Emotionally touching as it was, Captain America's return to the present at the end of Avenger: Endgame as an old man seemingly breaks the time travel rules the film established. Perhaps the most popular theory going around involves two Captain Americas living at once, but not even Endgame's writers and directors can give fans a clear answer. Luckily, since there's now a slate of projects for Phase Four of the Marvel Cinematic Universe, there are more than a few ways to fix this problem.
The root of this whole Captain America problem stems from, frankly, how poorly directors Joe and Anthony Russo and writers Christopher Markus and Stephen McFeely communicated the situation. The directors and writers aren't exactly on the same page regarding Endgame's ending and the time travel rules. The Russo brothers stand by the idea that, in living a life in the past with Peggy Carter, Steve Rogers created an alternate timeline.
Markus and McFeely claim that Steve was essentially always supposed to go back and live with Peggy (he effectively completed a time loop). This would mean that the husband Peggy references in Captain America: The Winter Soldier is actually Steve, and she kept one of the biggest secrets in human history. A lot of buzz was generated around the theory that one of the men carrying Peggy Carter's casket in Captain America: Civil War was actually old man Rogers.
While the directors and writers have given fans their contradicting answers, there's still nothing set in stone. And even if they were to give an affirmative answer to the overarching question, would that be enough? What would help this situation immensely is an in-universe answer, rather than competing answers from the directors and writers.
Looking at the upcoming fourth phase of the MCU, there are plenty of potential means to solve the "Captain America time travel" dilemma. First and foremost, there's The Falcon and the Winter Soldier series. Given that this series will likely deal with the implications of Steve passing his mantle to Sam, there's a strong argument to be made that the show will also, even briefly, discuss exactly what Steve did.
There might not be much discussion about the precise science behind Steve's time-traveling antics, but maybe a brief cameo from Chris Evans' Cap could clear a few things up. Maybe there's a conversation between Sam and Bucky to be had about whether or not they ever spotted an older Steve hanging around, thus confirming Markus and McFeely's answer.
Even the upcoming Loki series could give us some clues. Since the show is bound to follow the Loki that got away with the Tesseract in Endgame, it also lends itself to including a minor cameo from Cap. Yes, this Loki is in a branched timeline, but maybe events are meant to play out mostly the same in that universe, and old man Cap is still wandering around. Then, suppose Loki happens to briefly bump into him. This would imply something similar is happening in the prime MCU, with an older version of Steve having always been in the background.
One of the more obvious places fans might receive some concrete answers is in Doctor Strange in the Multiverse of Madness. With only a few details revealed about this film, it's already getting a decent amount of hype. It promises to be an interestingly different MCU flick. This movie could easily delve into the rules of MCU time travel a bit more. Along with that, as the name implies, the film will certainly peer into alternate realities. One of those could be where present day Steve lives in the past with Peggy after creating a new reality. This would confirm the Russo brothers' answer to the time-traveling Cap conundrum.
The What If...? series automatically lends itself to delving into alternate realities, as that's exactly what the show will be about. With infinite possibilities for how events in the MCU could take place, this series will have incredible flexibility to play with its vast array of pieces. Obviously, the show could observe a reality where Steve stays with Peggy Carter in the past.
This could go one of two ways, however. The show could implicitly tell viewers that they're looking into a completely separate reality with Steve and Peggy in the past, or events could eventually catch up with the ending of Endgame, meaning you've just been watching Steve in the background of the prime MCU. The former example proves the Russo brothers' point, while the latter proves Markus and McFeely's. Either way, the What If...? series is a hugely viable method of answering the burning question at hand.
If the Russo brothers' answer is correct, then any method for answering the question also needs to address how present day Steve crossed back into the prime MCU reality. Had he come back through the quantum tunnel, everything would be fine. But given older Steve just shows up, everything gets a little fuzzy. This is in part why the "two Caps" theory makes a little more sense; old man Steve was always in the background, and he was just waiting for everything to catch up with him.
Regardless of where this in-universe answer comes from, everyone involved at Marvel Studios needs to get on the same page about that answer, and about the time travel rules introduced in Endgame. A little more clarification will go a long way for fans.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Brie Larson, Paul Rudd, Don Cheadle, Karen Gillan, Danai Gurira and Bradley Cooper, with Josh Brolin as Thanos. The film is now available on Digital HD and arrives Aug. 13 on Blu-ray and 4K UHD.