“If there was any tampering, sir, it wasn’t at this end.”
“At this end?”
“Yeah, the cube – it’s a doorway to the other end of space, right? Doors open from both sides.”
– Clint Barton to Nick Fury
I will admit right off the bat that the first time I watched Thor I had to remind myself that the Casket of Ancient Winters was not the Tesseract. I mean, they’re both cube-ish, they’re both blue, they’re both MacGuffins. I mean, seriously. It’s almost like they were purposefully made to look similar or something crazy like that.
I will start off by saying that there is very little to no backing from the comics in this segment (that I know of at least). I would like to propose the theory that the Tesseract and the Casket of Ancient Winters are two sides of the same door.
“The Tesseract was the jewel of Odin’s treasure room.”
– Red Skull
The Red Skull makes that statement at the beginning of Captain America: The First Avenger after recovering a false Tesseract. The real Tesseract is found in a carving of Iggdrasil on the wall. For those who weren’t paying attention in Thor, Iggdrasil is the World Tree – the little space-tree picture that Thor draws for Jane Foster while they’re at the camp fire. Just like Thor and Odin and Loki, it also originates from Norse mythology. That quote establishes a connection between the Tesseract and the Nine Realms part of the movies.
Furthermore, the Red Skull finds the Tesseract in a village called Tonsburg, Norway. We also see Tonsburg, Norway in the beginning of Thor. We see it being attacked by the Frost Giants (who I will be calling Jotuns by the way). The Jotuns appeared to be teleported there by the Casket of Ancient Winters (I mean, Laufey’s kind of just standing there holding it). It is at this battle that the Jotuns fall to the Asgardians.
After the battle, Odin and the Asgardians take from the Jotuns the Casket of Ancient Winters, leaving them apparently stranded on Jotunheim.
“Our armies drove the Frost Giants back into the heart of their own world. The cost was great. In the end, their king fell and the source of their power was taken from them.”
I believe that the Tesseract is the Earth-side of the door and that the Casket of Ancient Winters is the Jotunheim-side.
I would like to point out that, at the finale in Thor, we see Loki use the Casket of Ancient Winters to freeze Heimdall and then he puts it back in whatever Bag of Holding he has. When he fell through the portal, he still had the Casket of Ancient Winters.