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Spoilers ahead! Plot points are revealed here, I'll put the deeper plot points in spoiler boxes.

Time travel features heavily in the plot of Endgame. Correct me if I'm wrong, but the MCU seems to subscribe to the many-worlds interpretation of quantum mechanics. This makes any alterations to the past fine, and the "present" people making those changes return to the future they left from unaffected.

So, the plan is:

pop into the past to gather the Infinity Stones, wield them to perform a reverse snap, then pop them right back where you found them as if they'd never left at all.

This plan succeeds. However, as a result,

the pre-snap Thanos journeys into the future to protect the job he has yet to do, and is defeated in the process. This means that, although the Stones are placed back in their respective times (the Cap's last job before retiring), there's now a universe short of Thanos.

So, my question: is there now a universe out there in which the snap never occurred as well as several of the Avenger's adventures (anything after 2014, I think that's where Thanos travelled from), while at the same time there is a universe (the one we see in the films) in which all of the events of the MCU films transpired?

A further point, we're told that traveling back to kill a baby Thanos wouldn't work because they'd still be in the timeline they left from upon return (the snap one), while the other timeline would exist to no benefit of their own. This is why the mission was never about killing a pre-snap Thanos - your own past is unchangeable, the snap is locked in. Therefore, the Thanos that travelled to 2023 is forever locked in to travelling to that point and being defeated, meaning the time he came from must no longer have him.

This could also explain how the universe that we watch through the movies has a Gamora now. Because she came from a timeline in which Thanos never sacrifices her, she can be alive (remember, we're told that there's no way to reverse the sacrifice made to retrieve the Soul Stone - the only way she can be alive is if she's from a universe where she was never sacrificed (i.e. no Thanos after 2014)).

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    Do we have confirmation spoiler died? He could just as easily been sent back to his 2014 spot in the timeline with no knowledge of the future – Jimmy M. Apr 26 at 6:26
  • MCU has not established any fixed mechanism of time travel yet. The dialogue from the film indicate that they don't follow the Back to the future model for sure, meaning there don't have to be 2 different timelines. – Stark07 Apr 26 at 13:14
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    @Stark07 I think the rules were relatively clear in the film, just awkward to understand. – TheLethalCarrot Apr 26 at 13:36
  • The many worlds interpretation of quantum mechanics says absolutely nothing about what happens if you go back in time. All it tells us is how to interpret the result of measuring a quantity in a basis of states that occupies more than one eigenstate of the operator representing that quantity. Please don't use scientific terms just for the sake of sounding sophisticated, it drives me crazy. – Bridgeburners May 9 at 13:30
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EDIT: After reading the Russo brothers' comments about the movie's rules on time travelling, and watching SpiderMan: Far from home trailer, I think is clear that there are actually different realities; I'll keep my original answer as it's yet to be defined which and how many realities are out there, and if these realities where created from the events of End Game or have always existed.


It did exist, but only for a short span of time.

Spoiler Alert (Duh!)

What we learned from the conversation between Bruce Banner and The Ancient One is that taking the infinity stones would create branches in the timeline, but putting them back where (when) they belong would erase such branches, preventing the existence of alternate universes. This means that, after every infinity stone was taken, one or many branches where created.

One of these branches was the one where Thanos travelled to the future to fight the Avengers and prevent them to undo what his other self did.

Now, these branches all exist and flow parallel to the "main" timeline, until Cap goes back in time and returns the stones to their respective places and times. At this point, all the branches cease to exist (including the one where Thanos travelled to the future and left the universe "decimation-free") keeping only the main timeline (the one we know). This doesn't deletes the moment when Thanos arrives to the future, it only deletes the branch that was left when Thanos travelled to the future.

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    "that taking the infinity stones would create branches in the timeline, but putting them back where (when) they belong would erase such branches, preventing the existence of alternate universes" I'm not sure this is confirmed, though I don't remember the exact dialogue, I was under the impression that all time travel creates a branching timeline. The Ancient One doesn't want to give up the time stone because their universe will need it later (for the events in Dr Strange). But Bruce convinces her that they'll bring it back before anyone misses it. – Calvin Li Apr 29 at 21:34
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    It make sense, but I agree with Calvin, We could use more details on the universe situation. The implication the Ancient one and Bruce made was that time stone is a weapon required to keep their reality safe from creatures like Dormamu, and if it is borrowed and not returned, then the past will change since it is required for Dr Strange to win. This means the many world interpretation is true, but can be prevented if the actions do not conflict with the main timeline. Problem is, Thanos does not return which does conflict, unless Tony made it so with his snap. – Ryan May 1 at 16:49
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    There are two things to take in mind: one is that what the Ancient One says (with more or less words) is that the stones create what we perceive as the flow of time, and removing them would create a branch, or a "secondary" time line. Is in one of these branches where Thanos travels to the future and never returns. Now, before that, Bruce explains that the past cannot be changed because going to the past means that past is now part of your future, so whatever they do, the main timeline stays the same. – Josh Part May 2 at 18:34
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    I think the Ancient One is using the removal of an Infinity Stone as an example of how an alternate timeline is created, not that interaction with an Infinity Stone is the only way an alternate timeline can happen. – SeanR May 3 at 2:02
  • FWIW the writers would agree with your original explanation here (as do I actually) so you might want to update to reflect on that as the Russos are not necessarily correct. – TheLethalCarrot May 15 at 15:17
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[Lot of Spoilers ahead!]

I think that there is one timeline after the events of Endgame.

Let me explain: we see at the end of the movie that Cap does one last mission before retiring. He goes back to the past to deposit the Infinity Stones exactly where the Avengers took them from.

Banner explained to the Ancient One that if they put back the gems exactly where they took them then the timeline will not change. That means that Thanos from the past will be there when Cap goes back to the past and puts the gems back. That exact timeline will happen again, but for the people from the past. That's also why Cap took Mjolnir back with him to the past - so he could return it.

(Another note related to the timeline: I think that Cap did not came back to the present using a time machine. I think he was already there, and that young Cap is gone forever. During all these years old Cap was living among them in secret so as not to alter the timeline. That's why he was not in the middle of the time machine and that he was sitting a little further away. Just after that we see young Cap kissing Peggy (I think in the same time as old Cap gives the shield to Sam). We see also that smile on Bucky's face when he turns back and sees the old Cap.)

So after Cap puts back the stones back in the EXACT time and place they were taken from the timeline will be restored to as it was before any time travel. This means that the Thanos in 2014 will not discover that the Avengers travelled to the past and will continue his quest to gather all the Infinity Stones. He'll do everything that he did in 2018; he will snap his fingers to eradicate half the population and die three weeks later when he's killed by Thor. The timeline continues and, five years later in 2023, Ant-Man gets out of the Quantum Realm and the rest of the story is Endgame all over again.

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    Totally agree with your last paragraph. – Lefteris008 Apr 26 at 7:12
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    Can you clarify how the events of this single timeline work around the period in which Thanos travels to the future? ie given thanos was erased from existence without travelling back to the past how is there a thanos after the point he disappeared to the future? Or are you suggesting that returning the stones meant that this change never happened and in fact thanos never in fact time traveled to the future? – Chris Apr 26 at 12:07
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    @DhonJoe Yes, but the gathering of the Stones from the past is what triggered Thanos to travel into the future, and even though they were put back a second after they were taken the events of them being taken still happened, meaning the earlier Thanos always travels to the future in a past where he's triggered to meaning he's always outside of his original timeline in a universe where he's aware of his future. – Ongo Apr 26 at 12:49
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    @DhonJoe you seem to be missing the question. The point is that Thanos leaves his timeline right before the events in the original continuum in which Quill gets the Power Stone. He doesn't return. So how does the Snap occur at all unless there are multiple timelines? This has nothing to do with whether or not the Stones were returned. Even though Peter presumably can still steal the Power Stone, there's no longer a Thanks in that stream. – Paul Apr 27 at 1:11
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    Your penultimate paragraph doesn't seem to be relevant. The question is about Thanos, not Captain America's timestream with Peggy. – The Dark Lord Apr 27 at 9:15
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I've read the conversation between the Ancient One and Banner in a different way than some of the answers here.

  1. The Ancient One was using the removal of an Infinity Stone as an example of an event that would cause a branching reality. She wasn't saying that the only way to create a branch would be to remove an Infinity Stone. In fact, the mere presence of a traveller from the future creates a branching reality.

    An example of this is Tony and Scott's bungled attempt to steal the Space Stone. Loki steals it instead and escapes Thor's custody. This creates a branching reality where Loki doesn't get imprisoned in Asgard's dungeons, which could ultimately lead to Malekith succeeding with his plan in Thor: The Dark World. Now, the Space Stone hasn't been removed from the timeline, it's just not where it was in the Prime Reality.

  2. Replacing the Time Stone back in 2012 would not "delete" a branched timeline. It merely means that with the Time Stone back, the branched timeline is not a horrible future where Earth no longer has the Time Stone. Without the Time Stone returned to the Ancient One in 2012,

    Doctor Strange would not have it to be able to save the world from Dormammu. So in returning it, history still carries on relatively the same.

    There is merely a timeline where Banner borrowed the Time Stone, and a timeline where he didn't come back at all.

    To go back to the Loki example, the Space Stone hasn't been removed so can't be replaced to erase the timeline where Loki escapes.

So to answer the original question:

Yes, there is a branched reality where that character and all his allies disappear

This would have several ramifications, but of note for the question:

  1. With the

    Power Stone returned to Morag, Peter Quill regains consciousness and successfully acquires it. He wouldn't be challenged by Korath, since Ronan wasn't alerted to its presence by Thanos. Quill would have most likely completed his negotiations with The Broker, and would not have met Gamora (I don't think the whereabouts (or when-abouts) of Gamora from this timeline are currently known). It is, however, possible that the remainder of the Guardians of the Galaxy would have still been brought together.

  2. Assuming that

    the Gamora that travelled with Thanos to 2025 did not return there, nobody in the Thanos-free timeline now knows the location of the Soul Stone on Vormir. In fact, since Gamora destroyed the map of the location, it may never be found. Though the fact that Vormir is a named planet must mean that someone has been there before... maybe Red Skull only appears to those seeking the stone, and if they just show up on Vormir there's no way they'd otherwise find it?

  3. Without the existence of

    Thanos, the Asgardian refugee ship would have uneventfully made its way to Earth.

So that means that in the timeline in question, the events of the following films either don't happen at all or are changed massively:

  • Guardians of the Galaxy
  • Guardians of the Galaxy: Vol 2
  • Avengers: Infinity War
  • Avengers: Endgame

    - and the post-credit sting from Thor: Ragnarok.

All other MCU films would remain the same.

1

I think what happens is that it creates a third alternate universe. Borrowing the terminology from LP154's answer:

  • Universe A = "Main" setting of MCU films including Endgame
  • Universe B = The universe created as a branch when the heroes travel back in time
  • Universe C = (Explained below)

Explanation:

  1. The heroes (from A) travelling to the past creates universe B which branches due to the fact the stones are taken from it
  2. Thanos from B comes to A and is defeated in 2023
  3. Steve Rogers goes back to put the stones exactly where they were
  4. In doing so, it creates another branch point: Universe C (from root universe B) where the stones were taken and returned as if they never left and therefore containing both the stones and Thanos. However, Universe B would continue without the stones (and without Thanos) and possibly be shredded apart due to the lack of stones to hold it in balance.

HOWEVER, this theory does NOT fit in with

  1. Steve Rogers going back in time and remaining in the same universe (A)
  2. Why the Ancient One in B would let Banner take the stones if there was a possibility that bringing the stones back only changes one branch (C) and the original branch (B) continues unchanged (Maybe she didn't think so...?)

So to answer your question: I think, yes, there IS a universe (B) which does not have Thanos (but doesn't have the stones either)

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Yes, a universe where Thanos never snapped now exists

Let's say our heroes are in universe A.

When they borrowed the stones from the past, it created another universe. A universe where Loki escaped after the battle of NY, where Sitwell thinks Cap' is with Hydra, where Thanos never snapped because he went in universe A and was killed etc. Let's call it universe B.

At the end, Cap' went in the past to give back the stones in universe B. After getting old, Cap' came back in universe A but :

  • In universe A, Peggy died alone without having married Cap'
  • In universe B, Loki escaped with the Tesseract after the battle of New York
  • In universe B, Thanos disappeared in 2014 (he went in universe B and died)
  • In universe B, Peter Quill has never met Gamora
  • And there may be a lot of other differences...

Based on my answer here, it's somewhat linked : https://scifi.stackexchange.com/a/210783/115560

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    I don't think this is correct the alternate reality for the past is the one we see in the film, when Cap puts the stones back that alternate reality is destroyed and things continue on as we saw them previously. – TheLethalCarrot Apr 26 at 13:45
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    How can it work ? By putting the stones back in that alternate reality, how could it fix the fact that Loki escaped ? Or that Thanos disappeared ? etc. The only way to avoid paradox in endgame is if the alternate reality still exists as of today. If there is no reality where Thanos disappeared, how could he be at the end of the movie fighting the Avengers ? – LP154 Apr 26 at 13:46
  • Thanos only disappeared after the stone left his timeline and therefore was in an alternative reality. I think a stone was taken from Loki's timeline to which would mean he could go back to "normal" but I can't remember properly. – TheLethalCarrot Apr 26 at 13:50
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    "The only way to avoid paradox in endgame is if the alternate reality still exists as of today." Except it doesn't exist the stones were put back so the alternate reality was destroyed. – TheLethalCarrot Apr 26 at 13:52
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    @TheLethalCarrot There's no such thing as travelling back in your own timeline. Per the MCU, you always travel to an alternate reality with time travel. It's the most paradox-safe method to time travel, since you literally cannot create paradoxes, just new realities. We have no indication that alternate realities created via time travel are ever collapsed - just that they get put on a better track. The Ancient One isn't saying that the timeline she's in would be destroyed, just that it'd be better (since sans Time Stone, Dormammu consumes Earth in Dr. Strange Movie). – Delioth Apr 29 at 18:26

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