After Infinity War, Avengers 4 will finally bring a closure to 10 years (and counting) of MCU movies. Thanos is expected to fall, the Avengers roster will completely change, and the most powerful superhero of the shared universe will finally join the team.
However, given the many deaths of Infinity War, fans are still wondering how the fallen heroes will be brought back. And, given leaked set photos, everybody seems to agree that Avengers 4 will feature time-travel.
But, in the absence of the Time Stone, how could time travel be achieved? The answer was provided by Ant-Man and the Wasp, the movie that followed Avengers: Infinity War. In it, it was explained that there are multiple layers to the Quantum Realm. Furthermore, the idea that time and space don't bear the same meaning as in our world was emphasised.
But, most importantly, Ant-Man and the Wasp introduced the idea of time vortexes. And, of course, this means that another way to travel through time was made available.
Speaking about this with Huffington Post, director Peyton Reed teased that time travel via Quantum Realm is possible in Avengers 4:
"Based on the science that we've set up in the first movie and this one, I won't rule out the possibility, because, again, we are dealing with a time vortex. That could happen."
Of course, this is in no way a confirmation of this fan theory. However, it does add credibility to it and explains how Ant-Man appears alongside and aged Tony Stark in battle of New York scenes.
There is another theory saying that the Quantum Realm will be used to travel between parallel Earths. However, this would be much more difficult to explain during a single movie.