i believe that the portal works like this.
it goes at light speed which is why it takes them such a short time to travel this distance, but my point being that at light speed you are sorta in a wormhole, which is a space tube. i believe it is a forced wormhole that in the middle of its use collapsed, and leaving master chief in between our universe and another, sorta like a sub universe where everything has similarities to both universes, the marathon universe is one thing and halo's is another, but in between them master chief becomes the marathon man, and fills in its part. also this could also be a time travel theory because a wormhole defies space and time, in that it brings short cuts throughout the entire universe in both cases