Avengers 4 Theory: How Memory, B.A.R.F. And The Quantum Realm Explain Time Travel
2. The Quantum Realm And The Key
The handiest thing for the Russos in their agenda to explain how time travel might be possible in the MCU is that they have what amounts to a narrative skeleton key in the shape of the Quantum Realm. So far, it's vaguely defined so there's scope to basically just use it to explain ANYTHING because it's such an alien concept.
Concept art for Ant-Man & The Wasp suggests the film was actually going to explain far more of the Quantum Realm, including the concept of the Nexus, which concept artist Jackson Sze revealed would have been the heart of the realm and would have allowed anyone to use their own memories as a time travelling device, essentially:
“Early on, there was almost a Nexus inside the Quantum Realm that they wanted to explore, which was a quantum memory palace. I think they were discussing ideas about how Hank might actually be able to go into his own memories and change history.”
Ultimately, that concept didn't make it into the film, but it shows that Marvel have entertained the idea of memory as the key to time travel. The other key point to remember here is that Quantum Energy is what defines the Quantum Realm's various "powers" and which probably powers both the Nexus and the time vortexes Janet Van Dyne warns Scott Lang about in the post-credits scene of Ant-Man & The Wasp. In other words, Quantum Energy and creating actual rifts in time go hand in hand.
What if the harvesting of Quantum Energy - as introduced in Ant-Man & The Wasp - is the most important thing for understanding how the heroes will time travel?
So, what if the surviving Avengers use Scott Lang's knowledge of Quantum Energy (when he finally escapes the Realm) in combination with Tony Stark's ability to project memories using B.A.R.F. to create a doorway through the Quantum Realm into those memories? What if Quantum Energy effectively becomes the plutonium that Stark powers his B.A.R.F. Delorean with?
The reliance on Quantum Energy for time travel would also provide Marvel with a means to close their own loophole too and ensure that everything in future MCU movies can't just be messed with through time travel, robbing it all of its real stakes...