But wait a minute, the complaints say — how can Cap be in the original main timeline as an old man? If he went back in time and stayed there, living a life with Peggy, that changes the past in some important ways, including the fact Peggy had originally gotten married to someone else and had two kids. That’s a pretty major change to history, you’d think.
Which is to say, by going back and staying in the past, Captain America must’ve changed the timeline and thus created a divergent timeline. Right?
Well, it turns out even if the answer to that is a resounding “yes,” it doesn’t matter. Because the film established something ELSE about time-travel, in a very important moment that taught Cap everything he needed to know to pull off his magic reappearance as an old man in the original timeline.
Remember when Cap, Iron Man, and Ant-Man go back to 2012 and the events of the first Avengers film to retrieve the Tesseract, and it all goes wrong? At that point, Cap and Iron Man have a discussion and it’s revealed that since they are traveling into the past rather than the future, they can use their hand-held Quantum Realm devices to pick a different time (the 1970s) in the past to time travel again. They even pick the precise location they want to travel back to. They simply type in the new date and coordinates, and poof! They vanish, traveling back to the new destination.
That’s where Captain America sees Peggy Carter by accident, and starts thinking about her. This is surely the moment when the idea enters his head that he can possibly eventually go back and spend time with her. And again, the film clearly establishes this additional special rule allowing time travelers to choose a different date in the past and travel there, without need for the big platform and someone running a machine to pull them into the future. Notice, too, that during this part of his mission, Cap only needs to grab a couple of vials of Pym Particles, but instead the film takes pains to show us he actually winds up grabbing FOUR vials — meaning he has extras that nobody else knows about. This is relevant later, so keep it in mind.
So what we know is going into the past can be done from any point in time, using the handheld Quantum Realm devices, while traveling back into the future again requires the use of the big platform and machine. Cap knows this too, and it’s how he figured out what to do to have his life with Peggy and then return to his own original timeline again.
Cap dropped off all of the Infinity Stones and Thor’s hammer, went back to the 1940s and lived a good life with Peggy Carter, and then stayed in that timeline all the way up until some time AFTER that day when the Hulk sent him back to drop everything off. By living in the alternate timeline to a point past the date he began his time travel, Cap was then able to use his Quantum device to send himself BACK IN TIME without need for the big platform and machine, and he could even pick the precise spot where he wanted to arrive — namely, that hill with the park bench, right beside the spot where Hulk, Falcon, and Bucky would be waiting for him. Or maybe he jumped back to that morning, or the day before, to spend a little time catching up on this original timeline. The point is, he used the handheld device and the special rule about traveling backward in time to arrive at the same moment there on that hill, where his friends would be waiting for him.
