Dimensional retroclosure

Dimensional retroclosure was the process of the barriers between different parallel universes closing. When Davros and the New Dalek Empire began stealing planets from the Medusa Cascade, the walls between the universes began to weaken, allowing inhabitants of Pete's World to cross over to the Doctor's universe and vice versa. After the reality bomb was prevented, the Tenth Doctor and Donna Noble took Rose Tyler, Jackie Tyler and the Meta-Crisis Doctor back to their parallel universe, with Donna explaining that the walls were closing again and once sealed would prevent any travel between the universes. (TV: Journey's End)