How does the Bifrost still work after the destruction of Asgard?

Question

We know that after Thor uses Mjolnir to destroy the Bifrost in Thor, travel to other worlds was rather difficult and required other means, such as Dark Energy mustered by Odin (as stated by Loki in The Avengers), use of the Tesseract, or (rather more prosaically) by ship. Once the Bifrost was rebuilt prior to Thor: The Dark World, their ability to travel to other worlds is restored. From this we know that use of the Bifrost required a physical presence in Asgard, so with the destruction of Asgard at the end of Thor: Ragnarok, the Bifrost should also have been destroyed.

Yet in Infinity War we see it used twice, and also in a point-to-point way that hasn't been previously (all previous travel seen was to/from Asgard):

First Heimdall summons it on the refugee ship to send Hulk to Earth, and Thor later uses Stormbreaker to summon it to travel to Wakanda

How is this possible?

Answer 1

It isn't the Bifrost but the power of Dark Energy, mustered by the All-Fathers

The Bifrost in the MCU is described as a "wormhole" or "Einstein-Rosen Bridge", allowing for near instantaneous travel between the Nine Realms. However after its destruction in the first Thor movie, other methods were revealed for travelling across space-time. As outlined in this answer the All-Fathers muster "Dark Energy" to transport Thor to Earth while the Rainbow bridge was destroyed (evidenced in this supplementary MCU comic panels). While initially met with confusion from fans, this ability is re-confirmed in Infinity War when the Hulk is sent flying back to Earth by Heimdall summoning Dark Energy from the All-Fathers.

"All-Fathers, let the dark magic flow through me one last time."
_{Heimdall - Infinity War}

With regards to Stormbreaker, we are led by the film to believe that Stormbreaker has some form of Dark Energy imbued into it or the ability to harness Dark Energy to act as a method of transport across the nine realms, acting similar to the Bifrost before it was destroyed.

Answer 2

Your assumption is that the Bifrost requires a projection system. It appears that way because Asgard only wanted one way into and one way out of Asgard. It is also likely the users of Dark Magic are limited or it requires great skill or personal power to utilize the Bifrost portal creation magic without technological support.

The Marvel Cinematic Universe Wiki defines the Bifrost as a type of energy:

• The Bifrost Bridge is a dimensional energy which allows instantaneous travel to any planet. It was used primarily by the Asgardians, who were able to use a Rainbow Bridge to harness the energy and access the Bifrost in order to travel to and protect the Nine Realms.

• The Bifrost was also able to be called upon using dark magic and was also imbued into Stormbreaker, allowing its wielder to access the bridge at any time. The immense dimensional energy, given enough concentration, could also be used to destroy planets.

The Bifrost isn't a thing, it is a technology/magick usable by people versed enough or powerful enough to create the portal (Einstein-Rosen bridge) between worlds. The power to bridge between worlds was an aspect of Asgardian magic (called Dark Magic) which was channeled through members of the Asgardians capable of utilizing it.

Odin using his personal energy stores and Heimdall who was also able to summon such magic, in extremis. Since Heimdall was one of the Vanir, the gods of Asgard associated with magic, it doesn't seem unreasonable he could summon the ability to create a portal between Earth and Asgard using his own personal energy.

The technology of the Bifrost system however meant no Asgardian had to tap their personal power to do so. Like all technology, the goal was to simplify their need and make it able to be done even if a user of Dark Magic wasn't available. All you needed was the key -- which was Heimdall's sword.

It should not surprise anyone that Stormbreaker can be enchanted to do this. In the comics, Thor's Mjolnir was also capable of creating bridges between worlds and teleporting Thor to Asgard, as needed. In Avengers #166, Thor tries to create a portal to teleport the now super-powered Count Nefaria to another realm of existence. Nefaria counters by choking off the portal with a skyscraper.

Answer 3

A source of dark energy is needed. If you remember from Avengers (1) when Loki and Thor met

With Bifrost gone, how much dark energy did the All-Father have to muster to conjure you here

So presumably the bifrost is a source of dark energy which means the real question is... How much dark energy did Heimdall have left... Just enough apparently

Answer 4

The Bifrost can be summoned everywhere but requires a vast amount of energy to work, Thor tells Jane Foster that magic and science are the same thing. Therefore you can see the ability for Asgard to call the Bifrost as no more then the appliance of science through a big machine the key part of which is Heimdall's sword which is potentially the source of the dark energy. Anyone can open the Bifrost using the machine as long as they have the key but normally the sword needs the machine to summon the Bifrost.

In Avengers Loki asks Thor, how much dark energy did the Allfather use to bring you here. It is possible the sword was used as part of the effort to send Thor to earth.

In infinity War, Heimdall, who we know has powers beyond other Asgardians, is able to call the Bifrost down this obviously takes a toll and possibly uses up the energy in the sword or takes a toll on the bearer hence they don’t do it normally.

Eitri then states that the weapon he is forging for Thor even has the power to call down the Bifrost potentially this is the same power Heimdall's sword has and you can imagine the dwarfs forged the sword and made the machine that used its energies. However unlike the sword it doesn’t require a machine to summon the Bifrost it can do it itself.