Dimension Cannon

The dimension cannon or the dimension jump was a device that could travel between dimensions, at the cost of gradually ripping a hole in the universe. In normal circumstances, it didn't work, it only worked if the walls between universes were weak. It was created in Pete's World by Torchwood. (TV: Doomsday, Journey's End)

First version (Battle of Canary Wharf)
After the Cybermen left Pete's World to travel to the Doctor's universe, Torchwood created a device that could hop to another universe. This first version resembled a yellow disc. It would hang by someone's neck like a necklace and could teleport at the same position on the chosen world where they teleported from the other world. The use of the dimension cannon opened the breach further and further and raised the temperature on Pete's World. Pete's version was capable of transporting an entire group of people wearing the medallions in the same area while some (like Rose's) could only teleport the single user. After the Tenth Doctor sucked the Cybermen and the Daleks in the Void, the breach closed itself because of the amount of Void stuff present. The devices stopped working as a result. (TV: Doomsday)

Second version (War in the Medusa Cascade)
Torchwood subsequently developed a new dimension cannon. Using the weakening barriers between universes, (AUDIO: The Endless Night) which had been caused by Davros’ reality bomb, (TV: Journey's End) the cannon punched a hole in reality to another universe which enabled the original discs to work again. The hole could only be maintained for a few days at maximum however. It was initially believed only one person could travel using it, however Rose proved it could transport two after using it to rescue Clive Finch from dying on his parallel Earth. (AUDIO: The Endless Night) The discs could also now travel to different parts in space, but needed to recharge for thirty minutes after each use. The dimension cannon could also measure timelines. (TV: Journey's End)

Rose volunteered to use the cannon first as Torchwood began searching for the Doctor’s universe, visiting numerous dying universes. She was accompanied on missions by Clive, Pete Tyler and Jackie (AUDIO: The Endless Night, The Flood, Ghost Machines, The Last Party on Earth) and eventually reached the Doctor’s universe. (TV: Partners in Crime, The Stolen Earth) Mickey and Jackie used the cannon to follow her, arriving on Earth in time to save Sarah Jane Smith from a pair of Daleks following their invasion of Earth. Jackie later used it to escape from the testing of the reality bomb aboard the Crucible.

Following the invasion and Davros being prevented from using the reality bomb in the Doctor's universe, the walls of the universe eventually started closing themselves, a process that the DoctorDonna called a dimensional retroclosure. (TV: Journey's End)