers, Tony Stark, Professor Hulk and Scott Lang return to the Battle of New York in 2012, Banner is tasked with visiting the Sanctum Sanctorum to obtain the Time Stone. It is here that the Ancient One reveals that the Infinity Stones are what create the flow of time. If someone were to remove an Infinity Stone from its place in time, it would create a divergent reality in which all manner of dark things could occur. This is the basis for why Steve has to return the stones to their exact place in time once the time heist has been completed.
However, before they begin the time heist, Professor Hulk explains time travel to Lang, James Rhodes and Clint Barton. In his explanation, Professor Hulk completely dismantles the previously established rules of time travel from movies like Back to the Future, Terminator and Hot Tub Time Machine, stating that the past cannot be altered by returning to it.
The reasoning? Going back to the past becomes a person's new future, and their former present becomes their past, which cannot then be affected by their future. It's here that viewing the flow of time as a continuous line becomes the easiest way to interpret the rules.
For the sake of simplicity, let's take a look at the Battle of New York as an example. When Rogers, Stark, Hulk and Lang return to 2012 from 2023, it's akin to taking a copy of the exact instant that they return in 2012 and pasting it on the continuously flowing line of time just after they left 2023. Depending on the choices they make, things could play out exactly as they did in the original aftermath of the Battle of New York in 2012, which they did until the plan to get the stones was enacted.
By returning to 2012, nothing has changed from what audiences have come to know within the Marvel Cinematic Universe. The use of time travel in Avengers: Endgame is in no way, shape or form a retcon of what's already happened in the MCU. That much should be made abundantly clear.
The four are acting within an alternate reality in which they can alter how they, and those within that reality, experienced those years within the main timeline, but it is not a divergent reality or a branched reality. It's a reality that exists in parallel with the main reality that's already happened. That being said, the flow of time experienced by the people traversing the parallel reality continues to flow forward, which explains why Steve was an old man when he appeared on the bench.
From this point of view, Markus is correct in the sense that Steve returning to 1945 to live with Peggy doesn't create a branched reality, but he is wrong to suggest that it does not create an alternate reality existing in parallel. If Steve is not living in an alternate reality when he returns to 1945, then that moment within the film acts in opposition of the established rules of time travel in the film. It would mean that either Steve was the unnamed husband hiding in the background all these years, or the entire MCU would be up for a retcon as a result of Steve going back. Therefore, the only possible explanation is that Steve lived with Peggy in an alternate reality.
So, how did he end up on that bench in the main reality? It's an interesting question, and one that may receive a definitive answer in the future. As for possible explanations, there seems to exist two, though one is far more likely than the other.
The first, and least likely, explanation is that when Steve returned to 1945, he let everything happen as it should have from 1945 through 2023 and merely just showed up on that bench on the day of Tony's funeral to hand off the shield to Sam Wilson. This one isn't likely because, as he's living in an alternate reality, even the smallest of actions could throw the timeline off course, and that moment wouldn't be with the same exact people he just left.
For all intents and purposes, they would be, but they'd be existing within his new reality and sharing the same experiences. Plus, it would require Steve to act as Peggy's unnamed husband in the background for nearly 80 years until the young version of himself that also exists in that reality went back to return the stones again.
The other, and far more likely, explanation is that, upon returning to be with Peggy, Steve lived out his life exactly how he wanted and managed to return to the main reality in 2023 for that moment with Sam. How did Steve manage to do that? The most likely answer would be through Doctor Strange. This explanation is not without fault, however, as there would still be a time frame in which there were two Steve Rogers in that reality, but it seems the more likely of the two given the nature of Steve Rogers.
It doesn't appear as though the Russo brothers or Markus and McFeely are all too concerned over the logistics of how Steve returned, though. Frankly, it's not all that important for the moment itself. Far more important was the passing of the torch, or shield in this case, to Sam, and the knowledge that after years and years of self-sacrifice for the betterment of others, Steve finally got his dance.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos.