WARNING! SPOILERS GALORE!
What are multiverses, timelines, dimensions and realms?
The Many Worlds quantum interpretation:
The concept of a multiverse is borrowed from the Many-worlds interpretation of quantum mechanics, in which all of reality splits into n universes when a random, quantum event of n possible outcomes happens. Instead of exactly one of the possible outcomes happening, all outcomes happen, one per newly-created universe.
From the start of time, till now, a near infinite amount of universes have been created, all stemming (presumably) from the same initial event (the Big Bang, for example), and all of these constitute the multiverse.
Branching timelines conceptualization of time travel:
On a far more fictional note, is a popular conceptualization of time travel found across fiction. It is quite handy, as it avoids all kinds of causal paradoxes. It is the conceptualization in which you never change the past of your own timeline; instead, the second you travel back in time, you start an off-shoot timeline from your own timeline. In this timeline, things go on differently due to your influence, whereas the timeline you came from stays on its usual path.
This is the conceptualization chosen by the MCU writers, as explained by Bruce Banner / Smart Hulk in Avengers: Endgame (2019).
How these fit together in multiverse of the MCU:
Universes and timelines:
They amount to the same thing, despite being created by different processes. There is, in function, nothing differentiating a universe created in the typical, quantum way, and a universe created by a time travelling influence. This actually makes perfect sense. If someone travels back in time to your time, that means they're coming from the future. If one is living in a quantum world of chance, your future is uncertain. Thus, whether or not any person from your future is going to arrive at any point in your present, is uncertain. As such, it amounts to a random, quantum event.
Now, if you're able to travel back to the timeline you came from, you're actually both time travelling AND multiverse travelling, since due to your time travel, you've created a new universe from which you're returning. That means that when 2014 Thanos travelled to the main MCU timeline in 2023, he was time travelling and multiverse travelling.
Dimensions and realms:
So, what about dimensions, like the Dark Dimension where Dormammu resides? Due to the lack of explicit explanation, they could be anything. There's two main possibilities: either they're entire universes within the multiverse, or they're a segment of a single universe.
If they are simply alternative universes, then they probably changed drastically early on in the universe's creation, or went through some great transformation. Perhaps the starting conditions of the Big Bang is some list of presets that were randomly selected; and so maybe different dimensions are (sub-)multiverses that differ in these presets. The same goes for realms, which seem to be used somewhat interchangeably with dimensions. Supportign this possibility is the fact that in Doctor Strange: Multiverse of Madness (2022), we see Dr. Strange and America Chavez falling through many different worlds, some of which are as different from the "normal" reality as the Dark Dimension is. I find this to be the directors telling us that there are no limitations to the universes found within the multiverse (as well as taking an excuse to dazzle us with acid-y visuals); the universes they show seem to differ in their fabrics of reality and laws of physics.
An argument for the other possibility, where dimensions are a part of a single universe, is the existence of incursions. Unless Dormammu has somehow circumvented the normal process of incursions (as seen in What If...? (2021—) and Doctor Strange: Multiverse of Madness), his ability to simply invade "other dimensions" seemingly without causing normal incursions (see Doctor Strange (2016)) would suggest these dimensions are parts of a single universe.
So, what are realms then? I think realms also refer to parts of a universe, though these parts aren't necessarily separated by anything special. From usage, it seems realms can refer to physical worlds within a dimensions, that are separated from each other simply by space. As an example, take "the Nine Realms", who are just different planets within the main, "normal" dimension.
So, assuming the definition of dimensions that sees them merely as parts of a single universe, being separated from each other through some kind of special barrier; what dimensions have been featured up-until-now?
- The normal dimension, where most movies take place.
- The Dark Dimension, featured in Doctor Strange.
- Ta Lo, featured in Shang Chi: The Legend of the Ten Rings (2021).
- The Mirror Dimension, featured in all of the Doctor Strange movies.
- The Duat, the underworld featured in Moon Knight (2022).
- The Field of Reeds (A'Aru), the paradise afterlife featured in Moon Knight.
- The Ancestral Plane, the afterlife feautred in Black Panther (2018).
- Valhalla, the Asgardian warrior afterlife, featured in Thor: Love and Thunder (2022).
- The Noor Dimension, featured in Ms. Marvel (2022).
- The Soulworld, where Thanos goes to in Avengers: Infinity War (2018) after the snap.
An eleventh addition is possibly the Quantum Realm, featured in Antman (2015) and Antman and the Wasp (2018), though whether this counts as a dimension in it of itself is debatable. Also, it is important to know that, both due to the uncertainty of what dimension even means in this franchies, and also due to the lack of explicit ontological descriptions of these places, the above list is debatable in it of itself. For all we know, Valhalla is just some server planet that your consciousness can be sent to, in theme with the "advanced aliens" description of Asgardian people that the early days of the MCU gave.
There is also what I call the "multiversal interrim", where Aatu the Watcher resides in What if...? (2022), which I guess can be seen as a dimension of the multiverse; as opposed to the normal dimension of the multiverse, in which all the universes exist.
See this article for a slightly different take on exactly this issue.
The Sacred Timeline:
In the 31st century, an Earth scientist, by the name of Nathaniel Richards, discovers time travel / multiversal travel, and so do the many different versions of him. This causes an all-out multiversal war, of which the aformentioned Richards reigns supreme; this is He Who Remains, which we see at the end of time, in the finale of the TV series Loki (2021).
There, we are explained that all the timelines and universes of the multiverse are bundled together in a so-called "Sacred Timeline", that conforms to a set of events as defined by He Who Remains. There is some freedom, and thus there are actually multiple universes/timelines in this Sacred Timeline, despite what the singular tense implies. The TVA (Time Variance Authority) are created in order to prune away any universes/timelines that stray too far off, whether this straying is caused by time travel, or a random quantum event that wasn't supposed to happen. Given that these random variations of events are always going to be happening from time to time, the TVA has infinite work ahead of them (and before them, hehe).
So, to be completely clear, this doesn't mean that time travel is illegalized by the TVA; it is only illegal if causes timelines that stray too far. So, what about the Avengers' time travel in Avengers: Endgame? Loki asked this same question, and Mobius' answer was, "that was supposed to happen". What I take this to mean, is the following:
The main MCU timeline, as He Who Remains defined it, involves the Avenger's time travelling. The alternate timelines they create are either pruned away, or, stay within the acceptable treshhold of variance. Given that Steve Rogers is able to travel back to the alternate timelines, and put the infinity stones back, the time lines were stones were taken weren't pruned immediately at least. As for the timeline that offshooted from the 1970s, we have no idea. It could have been pruned immediately after Steve Rogers and Tony Stark travel back for all we know. And the timeline where Rogers lives happily ever after with Peggy Carter was obviously not pruned away, given that Rogers was alive to travel back to the main MCU timeline.
Now, we see some pretty diverse universes in What if...?, Spider-Man: No Way Home, and Doctor Strange: The Multiverse of Madness, so what then happened to the Sacred Timeline? It was destroyed at the end of Loki, thus returning the multiverse to its former diversity, hence the universes that we see in the rest of Phase Four. It also led to Kang The Conqueror taking over the TVA, though he apparently did not chose to create a new Sacred Timeline. Exactly what his role will play, and how his existence impacts the way this mechanism works, remains to be seen.
The universes of the MCM:
From here on, I will refer to all the universes featured in Marvel's cinematic works as the MCM, an acronym for Marvel Cinematic Multiverse. This multiverse does not contain the various universes seen in the comics, and I'll explain why later.
Universes definitely a part of the MCM:
So, what are the universes of the MCM? Well, that depends, but here's my bet:
The main timeline of the MCU, 616 (see here for a list of all movies, and here for a list of all TV shows)
The MCU-offshoot timeline where the time stone, sceptre and tesseract are taken in NYC. (2012)
The timeline offshooted from the above timeline, where Pym Particles are stolen in New Jersey. (1970)
The MCU-offshoot timeline where the soul stone and power stone are taken from Vormir and Morag, respectively. (2014)
The MCU-offshoot timeline where the reality stone is taken from Asgard. (2013)
The MCU-offshoot timeline where Steve Rogers travels back to live with Peggy Carter. (Probably 1940s/1950s)
The "Marflix" (Marvel + Netflix) universe, where the TV series Daredevil, Jessica Jones, Luke Cage, Iron Fist, The Defenders and The Punisher take place.
The Marvel-ABC universe, where the TV series Agents of S.H.I.E.L.D, Agent Carter and Inhumans take place.
The Young Adult Marvel universe, where the TV series Runaways and Cloak and Dagger take place.
The Adventure Into Fear universe, where the TV series Helstrom take place.
Raimi's Spiderverse, where the Spider-Man Trilogy takes place.
The Amazing Spider-Man universe, where the The Amazing Spider-Man and The Amazing Spider-Man 2 take place.
Sony's Venomverse, where Venom, Venom: Let There Be Carnage and Morbius take place.
– n. Miscellaneous universes shown in What if...? and Doctor Strange: Multiverse of Madness, like 838.
The MCU timelines:
To make entries 1. through 6. a bit easier to digest, I've included this diagram:
Credit: MCU Exchange, Charles Villanueva
Bodies of works that possibly exist within the MCM:
The universe of Columbia's Spider-Man: Into the Spiderverse (2018), and the universes that its characters come from, might be a part of the MCM, given how much Disney and Sony have collaborated the last years with their superhero movies (especially those pertaining to Spider-Man).
The X-Men series, that consist of seven or eight universes/timelines, may be a part of the MCM due to Disney buying 21st Century Fox.
In the above picture, three of these universes/timelines are shown. In Deadpool 2 (2018), there's the timeline where Deadpool dies, and the timelines created by Deadpool's post-credit time travelling; the timeline where Vanessa is saved, the timeline where Peter is saved and the timeline where the Deadpool of X-Men Origins: Wolverine (2009) is killed (thus also requiring multiversal travel to his destination, instead of just for his return). Deadpool also kills the Green Lantern, though this might not be a universe within the X-Men multiverse/MCM at all, but rather a universe within the DC cinematic multiverse, thus meaning Deadpool performed omniversal travel. However, it could be that some universe in the X-Men multiverse/MCM just happens to have a person identical to the Green Lantern exisiting in the DC cinematic universe.
The two Fantastic Four universes produced by Fox, containing Fantastic Four (2005) and Fantastic Four: Rise of the Silver Surfer (2007) in the same universe, and Fantastic Four (2015) in another universe, could also be a part of the MCM, due to Disney's aforementioned purchase of 21st Century Fox.
This same argument can be applied to all of Fox's Marvel-derived content, like e.g. the 2003 movie Daredevil. Other than that, there's plenty of miscellaneous movies and series based on content from Marvel Comics not under Disney's partial or full ownership (like Universal's 1986 Howard the Duck), that could be considered a part of the MCM. That depends on what Kevin Feige considers "canon" for the multiverse, though any official statement on this could be legally problematic, considering that they're outside of Disney's ownership. That said, Netflix is also not under Disney's ownership, but Marvel Studios did co-produce the series, which may change things (not sure about the details of the contracts involved). Given that we see different versions of Marflix characters in the MCU (or possibly, it's just them), it seems that there is no legal issue in Feige considering Marflix as a part of the MCM. The same can probably not be said for other works based on Marvel Comics that aren't owned by Disney or that weren't (co-)produced by Marvel Studios.
Here's a comprehensive list of all Marvel-based movies, and here's a comprehensive list of all Marvel-based TV series.
Here's a link to a list of all of the non-MCU TV series in the MCM.
The 12th-listed universe was previously referred to as the SSU (Sony's Spider-Man Universe), though now that name is now used to refer to the 13th-listed universe.
The list of TV series in "the main timeline of the MCU" has one exception, that being What if...?, which is a multiversal series that doesn't even include the main MCU timeline.
In the list of universes, the Marflix universe is listed as separate from the main timeline of the MCU. This is not certain (see this article for a weighing of both possibilities, as my post is already very long), so I made a choice here. Given the focus on upholding continuity that Feige has, I don't think he would allow the myriad of plot holes that come with canonizing the Marflix series as a part of the main MCU timeline.
Some may wonder if the Venomverse, The Amazing Spider-Man universe and Raimi's Spider-Man Trilogy universe actually all exist in the MCM, given that Sony owns them. Well, Spider-Man: No Way Home (2021) ensures that, so that's the end of that.
19999 versus 616
The MCU has been referred to as 19999 on multiple occassions (see Marvel Database for an example), which apparently originates as a joke made by Feige some time ago. In the movies however, it has been referred to as 616 on three occassions. Firstly, it is labelled as "Earth-616" on Erik Selvig's blackboard in Thor: The Dark World (2013). Secondly, it is referred to as 616 by Mysterio in Spider-Man: Far From Home (2019). Thirdly, and most importantly, it is labelled as 616 by the multiverse scientists in Doctor Strange in the Multiverse of Madness. I'd say that the movies take precedence over what has been said previously, especially consider that such comments originated from a joke.
However, there's another universe called 616: the prime universe in Marvel Comics. So, how does one reconcile this? I see two possibilities:
- The scientists that label universes make mistakes/use different nomenclatures, and ended up naming two different universes with the same number.
- The MCM and Marvel Comics Multiverse are different multiverses, within the Marvel Megaverse.
If 2. is correct, then that could explain the clash between 19999 and 616. Perhaps 19999 is the label of the entire MCM, whereas 616 is the number of the main timeline of the MCU within he MCM. If 1. is correct, then in theory, you could have meetings between comic characters and cinematic characters. Personally, I believe 2. is correct. I reckon that the Marvel Megaverse is split into four multiverses; the comics multiverse, the cinematic multiverse, the video game multiverse and the cartoon multiverse; one for each medium, explaining their fundamentally different natures of existence.
Outside of this, there's the omniverse of all fiction. As a sub-set of the omniversal universes, we have the Marvel Omniverse, that contains all universes with content based on Marvel publications, some of which exists within the aforementioned Marvel Megaverse, and some of it potentially existing as outliers.
As an aside, some might be bothered by how Erik Selvig and/or Mysterio were able to know that their universe is called 616. This can perhaps be explained by them experiencing alternate versions of themselves (through dreams) that called their universe 616, thus giving them the idea. It doesn't make too much sense, with fan-pandering and easter-egging being far more likely; but if you've gotta force a logical explanation on it, this is the only one I know of.
How the Ancient One's explanation of time fit into all of this:
See 0.40 to 0.59 of this video for a source to the following quote:
The infinity stones create what you experience as the flow of time. Remove one of the stones, and that flow splits. Now, this might benefit your reality, but my new one, not so much.
—The Ancient One.
To some, it might seem like she is saying that the only action that can create new timelines, is that of removing an infinity stone (at least, that's what Sheev Talks seem to say in this video). I think she is saying something far simpler, just in a stupid way.
She knows that alternate timelines can be created in other manners. She is simply acknowledging that removing an infinity stone from her reality is going to have extreme consequences; far greater than those that have already been caused by the initial time travel. And that doesn't seem too unreasonable. You visit her time, and that's one thing. You steal a fundamental McGuffin of her reality, and that's another. Perhaps not returning the stone would amount to a Nexus event, meaning that her reality would be pruned. Or perhaps the absence of an infinity stone would just destroy the universe completely. Either way, it doesn't benefit her universe.
How America Chavez fits into all of this:
America Chavez is the only version of herself in the entire MCM, according to Doctor Strange: Multiverse of Madness. I believe that in comics terms, this makes her a multiversal constant. However, it also makes her a pain in the ass, conceptually speaking. From 16.50 to 18.14 of the linked-to video from Sheev Talks, this is brought up. He mentions that whenever America Chavez exists in a universe that is split into two, she too is also split into two, thus contradicting her existence as the sole version of herself.
Well, that's assuming she is in fact split into two. What if, whenever a quantum event happens, she just randomly, or perhaps non-randomly, "follows" only one of the outcomes. In the universe(s) that wound up with the outcome that she did not follow, she simply vanishes. That means that there's a gazillion versions of Doctor Strange that's chilling with America Chavez, before she suddenly poofs into thin air. That's a pretty comedic thought, and it works perfectly fine. It might bother some people that there are alternate stories of Doctor Strange and America Chavez in which the whole journey just randomly ends because she inexplicably disappears, but those people need to be reminded that there's a gazillion versions of the story where Doctor Strange is just randomly shot in the head too. The multiverse's diversity necessitates versions of events you don't like; the story that we follow is the one that the writers wanted to show, but it is just one version of a gazillion.
But hey, maybe she does split in two? How do we know if her claims are actually true? She bases it off the fact that she's gone looking, and never found another version of herself. Well, assuming every version of her has the same power... then duh! Maybe most versions of her are running around looking for her in an a nigh-endless multiverse, and thus she's pretty hard to find, as opposed to someone that just stays still in most universes.
I have too much time on my hands.