[@Priscilla]
There are alternate means of multiversal travel. All precedented methods are described here:

 - Terminals
the use of specific devices that house inter-universal connections, allowing any user to instantly move to the connected universe. Any one terminal connects to another specific terminal in that other universe. Terminals often adopt a form native to a particular universe. Known terminals are a nuclear power plant in Sweden, a toy train in New York, and I believe I recall a featureless orb being used by the Ethereal (although that may have been an orb containing a terminal). It is unknown if living beings can be terminals, nor if they would be able to 'use' themselves.

 - Adjacent Universe
Often, franchises contain multiple 'planes', 'universes', or 'dimensions'. Within that franchise one is able to travel between these universes in methods detailed in that particular franchise. These are usually considered adjacent universes, universes so close to one another that travel between them is possible without a terminal. Examples are Accel World, Narnia, and the Warp. Being able to move between such adjacent universes does not constitute true multiversal travel.

 - Barrier Breaking
All universes are surrounded by a sort of barrier. It's possible for immense amounts of energy to break through this barrier, ejecting people into the space between universes. Due to the anomalous nature of Arcadia, this usually causes people to end up in Arcadia. Most attempts at true multiversal travel therefore end up dumping people into Arcadia. Examples are Doomsday, who was sent to Arcadia after being launched out of his home universe by the Man of Steel, and the Emperor, whom's rebirth as the Star Child opened up a hole in the Warp and lead to Arcadia.

 - the Arcadia Anomaly
Something weird is happening in Arcadia. Whatever it is started a few years ago, and it caused Arcadia to take up the role of multiversal hub. This anomaly is the reason Barrier Breaking leads inevitably to Arcadia, and that the Outliners first came to investigate. This anomaly also appears to grab random beings from other universes, dragging them into Arcadia. Beings dragged here tend to be powerful and competent beings, although even the Outliners do not yet truly understand why. This phenomena is basically why everyone is in Arcadia, and why everyone wants to control it. Due to this phenomenon, there are also a strangely large amount of terminals in Arcadia, although almost all of them have been gathered by the Outliners and the Imperium, both of whom are able to detect such terminals. The anomaly has on occasion been known to drag entire fleets of ships and even empires towards Arcadia.