No final de 4x20, Worlds Apart ,
The bridge between the two universes was closed, permanently preventing anyone from either universe from going to the other one.
Em 5x04, A bala que salvou o mundo , Peter faz um comentário irritado a Walter:
"There has got to be a better option for getting us into that train station than a portal that lets us see into another universe."
So in theory, they could still use one of the various methods from prior seasons to travel between universes, but breaking the repaired barrier between them would leave them in a worse situation than in previous seasons: Both universes on a collision course where neither will survive, and the machine no longer exists/is no longer accessible to repair the damage.