Bifrost Bridge

"What you seek, it is a bridge." "Like an Einstein-Rosen Bridge?" "More like a Rainbow Bridge."

- Thor and Jane Foster

The Bifrost Bridge is an interstellar portal which allows instantaneous travel to any of the Nine Realms and planets beyond. The Bifrost Bridge is guarded and controlled by Heimdall and/or the King of Asgard. The Bridge can only be activated by either the sword of Heimdall or the spear Gungnir wielded by the King of Asgard. Once opened the bridge calls upon powerful energies from deep within Asgard to function, and could even destroy an entire planet if left opened.

History
Thor, Loki, Sif and the Warriors Three used the Bifrost Bridge to reach Jotunheim and attack the Frost Giants by surprise after being told by Loki they were going to attack Asgard; even though Heimdall was not happy letting them cross the bridge.

Odin, riding Sleipnir, appeared via the Bifrost Bridge and asked Laufey to excuse his son's and friends' incursion. When Laufey would not and created an ice blade, Odin signaled for the bridge's return and the Asgardians were swept away in its wake.

Upon their return to Asgard, Odin used Gungnir to activate the Bifrost, while he and Thor argued; Heimdall was dismissed from his post. Odin then stripped Thor of Mjølnir and his armor before banishing him to Earth to be hit by Jane Foster and her colleagues.

Later, Loki used the Bifrost Bridge again to Jotunheim to ally himself with Laufey, so that they could attack Asgard using the Bifrost Bridge rather than through one of Loki's secret pathways.

Once again Sif and the Warriors Three used it to reach Earth to rescue Thor and bring him back to Asgard after being banished to Midgard. In order to prove himself worthy of the throne of Asgard, Loki wanted to use the Bifrost Bridge to completely destroy Jotunheim and he was able to open it after freezing Heimdall and killing Laufey.

Upon returning from his exile, Thor chose to destroy the Bifrost Bridge in a duel against Loki, so preventing the annihilation of the Frost Giants.

After the Bifrost Bridge was destroyed it has been regenerated with the energy obtained from the Tesseract, and Heimdall continues to guard the Bridge.

Thor and his friends used the Bifrost Bridge multiple times to pacify the Nine Realms and during the War against the Dark Elves.

Thor used the Bifrost Bridge to reunite with Jane Foster days after the resolution of the Dark Elf Conflict and the death of Malekith.

Sif used the Bifrost Bridge to come to Earth when she was chasing the criminal Lorelei. After her capture, the duo used the bridge to return to Asgard.

A year later, she used the Bifrost Bridge to transport the Kree agent Vin-Tak from Earth.

Thor used the Bifrost Bridge so he could return home to research the Infinity Stones after Ultron's defeat. Tony Stark disdainfully commented on how the Bifrost burned the ground it touched.

Capabilities
The Bifrost Bridge is a transit system used by Asgardians to travel to the various realms among the Nine Realms and numerous other planets. The Bifrost uses a powerful, rainbow-like energy drawn from the center of Asgard to create Wormhole-like portals that can instantaneously transport people and objects between realms, over light-years of distance. On other realms, the Bifrost appears as a brilliant beam of white light that shines down on it's surface, often leaving behind a rune-like pattern burned into the ground.

The Bifrost Bridge is composed of two physical structures in Asgard: Heimdall's Observatory and the Rainbow Bridge, which act as two components of a large machine that channels energy in Asgard to create the portals that allow for transportation. The Rainbow Bridge draws energy from the center of Asgard to Heimdall's Observatory where it is directed to it's destination allowing transportation that is precise and accurate. The Bifrost also behaves like a tractor beam, drawing people and objects bathed in its light back to Heimdall's Observatory, such as people wishing to reach Asgard or pulling the Aether from Svartalfheim to prevent Malekith from using it. The beam of energy projected by the Bifrost is powerful enough to destroy a realm/planet if it is allowed to remain open long enough, as Loki tried to do to Jotunheim.