As Hawkeye put it, doors opens both ways.
Is there any indication how Loki is able to trigger the Tesseract and get to the Earth in the first place?
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In the end-credit scene of Thor, we can see that Loki was already influencing Selvig's mind.
Selvig: What is it?
Fury: Power, doctor. If we can figure out how to tap it, maybe unlimited power.
Loki: Well, I guess that's worth a look.
Selvig: Well, I guess that's worth a look.
We can't know how, but Loki made him to design the device and make it work so that it can open a door to let Loki teleport. It was planned near the end of Thor already.