Taken from a thread on B-net. I found this interesting.

As the Master chief and Arbiter fly away from the ark, the cataclysmic explosion from the ark and halo collapses the portal just as the ship enters, leaving MC and Cortana drifting in unknown space. We then see them drifting undeniably towards a forerunner planet (There are forerunner symbols on it, and the game file for the last cut scene is called "forerunner_planet"). The symbols upon the planet light up, whether this is due to the sun cresting the horizon of the planet, or other unknown factors is up for discussion.

As the portal collapsed, we have no idea what the repercussions of this force was, it is just as likely that MC and Cortana were teleported through time as them being teleported through distance. So the question just as viable as "Where the -blam!- is Master Chief?" is "When the -blam!- is Master chief?". We already know where he is, the forerunner planet. But I know when.

A large factor is Mendicant Bias, an intricate character of Halo 3, who we got limited insight and knowledge of his brilliant character and story. In the last terminal, he speaks of wishing to gain redemption of his sins against his masters (The forerunners), and has promised to help you and guide you. This will lead to attempted saviour of his masters. We didn't notice any help when we continued on to the end. What Mendicant Bias had in mind was saving the forerunners, the only way he could, by sending Master chief back through time to aid them in their futile battle against the flood. This isn't the first time he has done this though.

The theory of time being a loop works in this; Mendicant Bias has already sent Master chief back infinite times through older versions of the past, each time the Master chief has been successful in stopping the flood, yet unable to save the forerunner, and this cycle will happen for eternity. There is not only one Master Chief or Mendicant Bias though, there is one with every version of the past.

What of the evidence that Master Chief went back into the past? Why Guilty Spark of course.
Those memorable lines in Two Betrayals:

GUILTY SPARK:

But you already knew that. I mean, how couldn't you?

CORTANA:

Left out that little detail, did he?

343 GUILTY SPARK:

We have followed outbreak containment procedure to the letter. You were with me each step of the way, as we managed this crisis.

CORTANA:

Chief, I'm picking up movement!

343 GUILTY SPARK:

Why would you hesitate to do what you have already done?

CORTANA:

We need to go, right now.

(Sentinels float up behind 343 Guilty Spark)

343 GUILTY SPARK:

Last time, you asked me, if it was my choice, would I do it? Having considerable time to ponder your query, my answer has not changed.

First time you heard that you though "What?". Right there is undeniable evidence that Guilty Spark knows the Master Chief from the past. Apparently Master Chief and Spark have met before a even made conversation, have a relationship, and Master Chief has fired the halo (Maybe even Halos) before. The Master Chief at Halo 04 does not know this though because this has yet to happen. Ironically, he must continue on through the future to live out the events of the past.

Now you may say Guilty Spark is rampant, but why would Bungie give us this nugget of speech from guilty spark, the only part about Spark's speech that sounds almost "human" (Obviously in the metaphorical sense not the biological sense), this part of him speaks the truth. Bungie has done this to make connections through the plot, as they love to do, and these connections always lead to something, it's just that this connection has been left unfulfilled for so long, that is definetly is the most important and has something to lead to. Even when I first heard this, I knew this was the most important line in the game.

Also note that this level and scene is called Two Betrayals, the most ambiguous subtitle I have ever come across. Mendicant Bias' message in the last terminals speaks of two betrayals to previous masters, and here is where I feel the link is and the meaning behind the name Two Betrayals. It is linking back to Mendicant Bias and why he sent to back to the past.

We know Spark has not mistaken Chief for a forerunner, because he is able to label him because of his biology as a human (Reclaimer) not forerunner.
But why would Spark talk to a human as if he has talked with him in the past, Master Chief must have been in the past.

Furthering the theory with more broader speculations, Master chief comes into contact with forerunners, who either have no clue who he is, or recognize his importance as a human (The species they sought to save), and a vastly intelligent one at that, who has significant experience in fighting the flood. Master Chief immediately becomes an important figure, due to his abilities as a superhuman (which could dwarf those of of the forerunners if they have any at all) and his experience in fighting the flood. Just think, the forerunners are losing a war, against an unknown enemy they have no idea how to defeat, then this unknown man comes along, who has experience with the flood, and knows how to destroy them. He is they Saviour the forerunner were looking for.

We don't know what happens after, but it is likely Master chief is the difference between crushing defeat from the flood, and narrow victory. And as Mendicant Bias had planned, Master Chief stops the flood. However it is unknown whether he saves the forerunners, therefore not fulfilling Mendicant Bias' redemption.

The term forerunner does not mean a race of people, as we have become to view it as. It is the people Before. Before the time of humans, they are the forerunner. This means anyone from the forerunner timespan can be forerunner, and Master chief falls into this category, thus he is Forerunner, and a very prominent forerunner at that. A scene comes to mind at this point "You are forerunner", Guilty sparks exclaims before blasting Master chief with super hot red jelly. This has caused many debates and much controversy, humans can't be forerunner, because of the Cradle of Life comic, the terminals, and the IRIS server 05. Yet Master Chief can be forerunner, because he was at the time of the forerunners, and Guilty Spark knows this. You may say that Spark is wrong because he is rampant, but that is a poor argument to hide behind, as it is Bungie who write the lines not Spark, and they wouldn't make spark do a stupid lie. That would be like Spark saying "You are Hippopotamus! But this ring is Mine!", just stupid. Bungie wrote it for a reason, the reason is that they like to come back and connect the story together. So Spark tells the truth.

Now, many people feel that time travel is a cheesy Sci fi cliche, however I have to disagree, it can be very tasteful indeed (Think of Dejavu, a superb film which uses time travel magnificently). Now Joseph Staten may have said in interview that the idea of time travel is a horrible cliche, yet he does not say they would not use it in the halo series. It is even in used in the books; the forerunner crystal that can warp space and time. More evidence that time travel is a possible plot device in the series, as it has been used before. The comments that Joseph Staten uses to answer the interviewer just asks more questions than it answers in his ambiguous nature. You can almost imagine that as he is saying it, he has a cheeky grin covering his face, as if he knows something we don't.

A new point to make. How could Mendicant Bias transport you to the past? Well we know the forerunners have created an artifact capable of warping space and time, it's likely not the only crystal they created, and it's likely that more were left on the ark. Whether the forerunners understood it or not is up for debate. I think it is less likely though, before they could have used the power of time travel against the flood to defeat them. However Mendicant Bias has had an extremely ample amount of time to study its use and power. This is why he is able to transport the chief back in time. Maybe the crystal needs such huge amounts of raw energy, like a huge explosion for it to work, that it is the result of the explosion of the ark and collapse of the portal that MC is back in time.

So there you go, the theory. DEBATE!

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http://youtube.com/watch?v=mlUDSiHm8lg

holy crap

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takes over 20 shots to kill, has piss-poor range,

"you are totaly right with everything, exept that one.
I killed my friend with an AR with like 7 three shot bursts :P"
ROFL

Some parts seemed a tad bit flawed but it is very thought through and possible

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CK3RN53 (12:00:21 AM): i hope you are ok
CK3RN53 (12:09:20 AM): that came out gay

tldr

sry

Omfg. Insane. This kid, thought hard about this. It actually kind of makes me laugh. But it is extremely amazing.

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wtf is this in halo 1 cause i saw a thread like this in another forum talking about some halo 1 things that indicated about time travel and it was posted about 4-5 months ago in that thread.

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^
Rofl

that is long.........

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I could not STAND the font...

To annoying to read with that ****ty font

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So you don't know what a shnoon is?

There I fixed the font problem =[

Now to go post pictures in the People of Haloboards or w.e thread

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http://youtube.com/watch?v=mlUDSiHm8lg