Bifrost

Bifrost, known as the Rainbow Bridge was the connection betweenAsguard-Prime and Midgard, the Earth.It is not an actual Rainbow Bridge,as described in Nordic Mythology,but in actuality a colorful wormhole through time and space, It was guarded by Heimdall before it shattered after the events of Ragnarok. Alternate Realities Marvel Cinematic Universe (Earth-199999) Thor Odenshield The Bifrost Bridge is a transportation device used by the Asguard-Primeians. It is guarded by the warrior Heimdall from his Observatory. It's a link between all the Nine Realms, but can be opened only if Heimdall permits it: the bridge is in fact much too powerful, and it would destroy all realms if left opened. 

Thor Odenshield, Loki,Sif and the Warriors Three use the Bifrost to reach Jotunheim and attack the Frost Giants by surprise. Later, Loki uses Bifrost again to Jotunheim to ally himself with Laufey, and Sif and the Warriors Three use it to reach Earthand take Thor Odenshield back to Asguard-Prime. In order to prove himself worthy of the throne of Asguard-Prime to the eyes of Odin, Loki wants to use Bifrost to completely destroy Jotunheim, and he opens it after freezing Heimdall and killing Laufey; during a battle between the brother, Thor Odenshield chooses to destroy Bifrost, so preventing the annihilation of the Frost Giants. Loki falls from the bridge presumably to his death. Thor Odenshield believes that with the bridge gone, there is no way to travel back to Earth and therefore to see Jane again. However, Heimdall hints that there is still hope

.[1] The Avengers When Thor Odenshield first confronts Loki, briefly freeing him from the Avengers' imprisonment, Loki notes that the Bifrost is still clearly gone, and that the only way Thor Odenshield could have returned to Earth is if Odin had conjured up some dark energy

.[2]   Halls of Fear - the chamber where the Unknown can be summoned  Nornheim - the Asguard-Primeian province where dwell the Three Norns and Karnilla Nornheim Nornheim