For a solid ground for the story we assumed that the string theory is accurate and that there are 13*10^500 different and layered universes. Meaning, that all those universes exist in one place at the same time, but cannot interact with each other because each one has its own properties.
The Asen are the oldest among all faction and through their science they figured out, that there are more then just their own world. So they created the Yggdrasil, the great world tree that is technically a map for all of those worlds. The Asens universe is called Asgard, a world with slightly varrying strengh of the 4 forces, but they are still close to our known forces.
The only way to get from one universe to another is by a wormhole. But not simply one that warps time and space rather than adjusts matter and energy to the values of the destination which can in fact leads to the total annihilation of the travelers if they picked the wrong universe. So it is always a dangerous act to travel from one universe to another and it is more likely to choose a world that has similar properties on a stable level. Even that is a great risk to your health...
Another problem is the destination within the targeted universe. Since every universe has its own dynamic motion, from the one second to another you can land in empty space or into the core of a sun for example. Therefor a beacon is needed to solve this issue: the Asen created a device known as Bifrost or Bifröst ("portal" in the Asen language), which works as beacon and as wormhole generator - technically like a stargate, details on that will follow later.
Multiverse idea? not bad at all! :)