In Thor, it's stated that the Bifrost Bridge is how Asgardians travel between the worlds. When Thor destroys the Bridge at the end of his first film, Loki and Thor have the following exchange.
LOKI: Stop! What are you doing?! If you destroy the Bridge, you'll never see her again!
THOR: Forgive me.
Thor transcript via The Internet Movie Script Database
If the bridge was destroyed, how does Thor return to Earth in The Avengers? Is there any in-universe explanation?