Warning: major spoilers for Avengers: Endgame ahead.
If you're one of the millions who has seen Avengers: Endgame in the last nine days, you're very familiar with the concept of time travel. Sure, you may not completely understand how Hulk (Mark Ruffalo) explained it just before Clint Barton (Jeremy Renner) attempted the first trial run, but it actually all made sense. The concept was later explained in an exchange between The Ancient One (Tilda Swinton) and Bruce Banner, with their conversation focusing on branch realities and how a minor change in the past regarding the Infinity Stones can and will change the future.
One of the most debated time-travel moments in the film comes from Captain America's (Chris Evans) final mission to return the stones to their respective locations and timelines in the past. This was done to prevent those branch realities from being left without their respective Infinity Stones, which also included returning Mjolnir to Asgard in 2013. As we know now, Steve Rogers stays back in time to live a full life with Peggy Carter (Hayley Atwell), and this is where the debate begins. Is Steve living in a branch reality? Is he living alongside another Captain America? Well, even the film's writers and directors don't agree on the topic, according to Screen Rant.
Directors Joe Russo and Anthony Russo believe Cap is living in an alternate reality with a different Peggy in the past. This would somewhat conflict with how an elderly Steve would be back in 2023 to pass the Captain America mantle on to Sam Wilson (Anthony Mackie), but the Russos teased there is a story there to be discovered.
Screenwriters Stephen McFeely and Christopher Markus aren't on board with this interpretation of Cap's story and feel he is living in the prime timeline, alongside another Steve Rogers. The duo broke it all down when speaking with Fandango recently."We are not experts on time travel, but the Ancient One specifically states that when you take an Infinity Stone out of a timeline it creates a new timeline. So Steve going back and just being there would not create a new timeline. So I reject the "Steve is in an alternate reality" theory." Markus admitted. "I do believe that there is simply a period in world history from about '48 to now where there are two Steve Rogers. And anyway, for a large chunk of that one of them is frozen in ice. So it's not like they'd be running into each other."
With the writers and directors feeling two completely different ways on Cap's ending, that might mean there is no right or wrong answer when it comes to his timeline. This simply could be up to the viewer's interpretation, but Markus' recent explanation might hold a little more water than the Russo brother's sentiments... depending on how you interpret the movie's time travel rules, that is.