Avengers: Endgame involves time travel. This should not be a secret or a spoiler to anyone who has been following the development of the movie since last year's Infinity War. Earth's Mightiest Heroes have to go back in time to fetch the Infinity Stones in an effort to stop Thanos from Decimating half of the universe. As the movie shows, it's really not that simple, and the laws of time travel are very confusing, even for the people responsible for the movie. There are MAJOR SPOILERS for Avengers: Endgame below.
When Captain America goes back in time at the conclusion of Avengers: Endgame, he is placing all of the Infinity Stones to where they were prior to the movie. However, he decides to go back to 1948 and live the rest of his life with Peggy Carter, the love of his life. According to the Russo Brothers, this happened in an alternate reality. As Hulk says in the movie, time travel will not affect their past and future. The Ancient One speaks of time branching off of the Infinity Stones if they aren't returned. Screenwriters Christopher Markus and Stephen McFeely do not agree with the alternate reality theory.
While the Russo Brothers area claiming Captain America went to an alternate timeline when he went back with Peggy Carter, some Marvel Cinematic Universe fans disagree. Fans like to believe Steve Rogers is still in the current timeline, but with a different past and as it turns out, Avengers: Endgame screenwriters Christopher Markus and Stephen McFeely agree. They had this to say on the time travel matter.
"That is our theory. 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. 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."
While a lot of the science and terminology behind the Quantum Realm and time travel in Avengers: Endgame is based on real-life, the directors and screenwriters seem to be butting heads. This is understandable since the time travel rules of the movie are a little fuzzy for just about everybody. The Russo Brothers may have opened up a pretty big can of worms for the MCU moving forward with all of the time travel shenanigans.
Avengers: Endgame has brought about some pretty huge changes for the MCU and it's going to be interesting to see how everything works out from now. With Iron Man, Captain America, and Black Widow now gone, there are some pretty big shoes to fill for the future and Captain Marvel can only do so much in regard to where she spends her time. Regardless, it seems like the time travel debate just got a lot messier. The interview with Christopher Markus and Stephen McFeely was originally conducted by Fandango.