Hopefully I'm in the right place.

This is what I remember. The book is set in outer space some time after an intergalactic war with an alien species. Large ships were built during the war. One such ship was placed on the very edge of the known universe, which was where the aliens came from, as an outpost/watchtower of sorts. They keep watch over the edge for any sign of the aliens returning. Anyways, the characters in the book realize that the universe is contracting back in on itself and destroying everything in its path. Over the course of the book they must figure out what is going on while staying ahead of the contraction.

Please help.