Stephen Baxter's highly acclaimed first novel and the beginning of his stunning Xeelee Sequence.
A spaceship from Earth accidentally crossed through a hole in space-time to a universe where the force of gravity is one billion times as strong as the gravity we know. Somehow the crew survived, aided by the fact that they emerged into a cloud of gas surrounding a black hole, which provided a breathable atmosphere.
Five hundred years later, their descendants still struggle for existence, divided into two main groups. The Miners live on the Belt, a ramshackle ring of dwellings orbiting the core of a dead star, which they excavate for raw materials. These can be traded for food from the Raft, a structure built from the wreckage of the ship, on which a small group of scientists preserve the ancient knowledge which makes survival possible.
Rees is a Miner whose curiosity about his world makes him stow away on a flying tree - just one of the many strange local lifeforms - carrying trade between the Belt and the Raft. Accepted as an apprentice scientist, he learns that their world is dying, and that in order to live these survivors must contemplate a journey even more perilous and fantastic than that of their ancestors.