Comment by OkayPhysicist

The lifetime of human civilization problem is an odd one, because due to relativity, one-way trips are not an unsurpassable hurdle ( 2-3 generations on a 1 G spacecraft to get pretty much anywhere). But you can't come back, because it's basically guaranteed there'll be nothing left for you to come back to. Because while it might take "only" two hundred years from the passengers perspective to reach the edge of the (current) observable universe and come back, they'll be arriving 90 billion years in the future.