Since Avengers: Infinity War's cliffhanger ending, fans have speculated that the heroes of the Marvel Cinematic Universe will rely on time travel in Avengers: Endgame to undo Thanos' iconic snap that saw half of all life in the universe being wiped away. After all, with Peter Parker visibly alive and well in Spider-Man: Far From Home in a film reportedly taking place after Endgame, the reversal of Thanos' actions has always seemed like something of an inevitability.
Fan theories involving the Avengers resorting to time travel have been fueled by leaked set photos from Endgame, showing a recreation of The Avengers' climactic Battle of New York. This time, the carnage is revisited by a visibly older Tony Stark and Scott Lang, the latter of whom wasn't present at the battle at all. In the photos, both characters are seen sporting a mysterious device on their wrist, one also seen prominently on a leaked action figure of Thor as a piece of tie-in merchandise for Endgame. This device is likely the key for the Avengers to travel through time as they return to key moments in the history of the MCU, though how that might work is still unclear how.
Given Stark's older appearance, a time jump is a virtual certainty for the Infinity War follow-up. An older version of Lang's daughter, Cassie, is rumored to appear in some capacity during the film, and 13 Reasons Why actor Katherine Langford has been cast in a mystery role, widely speculated to be Tony Stark and Pepper Potts' teenaged daughter. As to why Tony appears older while Scott Lang appears relatively unaged since his appearance in Ant-Man and The Wasp, there have been theories that the passage of time operates differently in the Quantum Realm where he was last seen, as well as theories that Scott is an inadvertent time traveler himself.
Another possibility that has arisen based on set photos is that Tony uses one of his inventions, the Binary Auto Retro Framing (B.A.R.F.) device first introduced in Captain America: Civil War, to travel through time. Photos of the device seen in the Battle of New York recreation have leaked online, suggesting Tony may have modified the device to help him time travel with Scott in order to retrieve the Space Stone from Loki the last time it was on Earth.
The question, of course, remains how the characters will head back to the past, how Tony Stark will develop this previously unseen technology. It has been obvious since the Battle of Titan, at the end of Infinity War, that Doctor Strange is playing a longer game. No other MCU character has manipulated time as often as the magical superhero and, after glimpsing millions of possible futures and his connection to the Time Stone, the Sorcerer Supreme may have a hand in the theorized time travel despite his own dusting at Thanos' hands.
Another possibility involves the Quantum Realm. Mentioned by the cast and crew of Ant-Man and The Wasp as a key factor in Avengers: Endgame, Scott Lang was last seen trapped in the alternate dimension after the dusting of Hank Pym, Janet Van Dyne and Hope Van Dyne while he was exploring it. One of the last things Janet mentions to Scott before her death is the existence of time vortices within the Quantum Realm, suggesting the conventional laws of time can be bent or even broken within it. With the new suits worn by Thor, Black Widow, Hulk and Rocket Raccoon looking similar to Pym Tech, the Quantum Realm and Pym's mobile lab will both likely reappear in Endgame.
One of the biggest wild cards in Endgame is the arrival of Captain Marvel, despite her upcoming origin film being set over 20 years in the past of the MCU. While there was speculation that genetic enhancement by the Kree has slowed Carol Danvers' aging, allowing her to appear relatively unchanged decades later, comments by Samuel L. Jackson suggested the new MCU hero may be capable of time travel on her own. Perhaps the key for Tony Stark figuring out the secret to travel back in time comes from the Avengers' latest addition.
From set photos of previous moments in MCU history revisited by different incarnations of the characters to mysterious new devices, there are a plethora of theories supporting the idea that time travel playing a major role in Avengers: Endgame. With rumors that the film will have the heroes go as far back as the '70s, changing history is virtually guaranteed to be at least part of the Avengers' final desperate bid to stop Thanos and restore half of all life in the universe. The only real question that remains is how exactly the heroes will find a way to make that possible.
Avengers: Endgame will arrive on April 26. Directed by Joe and Anthony Russo, the film stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Chadwick Boseman, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Samuel L. Jackson, Cobie Smulders, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Tom Holland and Anthony Mackie.