It's no secret that the Quantum Realm was going to play a role in Avengers: Endgame. Let's just get it out of the way now. The promotional material for the movie has shown Earth's Mightiest Heroes in their Quantum Realm suits and we know that they are on a mission to retrieve the Infinity Stones to reverse Thanos' Decimation at the conclusion of Infinity War. With that being said, there is some real scientists backing up some of the dialogue used in Endgame. There are MAJOR SPOILERS for Avengers: Endgame below, so read ahead at your own risk.
Time Travel has been one of the constant rumors about Avengers: Endgame since the beginning and it's what our heroes use to go back and retrieve the Infinity Stones after Thanos admits to destroying them. One particularly humorous scene from the movie revolves around the group making fun of time travel movies like Back to the Future and The Terminator for their flimsy science. This is when things get a bit tough to follow in the movie and that's because Endgame screenwriters Christopher Markus and Stephen McFeely more than likely met up and studied with some physicists.
Avengers: Endgame contains talk of quantum mechanics, Deutsch propositions, The Planck scale, eigenvalues, and inverted Mobius strips, which are all based on real science. First, we need to look at the Grandfather Paradox, which involves a grandchild going back in time to kill his/her grandfather when he was a young man. Under this premise, the child would have never been born, which would mean they couldn't go back in time in the first place. Scientists sometimes call this closed time loops. Thanks to Einstein, we know about spinning black holes, which can twist time and space. This is where the quantum mechanics of Endgame comes into play.
Instead of time travel effecting your current future like the Grandfather Paradox, Avengers: Endgame goes with an alternative reality idea, which is based on the Many Worlds Theory. In short, going back in time and effecting the past would not disrupt your current timeline. Instead, it would create an alternate universe, which would branch off into multiple timelines. To avoid the Many Worlds Theory and different timelines, the Avengers plan to retrieve each Infinity Stone and place it back where it belongs when they are finished, ensuring no disruptions were made.
The quantum mechanics of Avengers: Endgame touches on modern scientific ideas about quantum time travel and the possibility of wiping out the Grandfather Paradox. According to Michael Milford and Peter Stratton, "In quantum mechanics, atomic particles are more like indistinct waves of probability." This basically means that there's a chance, or probability that something will occur during a certain time, which when combined with the Many Worlds Theory, could wipeout the Grandfather Paradox if everything is looked at probabilistically. Going back in time only gives a person a certain probability of killing their grandfather, which would, " break the causality loop." Avengers: Endgame based much of its premise on this theory.
According to Michael Milford and Peter Stratton, Avengers: Endgame did a fairly decent job of putting everything together and having it based on the science of time travel. However, there are a few hiccups and some jargon thrown in for dramatic effect, but for the most part, everything checks out and is a lot easier to understand than the actual real-life theories that are floating around. This information comes from Quartzy.