Since her debut in 2019's Captain Marvel, Carol Danvers has proven herself to be one of the most powerful players in the Marvel Cinematic Universe. From discovering the true nature of her cosmic abilities and mastering them in her solo film to her heroic eleventh-hour entrance in Avengers: Endgame, where she singlehandedly destroyed Thanos' ship, the Sanctuary II -- and held her own against the Mad Titan one on one -- there's no denying Danvers is incredibly powerful. There is one thing about her, however, that hasn't remained as consistent as her superpowers: her haircut.
First sporting a shoulder-length cut in Captain Marvel, Carol's hair had grown out slightly by the time Marvel fans saw her next at the beginning of Endgame when she rescued a stranded Tony Stark and Nebula in space and returned them safely to Earth to discover the aftermath of Thanos using the Infinity Stones. The next time she appeared, in a virtual meeting with the remaining Avengers after the Blip, Danvers was sporting a drastically-different short, choppy cut -- a detail that is glossed over aside from a quick throwaway line of dialogue from Rocket (Bradley Cooper). The next time Danvers appears in the film is in the final battle with Thanos, still with the same short haircut, which she keeps through the end of the film.
Marvel fans have their own speculations on why Danvers has gone through so many changes in her short amount of screen time in the MCU, and there could be many explanations that just haven't come to light yet. A reason for Carol Danvers' drastic chop could be for more practical purposes than anything; longer hair is more susceptible to getting in one's face while flying and also puts one at a disadvantage when fighting enemies if it isn't pulled out of the way. Even though she has a helmet for her intergalactic travels, it still might have been more convenient for Danvers to not have to worry about her hair always getting in her face, hence the chop.
Or, at the very least, it could just be a subtle way to reflect the amount of time that has passed since Danvers returned to Earth to answer Nick Fury's distress signal, similar to how Steve Rogers shaved his iconic beard and Tony Stark's hair lightened over time in the earlier parts of Endgame. Another explanation for Carol's shorter hairstyle in Endgame could be that directors Joe and Anthony Russo wanted to reflect Carol's shorter hairstyle in Marvel Comics, where she's often portrayed with a shorter, edgier hairstyle. There's no denying the shorter look definitely makes Danvers look tough.
There is one fan theory, however, that has more heartbreaking undertones. Reddit user CosmicBlooded speculates that perhaps Carol shaved her head before Avengers: Endgame to show support for her best friend Maria Rambeau, who fans learned in WandaVision had passed away from cancer two years after Thanos' snap. Maria and Carol's friendship was a huge part of Captain Marvel and largely the reason why Carol was able to remember her past and unlock the full strength of her powers, so there's no doubt Carol would do something like this as a sign of solidarity while her friend was succumbing to the deadly disease. Perhaps this may also be a theme explored in the upcoming Captain Marvel sequel The Marvels, which will feature Danvers, as well as Maria's daughter Monica, who was re-introduced as an adult (Teyonah Parris) in WandaVision.
Captain Marvel has featured in two Phase Four titles: Shang-Chi and the Legend of the Ten Rings and Ms. Marvel, though both appearances were brief cameo roles in the post-credit sequences. Despite having such a short amount of screen time in Phase Four, it's hard to miss the fact that her hair has grown out considerably since Endgame, though again, the scenes were so brief that her change in appearance wasn't mentioned. Ms. Marvel's post-credit sequence appears to tie directly into The Marvels, which will see Kamala Khan meeting up with her idol Carol Danvers and Monica Rambeau as Photon for what is sure to be a cosmic adventure and will hopefully shed some light on this plot hole.