Answer the following question given this paragraph: 

We know that the Universe is expanding. Astronomers have wondered if it is expanding fast enough to escape the pull of gravity. Would the Universe just expand forever? If it could not escape the pull of gravity, would it someday start to contract? This means it would eventually get squeezed together in a big crunch. This is the opposite of the Big Bang.


Q: Astronomers have wondered if the universe is expanding fast enough to escape the pull of what?


A:
gravity